BRUHN Et Al-1997-Sedimentology PDF
BRUHN Et Al-1997-Sedimentology PDF
BRUHN Et Al-1997-Sedimentology PDF
ABSTRACT
Successions of Early Eocene coarse-grained turbidites up to 400 m thick fill
fault-controlled canyons along the eastern Brazilian continental margin. They form
part of a Late Albian to Early Eocene transgressive succession characterized by
onlapping, deepening-upward sedimentation. In the Lagoa Parda oil field (Regencia
Canyon, Espirito Santo Basin) the turbidite facies consist mostly of unstratified
conglomerate and sandstone, with interbedded bioturbated mudstone and
thin-bedded, stratified sandstone. Within the main Regencia Canyon, the coarser
grained facies occur within 38 deeply incised channels. The fills are 9 to >50 m
thick, 2 1 0 to > l o 5 0 m wide and >1km long. The finer grained facies build
asymmetrical levees that are higher and thicker on the left side (looking
downstream) of their channels, probably as an effect of the Coriolis force (to the left
in the Southern Hemisphere). Nine levee successions up to 50 m thick are
associated with the 20 youngest channels. The deposits filling the low-sinuosity
Lagoa Parda channels record successive channel abandonment through relatively
rapid avulsions. Avulsions of unleveed channels took place randomly, but channels
with well-developed levees show preferential avulsion to the right (looking
downstream), opposite to the direction of preferential levee growth.
Lagoa Parda channels can be grouped into three complexes 20-100 m thick.
These complexes have an estimated duration of about 140 000 years. It is suggested
that control of the development of individual channel complexes was related to
variation in sediment supply, in turn probably related to climatic changes. The
deposition of each channel complex would have followed an increase in sediment
supply into the Regencia Canyon through deltdfan-delta and littoral drift systems,
which in turn would have responded to phases of higher denudation rates in the
high-relief, ancestral coastal ranges of south-eastern Brazil.
Overall, the three Lagoa Parda channel complexes form a turbidite succession
characterized by channel fills that become narrower, thinner and finer grained
upward. These trends were induced mostly by a longer term (>400 000 years)
decrease in sediment supply, which in turn resulted from the combined effects of a
long-term (second-order) trend of sea-level rise, and the decreasing fault activity at
the basin margin and source area.
MEGASEQUENCES:
0shales
and m h
siltstone,
carbonate
PR - Continental pre-rift megasequence
(Late Jurassic to Early Neocomian) evaporite
Fig. 1. (A) Location of the Espirito Santo Basin, and other important Brazilian sedimentary basins. Note the location
of the Early Eocene Regencia Canyon area (RC). which is shown in detail in Fig. 2. (B) Generalized geological section
for the eastern Brazilian basins.
larger Early Eocene Reggncia Canyon, on the turbidites, or in turbidites deposited in post-rift,
western side of the immature passive margin intracratonic sags or passive margins. The Lagoa
basin of Espirito Santo, eastern Brazil (Fig. 1A). Parda field requires detailed description so that it
The turbidites form oil reservoirs in the Lagoa can be used as a case history, or model, in future
Parda (LP) oil-field, located near the mouth of the exploration.
Doce River (Fig. 2). Immature passive margin basins with good
Turbidites form major petroleum reservoirs in core and well-log control make good examples
more than 80 sedimentary basins worldwide, and for studying the effects of various allocyclic con-
the combined volume of recoverable oil from trols on turbidite sedimentation. These basins
fields in the North Sea, California and eastern commonly have proximal sediment sources,
Brazil exceeds 4 x 109m3 (25 billion barrels; narrow and steep coastal plains and shelves, and
Weimer & Link, 1991). Passive margin basins, like intense tectonic activity contemporaneous with
those on the eastern margin of Brazil, represent sedimentation. Here, turbidite sedimentation is
one of the most important frontiers For oil explo- very sensitive not only to sea-level fluctuations
ration in turbidite reservoirs, and future discover- but also to variations in tectonics and sediment
ies are anticipated in synrift lacustrine or marine supply.
1997 International Association of Sedimentologists, Ssdimentology, 44, 17-46
Coarse-grained turbidite channel-levee complexes, Reggncia Canyon, Brazil 19
Immature passive margin basins also contain Turbidite systems tend to be only partially
examples of turbidite settings that have received preserved in outcrop (especially the mud-rich
little attention in the literature, particularly levee successions), making their internal
coarse-grained channel-levee complexes, and geometries very difficult to reconstruct. Many
coarse-grained canyon fills. Mud-rich channel- recent submarine fans provide entire systems that
levee complexes have been extensively studied in can be characterized seismically, but have poor
many modern submarine fans (Damuth et al., core and chronological control. There is conse-
1988; Weimer, 1989). In ancient rocks they have quently a need for sedimentological description
been termed type I11 systems (Mutti, 1985) or and interpretation of channel-levee complexes
slope fans (Van Wagoner et al., 1990; Vail et al., using subsurface data, especially if developed at
1991). The few coarse-grained channel-levee the oil-field scale. Oil fields such as Lagoa Parda
complexes described in the literature occur in the that have a large number of closely spaced wells,
ancient rock record (Walker, 1985; Bruhn & a comprehensive production/pressure history,
Moraes, 1989; Morris & Busby-Spera, 1990) and extensive coring of the reservoirs permit a
and have not been described from modern detailed geometry and facies characterization of
fans. Although many submarine channels are the turbidite reservoirs.
filled dominantly with fine-grained sediments
(Goodwin & Prior, 1989; Galloway et al., 1991),
there are others that include a thick succession GEOLOGICAL SETTING
of coarse, commonly conglomeratic turbidites
(Morris & Busby-Spera, 1988; Bruhn & Walker, Brazilian continental margin
1995) or have gravel on the canyon floor
(Malinverno et al., 1988; Hughes Clark et al., The basins lie beneath the present-day coastal
1990). Despite the economic importance of plain, continental shelf and slope of eastern Brazil
coarse-grained channel fills as petroleum reser- (Fig. 1A). Their tectonic and sedimentary evol-
voirs (Weimer & Link, 1991), these deposits have ution is linked to the Neocomian breakup of
not been incorporated in channel-feeding-lobe Gondwanaland and the subsequent opening of the
models (Mutti & Ricci-Lucchi, 1972; Walker, South Atlantic Ocean. The Late Jurassic to Recent
1978), nor in the eustatically driven models of stratigraphy of the basins can be subdivided into
Mutti (1985) nor in the sequence-stratigraphic six megasequences, as shown by arrows and dated
schemes of Van Wagoner et al. (1990) and in Fig. 1(B) (Bruhn, 1993). The basins are typical
Posamentier et al. (1991). of those on passive margins (McKenzie, 1978),
( 1997 International Association of Sedimentologists, Sedimentology, 44,17-46
20 C. H. L. Bruhn and R. G. Walker
Fig. 3. Geological cross-section of Regencia Canyon, located in Fig. 2. Dashed lines indicate position of the succession
studied. The canyon margins are controlled mainly by listric faults soling out on anhydrite beds of the transitional
evaporitic megasequence. Six major unconformities numbered 1-6 occur in the marine transgressive and marine
regressive megasequences (Late Turonian(?) to Middle Eocene). The marine transgressive megasequence is bounded
by unconformities 2 (or very locally by 1) and 5 . Early and Middle Eocene calcareous nannofossils zones are
indicated by lower-case letters, (a) Neochiastozygus chiastus, (b) Tribrachiatus orthostylus, ( c )Discoaster lodoensis,
(d) Discoaster kuepperi, (e) Chiasmolithus staurion and (f) Chiasmolithus grandis (Antunes, 1990). Combined
thickness of conglomerate and sandstone in the Early Eocene Neochiastozygus chiasfus zone is indicated (in
parentheses) for each well in the cross-section. Lithological symbols are explained in Fig. 4. Gamma-ray and
resistivity well logs for well A-1 are shown in Fig. 5. Vertical exaggeration x 3.
and result from a succession of thermomechanical It is one of several large submarine canyons (up
processes including continental rifting, crustal to 25 km wide and 100 km long) excavated along
extension and rupture, and subsequent sea-floor the eastern Brazilian margin after the Middle
spreading. Accommodation space for Mesozoic- Albian. It was filled by deep-water sediments up
Cenozoic successions over 10 km thick was to 1000 m thick, of mostly Early Maastrichtian,
created by Neocomian to Barremian mechanical Late Palaeocene and Early Eocene age (Figs 3
(rift) subsidence, followed by Aptian to recent and 4).
thermal-contraction subsidence (enhanced by Antunes (1990) recognized nine major phases
flexural loading of sediments; Chang et al., 1988, of erosion and filling in the Late Cretaceous to
1992). Middle Eocene section of the Regencia Canyon,
mostly based on missing calcareous nannofossil
zones. The six most important and extensive of
Stratigraphy of Lagoa Parda (LP) turbidites
the unconformities are shown in Figs 3 and 4.
LP reservoirs consist of sandy and conglomeratic LP reservoirs occur within the thickest (up to
turbidites that form part of the marine transgres- 650 m) and coarsest (up to 400 m of conglomerate
sive megasequence (Fig. 1B). This megasequence and sandstone) part of the Early Eocene Regencia
is characterized by onlapping, deepening-upward Canyon fill (Figs 3 and 5). These rocks were
sedimentation in all of the eastern Brazilian deposited in deep neritic to upper bathyal depths
basins (Chang et al., 1988). LP reservoirs are (200-500 m; Azevedo, 1985). They are included
contained within a thick succession of Early in the Neochiastozygus chiastus nannofossil
Eocene coarse-grained turbidites deposited in zone (Antunes, 1990), which has been tied to
Regencia Canyon (Fig. 2). This NE-orientated the chronostratigraphic chart of Berggren et al.
canyon is up to 6 km wide and 15 km long (Figs 2 (1985), and has a time span between 57.8 and
and 3), with a fill up to 1 km thick. 56.5 Ma (1.3 Myr).
c) 1997 International Association of Sedimentologists, Sedimentology, 44, 17-46
Coarse-grained turbidite channel-levee complexes, Regencia Canyon, Brazil 21
W A-I E
(2:) (ltl) (400) (3036) (707) (36)
0
- 1,000
- 1,500
MARINE REGRESSIVE
MEGASEQUENCE
SHALLOW CARBONATE
PLATFORM MEGASEQUENCE (?)
.2,000
r l SandstonelConglomerate
Sandstonel
Mudstone (lacustrine)
+ .+
High-degree metamorphic . 3,000
rock (m)
Fig. 4. Longitudinal cross-section of Regencia Canyon, located in Fig. 2. Dashed lines indicate the succession studied.
The reactivation (up to Early Eocene] of a fault transverse to the canyon allowed the exposure of Precambrian rocks
along the western canyon floor. Six major unconformities numbered 1-6 occur in the Late Turonian(?) to Middle
Eocene marine transgressive and marine regressive megasequences, but in this cross-section only unconformities 2-6
can be observed. The marine transgressive megasequence is bounded by unconformities 2 and 5. Lower-case letters
refer to calcareous nannofossils zones defined in Fig. 3 . Combined thickness of conglomerate and sandstone in the
Early Eocene Neochiastozygus chiastus zone is indicated (in parentheses] for each well in the cross-section. Vertical
exaggeration x 6.
Very little is known about Early Eocene tur- was reactivated up to the early Eocene, when
bidites further down Regencia Canyon from the Precambrian high-grade metamorphic rocks were
LP field. The two wells offshore contain some exposed along the western floor of the canyon
coarse-grained turbidites, but the overall succes- (Fig. 4).
sion is thinner (Figs 2 and 4). Recurrent Coniacian to Eocene magmatic
activity was very important in the Espirito Santo
Basin. Basaltic lava flows are found in many
Tectonics and magmatism
offshore and onshore wells, and the large
The structural framework of the Reghcia Canyon Abrolhos volcanic complex about 85 km north-
area is characterized by two sets of normal faults east of the LP field was built by Late Cretaceous to
orientated N-S and NE-SW. Reactivation of the Eocene volcanic rocks and sediments derived
NE-SW-orientated faults induced the develop- from a pre-existing narrow continental shelf
ment of listric normal faults that sole out on the (Ponte & Asmus, 1978).
transitional evaporitic megasequence (southern
side of Fig. 3). The basement-detached faults, in
Data base
turn, controlled the evolution of the Reghcia
Canyon boundaries (Fig. 3 ) , and the development The LP field (Fig. 2) is about 2.8 km long and
of smaller tributary troughs along its northern 2.5 km wide. There are 70 wells with an average
margin (Fig. 2). The N-S-orientated fault system spacing of 200-250m (Fig. 6). Each well has a
( 1997 International Association of Sedimentologists, Sedhentology, 44, 17-46
22 C. H. L. Bruhn and R. C. Walker
were calibrated with well-log responses, in an
Well A-1 attempt to characterize facies in uncored wells.
t MARINE REGRESSIVE
MEGASEQUENCE
P
5 L
L [Late] Early Eocene:
Discoaster lodoensis zone
FACIES DESCRIPTIONS
MARINE TRANSGRESSIVE [Middle] Early Eocene:
1MEGASEQUENCE Tribrachiatus orthostylus zone
Four major facies have been distinguished on the
basis of texture, physical sedimentary structures,
cc3 +@ bed thickness and bioturbation. A typical core is
shown in Fig. 7, in which the various beds are
related to gamma-ray and resistivity logs.
NORTHERN MARGIN
0 r
OF REGENCIA CANYON
LAGOA PARDA FIELD
0 Fir
Amalgamation of individual beds is common, incorporated into the deposits of the same tur-
forming sandstone packages up to 55 m thick. bidity current that induced channel-wall col-
Within these packages, the contacts between indi- lapse. Closely spaced well control and detailed
vidual graded beds are typically sharp and in correlations (described below) show that the
places clearly indicate erosion of the underlying coarse beds of Facies 1 always form channel-fill
bed. Facies 1 successions typically have sharp deposits in the LP field (Fig. 9).
contacts with underlying or overlying succes-
sions of finer grained sandstone and mudstone of
Facies 2-4. Some Facies 1beds grade upward and Facies 2: unstratified coarse-grained
laterally to the finer grained sandstone of Facies 2. sandstone and parallel-stratified medium- to
fine-grained sandstone
Interpretation Description
In the deep neritic to upper bathyal (200-500 m) Facies 2 consists of graded sandstone beds 0.3-
environment suggested by the benthic foramini- 3.8 m thick, interbedded with bioturbated mud-
fera (Azevedo, 1985), the thick, unstratified, stone beds thinner than 0.3m. The sandstone
coarse-grained graded beds of Facies 1 are beds are sharp-based and normally graded, with
best interpreted as turbidites, deposited rapidly unstratified, very coarse- to (mostly) coarse-
from suspension by high-density turbidity cur- grained sandstone at the base, grading up into
rents (Lowe, 1982). The common occurrence of parallel-stratified, medium- to fine-grained sand-
boulder- to pebble-sized clasts suggests that many stone (Tab,Bouma, 1962; Figs 7 and 10). A few of
clasts were supported by the combined effects of the graded beds are capped by thin m),
fluid turbulence and dispersive pressure resulting ripple cross-laminated, fine-grained sandstone
from grain collisions (Lowe, 1982). The large (Tabc;Fig. 10, turbidite b l ) . The stratified sand-
intraclasts were probably derived from the ero- stone beds may also show a pervasive bioturba-
sion and collapse of channel margins, and were tion in their uppermost parts, with traces of
deposited within a very short distance of their Thalassin oides, Ophiom orph a, Plan olites and
origin. These large intraclasts were probably Helminthopsis. The sandstone is characterized by
91997 International Association of Sedimentologists, Sedirnentology, 44, 17-46
24 C. H. L. Bruhn a n d R. G. Walker
poor to moderate sorting, subangular to angular
WELL C-7 grains, and low mud matrix content (<5%). The
Gamma-ray log Resistivity log contacts between superimposed graded beds, or
between graded beds and underlying mudstone,
are typically sharp (Fig. 7 ) , but they may be
deformed by loading, injection structures and
centimetre-sized ball-and-pillow structures. The
interbedded bioturbated mudstone is similar to
that described below in Facies 3. Facies 2 succes-
sions, typically 5-25 m thick, may grade upward
or laterally into Facies 3 , or be sharply underlain
or overlain by Facies 1 or 3 successions.
The lack of conglomerate with exogenic clasts
and the abundance of stratified sandstone distin-
guish Facies 2 from Facies 1. However, boulder-
to granule-sized mud intraclasts are common in
Facies 2 and locally form unstratified, intraclastic
sandstones (Fig. 7) or intraformational conglom-
erate beds. The clasts are concentrated at the base
of beds, or can be widely and randomly distrib-
uted through the graded beds of Facies 2. Average
porosities and permeabilities for the unstratified
sandstone are 25% and 290 mD. For the stratified
sandstone they are 23% and 84 mD.
Interpretation
The Tab and Tabc sandstones of Facies 2 are
interpreted as turbidites. The finer grain size in
the unstratified sandstones and the presence of
lamination and cross-lamination in Facies 2 sug-
gest that it was deposited by turbidity currents of
lower density than those which deposited Facies
1, Slower deceleration permitted the transfer of
sediment from suspended to bed load, followed
by traction sedimentation in Facies 2. The
boulder- to granule-sized mud intraclasts were
probably derived from the erosion and collapse of
channel margins, with very little transport of the
clasts. Detailed correlation between the closely
spaced wells shows that Facies 2, like Facies 1,
forms channel-fill deposits in LP field (Fig. 9).
Most of the sandstone beds are 2-8cm thick, In the interbedded bioturbated mudstone, the
but a few thicker (up to 1m) sandy successions trace fossil assemblage is largely dominated
also can be recognized (Fig. 11).Individual beds by Thalassinoides (Fig. 11). Planolites and
contain Bouma's (1962) divisions, mainly Tbc,T, Helminthopsis are widespread but less common
and T,. Thicker sandy successions ( > l o cm) are (Fig. 1 2 ) . Thin (<5 cm), reddish-brown concre-
composed of amalgamated T,, beds (Fig. 11).The tionary horizons containing siderite, rhodo-
ripples are commonly climbing, and convolute chrosite and pyrite are commonly found in Facies
lamination is also abundant. In some cores, the 3 mudstone (Fig. 1 2 ) , and can be correlated over
thin-bedded sandstone beds dip as much as 10". large areas of the LP field.
The successions containing dipping beds may Mudstone-rich successions of Facies 3 also
overlie subhorizontal successions of Facies 1,2 or show gentle depositional dips. The dipping beds
3. Because there was no fault activity during depo- may contain horizons of deformed mudstone up
sition of the turbidites in the LP field, and because to 50 cm thick, and disorganized, intraformational
the wells are vertical, the dips are interpreted as conglomerate beds. The conglomerate contains
original depositional dips. The most common contorted, cobble- to granule-sized mud intra-
trace fossil within the sandstone is Ophiomorpha. clasts dispersed in a poorly sorted matrix
Average porosities and permeabilities for Facies 3 composed of variable proportions of mud and
sandstone are 23% and 72 mD. sand.
C) 1997 International Association of Sedimentologists, S d m e n t o l o g y , 44, 17-46
26 C. H. L. Bruhn and R. G. Walker
LAGOA PARDA CHANNEL COMPLEXES
NW Tribmchmtusorfhosfyluszone + SE
Neochiastozygus ChiasNs zone + v.
loom-
50 -
0-
Fig. 9. Schematic cross-section showing the stratigraphic relationships between the levee successions and the most
important channel-fills of the Lagoa Parda turbidite system. This section was constructed by projecting 25
channel-fills into a single cross-section at a position and orientation similar to that of Fig. 13(A).The section
overestimates the sand-to-mud ratio present in the Lagoa Parda turbidite system because it contains a larger number
of channel-fills than any other section (compare with Fig. 13), and also because the channel-fills are represented by
their maximum width and thickness. Note the levee asymmetry and the overall tendency for channel-fills to become
narrower, thinner and finer grained upward. Datum is a high-resistivity mudstone horizon rich in siderite,
rhodochrosite and pyrite, which is subparallel to the contact between the Early Eocene Neochiastozygus chiastus and
Tribrachiatus orthostylus biozones. Vertical exaggeration x 5.
sandy and/or gravelly turbidity currents capable Neochiastozygus chiastus zone, which is up to
of deeply eroding the canyon substrate and 650 m thick (Fig. 3).
depositing coarser grained sediment.
Definition and mapping of individual
channel-fill deposits
GEOMETRY OF CHANNEL-FILL A N D
LEVEE DEPOSITS LP channels filled with Facies 1-3 were incised
up to about 50 m into mudstone-rich successions
Stratigraphic relationships within LP reservoirs (Facies 3 and 4) or into older channel-fill deposits
are shown in the cross-sections (Figs 9 and 13). (mostly Facies 1 and 2; Figs 9 and 13). At least 38
The datum is a high-resistivity mudstone horizon, channel-fills can be mapped in the study area
containing concretions of siderite, rhodochrosite (Figs 9 and 13, and Table 1). The definition and
and pyrite (Fig. 12). This datum overlies the mapping of individual channel-fills (Figs 14-16)
channellized sandstone and is subparallel to the is easier where coarser-grained turbidites (Facies
contact between two early Eocene calcareous 1 and 2) are laterally and vertically bounded by
nannofossil zones (Neochiastozygus chiastus and mud-rich facies (Facies 3 and 4). This can be seen
Tribrachiatus orthostylus; Figs 5 and 13). The in channel-fills 5, 9, 18 and 33 (Fig. 13A), and 30,
sections (Figs 9 and 13) illustrate detailed corre- 33, 37 and 38 (Fig. 13C). Where coarse-grained
lations within the uppermost 230m of the channel fills are partially amalgamated, their
f, 1997 International Association of Sedimentologists, Sedirnentology, 44, 17-46
28 C. H. L. Bruhn and R. G. Walker
separation is more difficult and involves three- conglomerate, whereas channel-fills 5 and 9 are
dimensional correlations using all available wells mostly composed of lower resistivity, very coarse-
(Fig. 6). The most important criteria for the differ- to coarse-grained sandstone. Thus, channel-fills
entiation and mapping of amalgamated channel- 6 and 1 0 (with the very coarse-grained facies)
fills include: (i] abrupt facies changes; (ii) are thought to be separate from, and truncate,
different stratigraphic positions of their upper- channel-fills 5 and 9, respectively. Their lateral
most sediments; and (iii) diverse sand-body separation is suggested in Fig. 13(C) by an abrupt
orientations. These three criteria, almost always facies change between wells B-7 and D-3.
used in combination, are illustrated in the
following examples.
Different stratigraphic positions of their
uppermost sediments
Abrupt facies changes
The effects of differential compaction in LP
Although channel-fills 5, 6, 9 and 1 0 all have appear to be minimal, as indicated by the flat
blocky SP log patterns in wells B-7 and D-3 (Fig. tops of channel-fills 5, 9, 10, 1 7 , 30, 33 and 37
13C), the core-calibrated well logs show different (Fig. 13C), which are subparallel to the datum and
facies. Channel-fills 6 and 10 contain many hori- to the high-resistivity markers a , p, y , and 6
zons of high-resistivity, boulder- to pebble-rich (Fig. 13A). We therefore suggest that the different
?) 1997 International Association of Sedimentologists, Sedimentology, 44, 17-46
Coarse-grained turbidite channel-levee complexes, Reg&ncia Canyon, Brazil 29
Channel mapping
Net sand maps for 20 channel-fills composed of
Facies 1-3 are presented in Figs 14-16. Many
of these channel-fills were partially eroded by
younger channels. Most of the channels were
mapped by isopaching their coarser grained fills
(Facies 1 and 2), which have different well-log
responses from levee deposits (Facies 3) or back-
ground facies (Facies 4). However, the portions
of these channels filled with Facies 3 cannot
be differentiated from nonchannellized, laterally
associated mud-rich facies, except in unusual
cases. For example, in channel-fills 34 and 35
(Figs 13A and 15C), the well logs show
decreasing-upward resistivities, contrasting with
the increasing-upward resistivities shown by
laterally associated levee succession F (Fig. 13A).
1-3 L-
......(
4 1
SP IES 0
.. . . . . , ,
., . . . ....
'IES 0 200m
Fig. 13. Geological cross-sections of the Lagoa Parda turbidite system (channel complexes CC1 to CC3). Datum is a
high-resistivity mudstone enriched in siderite, rhodochrosite and/or pyrite, which is subparallel to the contact
between the Early Eocene Neochiastozygus chiastus and Tribrachiatus orthostylus biozones (Fig. 12). Thicker lines
indicate boundaries of individual channel-fills. High-resistivity well log markers u, /j', y and 6 are subparallel to the
datum. Spontaneous potential (SP) and resistivity (IES) well logs were used to construct the sections (see wells B-1,
C-1 and D-1). A resistivity curve in amplified scale is provided for the uppermost, mud-rich successions (levee
successions A to I). Small arrows indicate trends of resistivity variations (i indicates increasing upward resistivities;
d indicates decreasing upward resistivities). Vertical and horizontal scales are the same. Location of sections is
shown in Fig. 6. (A) Typical cross-section of the central part of the Lagoa Parda field, which shows the entire width
of CC1, and most of the lateral extent of CC2 and CC3. Section also illustrates deep channel incision and the
uppermost mostly mud-filled channels of CC2 (34 and 35). Note also the tendency of CC3 levee successions (G-I) to
smooth the topography inherited from CC2 levee successions (A-F), (B) Section illustrates levee asymmetry [higher
and thicker levee successions on the left side (looking downstream) of associated channels] in CC2, and also a
possibly related trend of systematic channel shifting to the right (channel fills 28-33). Note also the tendency of CC3
levee successions ( G I ) to smooth the topography inherited from CC2 levee successions (A-F). This section also
shows deeper incision of lowermost CC2 channels into the uppermost CC1 channel-fills; this situation is typical of
the north-eastern part of Lagoa Parda field. (C) Section showing the maximum lateral extent of CC3; it provides a
transverse view of CC3 levee successions ( G I ) , and a mostly longitudinal view of CC2 levee successions (A-F).
. . . . . . . . . . . . . . a.. ., . . , _ ,....
I
I
Facies 3 @ Facies 4 @
Fig. 13. Continued.
fine-grained sandstone within very coarse-grained out from one single channel may have contributed
successions of Facies 1 (Fig. 7). Two types to the formation of more than one of the mapped
of stratigraphic relationships between coarser levee successions. For example, sediments spilled
grained channel-fills and laterally associated finer out of channel 30 may have formed part of levee
grained levees are proposed, in which (i) channels successions C and D (Fig. 9). Table 2 gives tenta-
erode levee successions, and the fill is therefore tive correlations between levee successions and
younger: or (ii) some channels filled simul- possibly contemporaneous channels.
taneously with the growth of laterally associated
levees.
Description and interpretation of channel
Levee successions may have been built by finer-
grained sediments spilled out from more than one
complexes
of the mapped channels. For example, levee The 38 channels recognized in the study area can
succession C may have accumulated during the be grouped into three channel complexes (CC1 to
cutting and filling of channel-fills 29 and 30 (Fig. CC3; Table 1; Figs 9 and 13). Overall, the channel-
9). Alternatively, fine-grained sediments spilled fills become narrower, thinner and finer-grained
f>1997 International Association of Sedimentologists, Sedimentology, 44, 17-46
32 C. H. L. Bruhn and R. G. Walker
Table 1. Channel fills of the Lagoa Parda turbidite Complexes CC1 to CC3 represent the youngest
system. of at least nine channel complexes (Fig. 5)
Max. Max. Sand-body included in the Early Eocene Neochiastozygus
Channel- thickness width orientation Dominant chiastus zone (time span of 1.3Myr; Antunes,
fill (m) (m) (azimuth) facies 1990). The older complexes do not form reser-
voirs; only two wells penetrate all nine com-
Channel complex CC3 plexes, and there is no core control. However, if it
38 13 230 128" 2 is assumed that each of the nine channel com-
37 14 270 110" 2 plexes had a similar duration, the average would
36 15 280 134" 2 be about 140 000 years.
Channel complex CC2
35 28 210 093" 3
34 25 220 106" 3 Channel complex CCl
33 16 430 090" 2
32 23 250 098" 2 Description. CC1 comprises the lowermost 1 2
31 9 240 100" 2
channel-fills, including the thickest (average
30 17 560 118" 2
29 25 400 095" 1-2 33 m) and widest (average 690 m) in the LP field
28 20 350 081" 1-2 (Table 1 and Fig. 9). The channels are deeply
27 26 380 040" 2-3 incised, mostly into mudstone of Facies 4, and
26 14 440 133" 1-2 have no laterally associated levees (Figs 9 and 13).
25 15 260 ? 3
Several high-resistivity horizons (markers a, /I, y
24 26 600 094" 1-2
23 16 270 083" 2 and 6; Figs 9 and 13) can be widely correlated in
22 14 470 098" 1-2 the muddy succession eroded by channels. These
21 15 >300 ? 1-2 markers appear to be planar and parallel to the
20 13 290 108" 2 datum located above the LP turbidite succession.
19 27 430 148" 1-2
The best developed CC1 channel-fills are 1,3 , 4 ,
ia 35 450 132" 1-2
17 9 240 081" 2 6, 7 , 9, 10 and 11 (Fig. 9). Channel-fills 2, 5, 8 and
16 9 390 080" 1 1 2 are smaller and/or poorly defined in the study
15 48 600 loan 1-2 area. Channels 1, 4, 6, 9 and 10 have mean
14 26 430 111" 1 sand-body orientations of 93-156", with palaeo-
13 42 660 112" 1
flow mostly from NW to SE (Fig. 14A,C,D).
Channel complex CC1
12 13 a40 066" 1 Channel-fills 3, 7 and 11 have different orienta-
11 40 780 038" 1-2 tions that range between 30" and 41", with palaeo-
10 31 >750 126" 1 flow from SW to NE (Fig. 14B,C,D).
9 43 >750 156" 1-2 CC1 channels were mostly filled with Facies 1
a 21 350 0850 1
(Fig. 9). Very high-resistivity horizons composed
7 35 650 030" 1-2
6 43 >lo50 110" 1 of conglomerate with boulders to pebbles of
5 24 >5ao 090" 1 high-grade metamorphic rocks are typical (e.g.
4 36 5 70 093" 1-2 channel-fills 3 and 9 in Fig. 13A). Facies 1 grades
3 >50 aoo 041" 1-2 to Facies 2 at the top and margins of a few
2 >30 >300 ? 1
119"
channel-fills (e.g. channel-fill 3 in Fig. 13A). The
1 35 870 1
fills of CC1 channels pinch out relatively abruptly
upstream and downstream, where Facies 1 can be
interbedded with Facies 4.
from CC1 to CC3 (Table 1 and Fig. 9). In the
terminology of Damuth et al. (1983), an individ- Interpretation. The orientations in the 93-156"
ual channel and its associated levee deposits form range suggest that most of the turbidity currents
a channel-levee system, and the overlapping, responsible for channel erosion and filling origi-
coalescing or lateral interfingering of channel- nated from the smaller tributary troughs at the
levee systems form a channel-levee complex. northern margin of Regencia Canyon (Fig. 2). In
Because only the youngest LP channels have contrast, the orientations in the 30- 41" range
levees, the abbreviated designation channel com- indicate turbidity current flow along the main
plex is used here. Palaeoflow directions along the thalweg of Regencia Canyon. These two distinct
LP channels were interpreted from sand-body channel orientations, and the stratigraphic rela-
orientations (Table l), assuming eastward (sea- tionships exhibited by their channel-fills, suggest
ward) flow down Regencia Canyon (Fig. 2). that only one major channel was active at any
1997 International Association of Sedimentologists, Sedimentology, 44, 17-46
Coarse-grained turbidite channel-levee complexes, Regencia Canyon, Brazil 33
1 CHANNEL FILL 1 -
0
CHANNEL - FILL 3
0
0 i
0
0
0
0
0
0
0
0
0 0
0 0
0
o o o o A 0
0 0 0
0 0
o o
0 0
0 0
0
0
0
0
0 Well control
-
A Cored well
-
0
-c Paleoflow orientation 0
-
A , Erosion by younger channels
, 0 500m oo 0 500m
ri
0
C.I. = 20m C.I. = 20m
~~~ ~
0
0 0
0 0
0
0 .
A-1 o 0
A
0
0
- -
0
0 500m 0
0 500m 0
C.I. = 20m C.I. = 20m
Fig. 14.Isopach maps for CC1 channel-fills. Average palaeoflow orientations are indicated by arrows, and are
interpreted on the basis of sand-body orientations and generalized turbidity current flow down canyon (eastward).
Legend shown in Fig. 14(A).
given time in the LP field. The orientation of from CC1 by a mud-rich succession up to 4 8 m
the active channel switched irregularly from thick, which can be traced over most of the LP
the 93-156" to the 30-41" trends, suggesting field. However, in the north-eastern part of the
intermittent activity in the two major source field, some of the CC1 channel fills (6, 9 and 10)
areas. are partially truncated by CC2 channel-fills (13
and 14; Fig. 13B), with no preservation of the
Channel complex CC2 mud-rich succession.
The CC2 channel-fills have relatively constant
Description. CC2 comprises 2 3 channel-fills (nos. sand-body orientations, mostly clustering be-
13-35; Table l),with an average thickness of 22 m tween 80" and 118" (palaeoflow from west to
and an average width of 390 m. CC2 is separated east; Fig. 15A-C). The channel-fill deposits
(2 1997 International Association of Sedimentologists,Sedimentology, 44, 17-46
34 C. H. L. Bruhn and R. G. Walker
-
CHANNEL FILLS 13,15,18, AND 19
0
i -
CHANNEL FILLS 28,29,32, AND 33
0
r
0
0
0 0
0
o o o o o o o 0
0 0
0 0 0
0 0 33
0 0 -L
0 0
0
0 Well control 0
A Cored well 0 0 0
+ Paleoflow orientation
-
0
-.t Axis of associated channe
0
w Erosion by younger channe
0 0 500rn
0
C I =20m C.I. = 10m E
0
r LEVEE SUCCESSION D
0
0 0
0
0 0
0 0
0 0 A
0
-
0
0
35
-
0
C.I. = 10m
500m
0
Fig. 15.Isopach maps for channel-fills and a levee succession of CC2. Palaeoflow arrows explained in Fig. 14. Note
- C.I. = 5rn
thicker levee deposits on the northern side (left, looking downstream) of the associated channels (Fig. 15D; channels
29-32). Legend shown in Fig. 15(A).
commonly begin with Facies 1 and grade upward coarse-grained facies (Facies 1 and 2 ) , which are
into Facies 2 (e.g. channel-fills 1 5 and 18 in Fig. replaced upstream by Facies 3. Channels 34 and
13A). However, in some channels the fill is exclu- 35 were filled with Facies 3 (Figs 13A and 15C);
sively Facies 2 (e.g. channel 33 in Fig. 1 3 c ) or however, they can be distinguished from the lat-
Facies 3 (e.g. channels 34 and 35 in Fig. 13A). The erally associated levee succession F by different
downstream terminations of CC2 channel-fills are trends in resistivity logs. The channel-fills show
rarely drilled and cored in the study area, but in upward-decreasing resistivities and levee succes-
one example (channel 33 in Fig. 15B), Facies 2 is sion F shows an upward-increasing resistivity
replaced downstream by Facies 3. The upstream (compare wells B-5, B-6, B-7, B-8 and B-9 in Fig.
terminations of most of the CCZ channel-fills are 13A). The origin of these well-log trends and the
characterized by a relatively abrupt pinch out of filling of channels 34 and 35 is discussed below.
'$2 1997 International Association of Sedimentologists, Sedirnentology, 44,17-46
Coarse-grained turbidite channel-levee complexes, Regencia Canyon, Brazil 35
-
CHANNEL FILLS 36,37,AND 38 i LEVEE SUCCESSION H 0
0 0
0
0 0
0
0 0
0 0
0 0 A
0
0
0
0 0 ' A - l o o
-
A
0 0
o o o o A 0 o o o o
0 0 0
0 0 0 0 0
0 0
0 0 0 0
0 0
0 0
0 0
0
0
Well control
0 0
A Cored well
-
+ Paleoflow orientation 0
0 500m
0
C.I. = 10m rz C I.= 10m \ \
Fig. 16. Isopach maps for channel-fills and a levee succession of CC3. Palaeoflow arrows are explained in Fig. 14.
Note thicker levee deposits on the north-eastern side (left, looking downstream) of the associated channel (38) (Fig.
16B). Legend shown in Fig. 16(A).
CC2 also differs from CC1 by the development Levee slopes are up to lo", with the slopes
of levees, which are prominent during and after facing the channels being typically steeper than
the filling of channel 1 9 (Fig. 9). The levees are those facing away from the channel (Table 2).
higher and thicker on the northern side (left side Levee asymmetry is also manifest by steeper
looking downstream) of associated channels slopes facing the channels on the left-side levees
(Table 2 and Figs 9, 13A,B and 15D). (Table 2).
c 1997 International Association of Sedimentologists, Sedimentology, 44, 17-46
36 C. H. L. Bruhn and R. G. Walker
Interpretation. The channel-fill deposits range in GROWTH OF ASYMMETRICAL LEVEES
orientation from 80 to 118". This suggests that AND CHANNEL AVULSION
(unlike CC1) CC2 was built almost exclusively by
channels originating from the smaller tributary Levee development
troughs located along the northern margin of The levees are discussed before the channels
Regencia Canyon (Fig. 2). The flows were prob- because asymmetrical levee growth appears to
ably muddier than those that built CC1, and have influenced channel switching. Levee top-
overspill of channel banks led to the construc- ography is systematically related to channel
tion of levees, as well as the passive filling of topography and the nature of the channel-fill.
abandoned channels (e.g. channel fills 34 and 35; Most of the deeper channels filled with coarser
Fig. 13A). sediment do not have associated levees (Facies 1;
CC1 and lowermost CC2 channel-fills; Fig. 9),
whereas the narrower, thinner and finer-grained
Channel complex CC3 channel-fills (Facies 2 and 3; uppermost CC2 and
CC3; Fig. 9) have prominent, higher levees. In
Description. CC3 comprises only three channel- modern systems, levee development is at least
fills (36-38), which are the narrowest and thin- in part related to the grain size and velocity of the
nest channel fills in the LP field (Table 1 and Fig. turbidity currents. For example, on Navy fan
9). Average thickness is 1 4 m, and average width (Piper & Normark, 1983; Bowen et al., 1984) and
is 260m. CC3 channel-fills are almost always Laurentian fan (Stow & Bowen, 1980; Piper et al.,
separated from the CC2 coarse-grained channel 1988) the turbidity currents that carry a high
fills by a mud-rich succession up to 24 m thick proportion of mud are slower and thicker than
(Fig. 13A,C). The lowermost channel of CC3 those that are predominantly sandy. Turbidity
truncates levee succession F and the essentially currents with high proportions of coarse-grained
mud-filled channel 34 (Fig. 9). sediments tend to be faster and more erosive,
Channel orientations vary from 128" to 134" particularly if moving on relatively steep slopes
(palaeoflow from NW to SE; Fig. 16A), and the (Normark & Piper, 1991).
fills are composed mostly of Facies 2. The These examples suggest that CC1 and the lower
upstream terminations are characterized by a rela- part of CC2 were characterized by strongly erosive
tively abrupt pinch out of Facies 2, which is turbidity currents enriched in coarser-grained
replaced upstream by Facies 3. The downstream sediments. The flows were probably relatively
terminations of CC3 channels cannot be observed thin, and only small volumes of sediment spilled
in the study area. out of the channel to build levees. Levees are first
CC3 also includes asymmetrical levees, which preserved in the LP area where they are associ-
are slightly higher and thicker on the north- ated with thinner and finer grained channel-fills.
eastern side (left side looking downstream) of At this time, the turbidity currents were probably
associated channels (Table 2, and Figs 9 and 16B). thicker, slower and enriched in finer grained
CC3 levees show gentler maximum slopes ( 5 4 " ) sediments. They had less power to erode the
and a less pronounced asymmetry than CC2 substrate, and hence the channels tended to be
levees (Table 2). The two lowermost levee succes- narrower and shallower. Initially, levee relief was
sions of CC3 (G and H) tend to smooth the levee very subtle (levee successions A and B; Fig. 9),
topography inherited from CCZ (Figs 9 and but following the development of channel 2 8
13A,B). (uppermost part of CC2 and CC3), a more promi-
nent levee topography was established (levee suc-
Interpretation. The channel orientations are in cessions C-I; Fig. 9). In general, the narrower,
the range 128-134", which differs from those of thinner and finer grained (Facies 2 and 3)
the uppermost channel fills of CC2 (80-118"). channel-fills are related to higher levees (Fig. 9).
However, CC3 also appears to have been built The lowermost levee successions of CC2 (A-D)
by turbidity currents that originated from the and CC3 (G,H; Fig. 13) typically show upward-
smaller tributary troughs located at the northern decreasing resistivities, owing to fining- and
margin of Regencia Canyon (Fig. 2). The detailed thinning-upward successions of sandstone beds
cross-sections (Fig. 13A,C) show that only one and/or an overall muddier-upward succession.
channel was excavated and filled in the study Smaller-scale trends of upward-decreasing resis-
area at any given time during the growth of tivities can be recognized within some of the
cc3. levee successions, and are probably related to
$) 1997 International Association of Sedimentologists, Sedimentology, 44,17-46
Coarse-grained turbidite channel-levee complexes, Regencia Canyon, Brazil 37
fluctuations in flow energy, flow thickness and associated channels (Table 2 ; Figs 9, 15D and
grain-size of the sediment in the turbidity current. 16B). This asymmetry is probably due to the
The presence of thinner, muddier-upward succes- Coriolis force, deflecting flows to the left in
sions may also result from the building of many of the Southern Hemisphere, and hence favouring
the levee successions by sediments spilling out of deposition on the left levee.
more than one channel (Table 2). The Coriolis force (F) is given by
The uppermost levee successions of CC2 (E,F)
F =2muR sin Q,
and CC3 (I; Fig. 13) are mostly mud-rich. They
have upward-increasing resistivities owing where m is the mass of the sedimentlwater mix-
largely to upward-increasing concentrations of ture in the turbidity current, u is the velocity of
siderite, rhodochrosite and pyrite in Facies 3 the main body of the turbidity current, R is the
mudstone (Fig. 12). The origin of these minerals is Earth's angular velocity (= 7.29 x 10 rad s '),
related to the degradation of organic matter in the and @ is the latitude. The Coriolis force decreases
sediments by sulphate-reducing bacteria, which toward the Equator and becomes zero as Q,
takes place preferentially in suboxic environ- reaches zero.
ments (Coleman, 1985; Curtis, 1987). The sul- Asymmetrical levees associated with modern
phate reduction zone is overlain by the submarine channels have been recognized both in
oxygenated (respiration) zone, which is rarely the Northern Hemisphere (Chough & Hesse, 1976,
thicker than the uppermost 10 cm of accumulated p. 530; O'Connell et al., 1991, p. 266) and in the
sediment. Therefore, slower rates of sedimen- Southern Hemisphere (Droz & Mougenot, 1987,
tation favour more extensive degradation of p. 1358; Carter & Carter, 1988, p. 190; Viana eta).,
organic matter and the generation of larger 1990, p. 322). Palaeogeographical reconstructions
amounts of CO, and H,S, which in turn favours (Scotese et al., 1988) suggest a palaeolatitude of
the precipitation of sideritehhodochrosite and only 20-25"s for the Early Eocene LP turbidite
pyrite, respectively (Curtis, 1987). The prov- system, suggesting that Coriolis deflection of
enance of the iron and manganese is probably flows might be minor. However, levee successions
related to the intense Early Eocene volcanic with well-developed asymmetry have been recog-
activity, particularly in the Abrolhos volcanic nized in present latitudes as low as 19"s(Droz &
complex (85 km north-east of the LP field). Mougenot, 1987) and 2 2 " s (Viana et al., 1990).
The muddier-upward character and the pres- Bowen et al. (1984) expressed the difference in
ence of early diagenetic minerals within the levee height due to the Coriolis effect ( V hcoriolis)
uppermost deposits of CC2 and CC3 suggest that as
the terminal stages of sedimentation in these two
Vhcoriolis=Wh . 2R sin @lu
channel complexes were characterized by lower
rates of sedimentation and progressive abandon- where W is the channel width and h is the
ment of channel complexes. The youngest sedi- thickness of the main body of the flow. This
ments of CC2 fill channels 34 and 35, which were equation is valid only for the straight portions of
cut after the accumulation of levee succession F channels, where centrifugal effects are absent. It
(Figs 9 and 13A). Their fine-grained filling facies follows that thicker, slower and finer-grained tur-
(Facies 3) and higher sinuosity (Fig. 15C) suggest bidity currents could give rise to levees with more
cutting and filling by lower energy, quasisteady prominent asymmetry than thinner and faster,
flows of longer duration (Damuth et al., 1988, coarser-grained turbidity currents, even at lower
pp. 904-909), although they also may have latitudes.
received sediments overspilled from channels
located outside the study area. The end of the
Channel avulsion
growth of CC3 is characterized in the LP area by
levee succession I, but this succession is probably The change from one low-sinuosity LP channel-
related to channel(s) located outside the LP field fill to the next indicates channel abandonment,
area. probably due to relatively rapid avulsions that
may have involved levee breaching. LP channel-
fills do not appear to have the lateral accretion
Levee asymmetry
surfaces or mega-foreset bedding that has been
The levee successions in the LP turbidite system described from other meandering channel-fill tur-
are typically asymmetrical, being higher and bidite sandstones (Bouma & Coleman, 1985; Mutti
thicker on the left side (looking downstream) of & Normark, 1991).
Chiasmolithus
Mid Eocene
staurion
Mid Eocene
47.0-
Chiasmolithus
gigas 49.0
50.0-
Discoasteroides
kuepperi Early Eocene
52.0 52.0-
Discoaster
lodoensis
-53.7-
54.0
Early Eocene Tribrachiatus
orthostylus
Late
57.8 Paleocene
Late
Heliolithus 60.2.
Paleocene kleinpellii Ma
62.0 la I 6 2 . d Ma
Fig. 17. Comparison between the global sea-level curve of Haq ef ul. (1988) and the chronostratigraphy, biostratigra-
phy and simplified lithostratigraphy of the Late Palaeocene to Middle Eocene successions in the Regencia Canyon
area. Unconformities 4-6 are also shown in Figs 3 and 4. Biozones recognized in the Cenozoic successions of the
eastern Brazilian basins are traditionally tied to the chronostratigraphic column proposed by Berggren ef al. (1985).
The chronostratigraphic schemes of Berggren et al. (1985) and Haq et al. (1988) show differences of 0.6-3.8 Myr in
the time boundaries assigned for the various Early Tertiary successions.
p. 95). Correlations between the Late Palaeocene the (claimed) eustatic, third-order sea-level falls
to Early Eocene biozones (Antunes, 1990) and the should consider the uncertainties involved. For
global, third-order sea-level cycles (Haq et al., instance, the curves of Haq et al. (1988) provide at
1988) are very difficult, and involve two prob- least three closely spaced ( I 0.5 Myr.) sea-level
lems: (i) different time boundaries have been falls in the transition from Palaeocene to Eocene
assigned for the same biozones in the widely used that could be correlated with the unconformity at
chronostratigraphic charts published by Berggren the base of the Neochiastozygus chiastus zone
et al. (1985) and Haq et al. (1988) (Fig. 17); and (Fig. 17).
(ii) the time spacing of third-order sea-level The major contribution of global sea-level
falls in the curves of Haq et al. (1988) is less than curves is to show that the Late Palaeocene to Early
the potential error involved in dating these Eocene overall transgressive trend was punctu-
events. ated by shorter-term sea-level falls (Fig. 17),
The Late Palaeocene to Early Eocene third- which may have induced seaward shifting of
order sea level falls identified by Haq et al. (1988) coastal depocentres, followed by submarine ero-
may have been responsible for some of the uncon- sion (by turbidity currents) along the Reghcia
formities recognized i n the RegGncia Canyon Canyon. One important phase of erosion took
deposits (Figs 3 and 4), including the unconform- place in the transition from Palaeocene to Eocene,
ity placed at the base of the Neochiastozygus allowing the Early Eocene, Neochiastozygus
chiastus zone. However, any attempt to correlate chiastus zone to unconformably overlie Late
these unconformities and overlying turbidites to Maastrichtian sediments (Fig. 3).
C 1997 International Association of Sedimentologists, Sedirnentology, 44, 17-46
40 C. H. L. Bruhn and R. G. Walker
Source of sediments The Early Eocene Reghcia Canyon represents a
very proximal depositional setting, as suggested
The sandstone and the sandy matrix in the con- by its very coarse turbidite facies. The LP tur-
glomerate beds of the LP turbidite system are bidite system is characterized by deeply incised
characterized by poorly sorted, subangular to channels, filled in large proportion by boulder- to
angular grains, composed largely of feldspar (35- pebble-rich conglomerate. The proximal part of
40%) and feldspar-rich rock fragments (Bagnoli, Regencia Canyon was probably incised into the
1984). The source rocks are mostly Late Protero- shelf, and could capture large amounts of coarser
zoic granites, granulites and migmatites. These grained sediment. This was probably introduced
rocks form most of the subparallel, coastal moun- rather directly into the canyon head from deltas
tain ranges of the Serra do Mar and Serra da (or possibly fan deltas), as indicated by the high
Mantiqueira (Fig. 1A). proportions of plant material.
The climate along the eastern Brazilian coastal
areas was humid and warm during the Early
Tectonics and sediment supply
Eocene, as suggested by (i)the high content (up to
35%) of carbonaceous plant fragments in Facies 2 During the long-term, eustatic sea-level rise of the
and 3 , (ii) the small number of individuals in the Late Cretaceous and Early Tertiary, the Espirito
calcareous nannofossil assemblage (Antunes, Santo Basin experienced a generalized eastward
1990) and (iii) global palaeogeographical recon- (seaward) tilting in response to the combined
structions (Parrish & Curtis, 1982). Chemical effects of thermal-contraction subsidence and
weathering under such conditions would elim- sedimentary loading (Costa, 1988). The Early
inate most feldspathic grains, unless the bedrock Eocene Neochiastozygus chiastus zone (57.8-
topography was steep (Folk, 1974; Basu, 1985). In 56.5 Ma) reaches a maximum thickness of 650 m
a source area characterized by rugged topography, in the thalweg of the Regencia Canyon (well A-1;
the commonly vigorous and erosive streams Figs 3 and 5). This suggests an average undecom-
would decrease the residence time of feldspathic pacted sedimentation rate of 65-70 cm per 1000
grains in soil profiles, and also cut more deeply years for this turbidite-rich succession, one of the
through fresh bedrock. Consequently, a large highest sedimentation rates recognized in marine
amount of feldspar-rich, coarse-grained, angular successions in the eastern Brazilian basins.
material would be available for sedimentation. Similar or higher sedimentation rates have been
Steep slopes in the cratonic area adjacent to the proposed only for the last stages of lacustrine rift
Espirito Santo Basin or along its margin were sedimentation (Barremian, 122-119 Ma; Chang
maintained by continued fault reactivation and et al., 1992). The high sedimentation rates indi-
relative uplift of Precambrian rocks during the cated for the Neochiastozygus chiastus zone sug-
Early Eocene. This tectonic scenario is suggested gest a vigorous sediment supply from a relatively
by reactivation of NE-SW- and N-S-orientated, close and rapidly uplifting source area. This is
basement-attached, normal faults in the Regencia also suggested by the very coarse-grained, and
Canyon area (Fig. 4). The N-S-orientated fault compositionally and texturally immature LP
system allowed important tectonic subsidence turbidites. Rapid continental runoff, probably
along the basin margin, and its successive re- induced by the mostly warm and humid Early
activation up to the Early Eocene exposed Pre- Eocene palaeoclimate, is suggested by the high
cambrian high-grade metamorphic rocks along content of carbonaceous plant fragments (up to
the westernmost floor of Regsncia Canyon (Fig. 4). 35%) in LP finer-grained sandstones (Bagnoli,
The compositional and textural immaturity of 1984), the high content of kaolinite (up to 35%)
the LP turbidite system also implies that the in the interbedded mudstones, and also by the
sediments derived from rapidly uplifted, faulted small number of individuals in the calcareous
highlands were not submitted to prolonged abra- nannofossil assemblage (Antunes, 1990).
sion (if any) in a transitional, high-energy deposi- Despite the high sediment supply, no
tional setting such as a beach, shoreface or wave- shallower-water facies were developed and/or
dominated delta. We therefore suggest that deltas/ preserved in the Early Eocene deposits of
fan deltas fed directly onto a relatively narrow, Reg6ncia Canyon. This results from high rates of
faulted shelf. Evidence for shelf sedimentation tectonic subsidence along the faulted, canyon
was removed by (later) Early Eocene erosion of boundaries (Fig. 3), and the generalized trend of
possible shallower-water deposits time-equivalent sea-level rise that did not allow the progradation
to the (earlier) Early Eocene LP turbidites. of coastal depositional systems. Tectonics not
( 1997 International Association of Sedimentologists, Sedmentology, 44, 17-46
Coarse-grained furbidite channel-levee complexes, RegEncia Canyon, Brazil 41
only increased the sediment supply to Regencia been influenced by short-term climatic changes
Canyon, but also provided accommodation space (Milankovitch cycles; Imbrie & Imbrie, 1980).
for sediment deposition and preservation. In the Interaction of various astronomical cycles defines
Espirito Santo Basin, sediment input overcame climatic cycles by influencing the seasonal distri-
the effects of tectonic subsidence and long-term bution of incoming solar radiation on Earth. A
sea-level rise only from the latest Early Eocene mostly warm and humid palaeoclimate is inter-
onwards, giving rise to the marine regressive preted for the eastern Brazilian coast during the
megasequence (Figs 3 and 4). Early Eocene (Parrish & Curtis, 1982), but it is
very probable that climatic fluctuations took place
during the sedimentation of the Neochiastozygus
DEVELOPMENT OF THE INDIVIDUAL LP chiastus zone (time span of 1.3 Myr). The average
CHANNEL COMPLEXES duration of the LP channel complexes (140000
years) is possibly related to changes in the Earths
During the early Eocene, Espirito Santo was still orbital eccentricity (100 000-year cycle; Imbrie &
an immature passive margin basin, with active Imbrie, 1980). Milankovitch cycles probably did
uplift in the adjacent hinterland and along not induce eustatic sea-level fluctuations in the
its faulted margin, probably characterized by a Early Eocene because of the absence of continen-
narrow, steep shelf. In this setting, the movement tal ice sheets (Prentice & Matthews, 1988), but
of clastic depocentres landward and seaward cyclic periods of higher seasonal precipitation
across the shelf in response to fluctuations of sea may have led to higher denudation rates in the
level would be minor. Coastal depocentres would source area, and hence higher rates of sediment
remain close to the shelf/slope break, even during supply at the basin margin. The greatest mechan-
highstands. ical denudation rates occur in temperate humid
It has been estimated that each of the nine zones when seasonal or annual rainfalls surpass
channel complexes has an average duration of their average levels. Here, denudation is even
about 140 000 years; there is no way of estimating higher than that shown by poorly forested semi-
the variability of this duration for individual arid areas. Rivers with the highest sediment yield
channel complexes. Growth and melting of conti- are in high-relief, tropical regions (e.g. Taiwan,
nental ice masses is the only well-known mech- New Guinea and Hawaii), owing to the combi-
anism that can induce eustatic sea level to change nation of rapid weathering and abundant running
at this frequency (Revelle, 1990). However, the water (Einsele, 1992). In areas with high seasonal
early Eocene has been considered the warmest precipitation the erosion rates may be twice those
part of the Tertiary (Shackleton & Boersma, 1981), in semi-arid areas with the same relief (Stow
and most palaeoclimatic studies based on the et a]., 1985).
fossil record and oxygen isotopes suggest that We suggest that changes in the rate of climati-
important development of continental ice sheets cally controlled sediment supply at the basin
did not take place before about 40Ma (Eocene/ margin were probably the most important control
Oligocene transition; Prentice & Matthews, 1988). on the development of the individual LP channel
It therefore seems unlikely that the channel com- complexes. The initiation of each channel com-
plexes are controlled by global eustatic sea-level plex would have followed an increase in sedi-
changes. ment supply through delta/fan delta systems,
Tectonic activity in the Early Eocene Espirito which in turn would have responded to higher
Santo Basin may have exerted an important con- denudation rates in the high-relief source areas
trol on sediment supply and local changes in (ancestral Serra do Mar and Serra da Mantiqueira;
relative sea level. However, tectonic reactivations Fig. 1A). These pulses of increasing sediment
are typically spaced at larger time intervals than supply would have been able to shift coastal
the average duration of the channel complexes depocentres seaward, thus influencing the
included in the Neochiasfozygus chiastus zone number and size of turbidity currents generated at
(Vail et al., 1991). Therefore, the effects of tec- the canyon margins. These pulses of increasing
tonic activity are better recognized in larger scale sediment supply could have induced short-term
successions composed of more than one channel falls of relative sea level. However, the lack of
complex (CCl-CC3), developed over a larger time regional unconformities or local erosion surfaces
span (>400 000 years). bounding the LP channel complexes suggests
Variations in sediment supply along the that the falls of relative sea level were small.
Espirito Santo Basin margin could also have Some of the eustatic sea-level falls resulted in the
(, 1997 International Association of Sedimentologists, Sedimmtology, 44, 17-46
42 C. H. L. Bruhn and R. G. Walker
development of widespread unconformities not into the Regencia Canyon would gradually
only in Regencia Canyon but also in large parts of become smaller, less frequent and finer grained,
the Espirito Santo Basin (e.g. unconformities 2, 4 giving rise to finer grained turbidity currents with
and 5 in Figs 3 and 4). longer recurrence interval and decreasing ero-
Despite proposed changes in the rate of sedi- sional power. Such a decreasing capability in
ment supply, ongoing fault-controlled subsidence eroding the substrate would favour a gradual
and the long-term trend of sea-level rise kept reduction in channel cross-sectional area. With
coarse-grained turbidites confined to the canyon. decreasing fault activity and sediment supply
The Espirito Santo Basin did not change from there would also be less coarse-grained sediment
overall transgressive to overall regressive con- to be incorporated into the littoral drift systems
ditions until the latest Early Eocene. Deposition and intercepted by Regencia Canyon.
from the turbidity currents within the canyon and The long-term (second-order) eustatic sea-level
close to the canyon head must have required a rise of the Late Palaeocene/Early Eocene (Fig. 17)
reduction in gradient of the canyon floor. This may also have influenced the development of
would first have taken place where the slopes of successively narrower, thinner and finer grained
the canyon head and walls flattened in the axial channel-fills, by moving coastal depocentres
region of the canyon. The reduction of axial slope landward and contributing to the reduction in
may also have been fault influenced. sediment input. The very proximal LP turbidite
system does not display retrogradational stacking,
probably due to structural confinement and lack
Decreasing grain size, thickness and width of
of available space for retrogradation. During the
channel fills
Early Eocene the LP field was always close to the
The stack of channel complexes CC1-CC3 forms a head of Regencia Canyon (<8km; Fig. 2) and to its
turbidite succession in which individual channel- northern margin (<1km; Fig. 6). The study area
fills become narrower, thinner and finer-grained also lay about 2 km from a N-S-orientated, very
upward (Table 1 and Fig. 9). These trends devel- active fault that established an important step
oped during a period of more than 400 000 years, within the canyon (Fig. 4).
a time span longer than the climatically induced The shorter-term, climatically induced in-
fluctuations in sediment supply that may have creases in sediment supply would have been able
controlled the development of individual channel to re-establish channel complexes in the LP field,
complexes. but these changes are merely overprinted on the
The faults that define the margins of the overall trends of narrower, thinner and finer
Regencia Canyon (Fig. 3) and the fault responsible grained channel-fills (CC1-CC3; Fig. 9) controlled
for exhuming Precambrian basement along its by the longer-term decreasing fault activity and
western floor (Fig. 4) suggest a gradually decreas- rising sea level.
ing tectonic activity during the deposition of the
uppermost part of the Early Eocene Neochiasto-
zygus chiastus zone. This biozone is very thin SUMMARY A N D CONCLUSIONS
in the westernmost part of the Regencia Canyon
(Fig. 4) and beyond the fault-controlled northern 1 The Early Eocene Lagoa Parda (LP) turbidite
margin of the canyon (Fig. 3). To the south, the system fills a canyon that was shaped by the
turbidite-bearing Neochiastozygus chiastus zone combined effects of subsidence along listric faults
is confined to Reg6ncia Canyon, and is truncated and erosion by high-density turbidity currents. It
by uppermost Early Eocene sediments of the forms part of a Late Albian to Early Tertiary
marine regressive megasequence (Fig. 3). transgressive succession, which is characterized
The decreasing tectonic activity in the Reg6ncia by onlapping, deepening-upward sedimentation
Canyon area suggests a similar decrease in tec- throughout the eastern Brazilian continental
tonic activity in the source area to the west. This margin.
would have influenced turbidite sedimentation 2 The turbidite succession studied comprises the
by reducing the longer-range sediment supply. uppermost 230 m of the up to 650-m-thick, Early
The progressive erosion of uplifted continental Eocene (Neochiastozygus chiastus zone) succes-
blocks and the decreasing slope in the source area sion of conglomerate, sandstone and mudstone
would decrease the amount and rate of accumu- deposited in deep neritic to upper bathyal depths.
lation of coarser grained sediments in areas more 3 LP turbidites consist of (i) graded beds up to
susceptible to failure. As a result, slope failures 6 m thick of unstratified, bouldery to pebbly
1997 International Association of Sedimentologists, Sedimentology, 44, 17-46
Coarse-grained turbidite ch ann el-levee complexes, Reg&ncia Canyon, Brazil 43
conglomerate and very coarse- to coarse-grained order) trend of sea-level rise, and decreasing fault
sandstone, and parallel-stratified, medium- to activity at the basin margin and source area.
fine-grained sandstone beds; and (ii) interbedded 9 The channel orientations cluster into two major
bioturbated mudstone and thin-bedded (<1m), groups: 30-41" (palaeoflow from SW to NE),
parallel- to ripple cross-laminated, fine- to very and 80-156" (palaeoflow mostly from NW to
fine-grainedsandstone. SE). These orientations suggest that LP channel
4 The coarser grained facies fill at least 38 deeply complexes were built by turbidity currents devel-
incised channels; the channel-fill deposits are 9 to oped along the main thalweg of Regencia Canyon
>50m thick, 210 to > l o 5 0 m wide and >1km (from SW to NE), or by turbidity currents that
long. originated from smaller tributary troughs at the
5 The finer grained facies typically build asym- northern margin of the canyon (mostly from NW
metrical levees, which are higher and thicker on to SE).
the left side (looking downstream) of their associ- 10 Most channel-fills are at least partially trun-
ated channels. Levee asymmetry probably results cated by younger channel-fills, and the channel
from Coriolis deflection of turbidity currents. complexes are characterized by extensive amalga-
Nine levee successions (up to 5 0 m thick) are mation of conglomerate/sandstone bodies. This
associated with the 20 youngest channels. high degree of cannibalization seems to be related
6 The low-sinuosity channel-fill deposits record to the combined effects of the large sediment
successive channel abandonment, probably supply, the erosional power of high-density tur-
through relatively rapid avulsions. Avulsions of bidity currents moving on relatively steep slopes,
unleveed channels took place randomly, whereas and the reduced width (<6 km) of the Early
leveed channels show preferential avulsion to the Eocene Regencia Canyon.
right (looking downstream). This is opposite to 11 The relatively restricted channel-fill deposits
the direction of preferential levee growth. Levee have variable width, thickness and sand-body
topography was therefore an important control on orientation; these sand bodies now act as reser-
channel avulsion. voirs. As a result of the common amalgamation
7 Individual channel-fills can be grouped into of many channel-fills and the partial preserva-
three channel complexes (20-100 m thick with an tion of levee deposits between channel-fills, the
estimated average duration of 140 000 years) on channel complexes show a complicated, multi-
the basis of thicker interbedded mudstone succes- storied sand-body geometry. A detailed three-
sions, different filling facies and changes in chan- dimensional representation of this type of reser-
nel orientation. Individual channel complexes are voir can be obtained only from oil fields which
interpreted to have been formed in response to have a large number of closely spaced wells.
climatically controlled phases of increasing sedi- 12 Comparison of the LP turbidite system with
ment supply via deltdfan-delta and littoral drift the channel-feeding-lobe models of the 1970s
systems. These in turn responded to higher denu- (Mutti & Kicci Lucchi, 1972; Walker, 1978) and
dation rates in the high-relief, ancestral coastal 1980s (Mutti, 1985), and with models derived
ranges of Serra do Mar and Serra da Mantiqueira. from studies of modern submarine fans (Damuth
These pulses of increasing sediment supply et al., 1988; Weimer, 1989), shows that none of
shifted coastal depocentres seaward across a these models can explain the geometrical charac-
narrow and steep shelf, resulting in short-term teristics and stratigraphic relationships displayed
falls of relative sea level. These relative sea-level in the LP field.
falls would have had a smaller magnitude, as 13 Influential schemes such as Exxon's sequence
suggested by the lack of regional unconformities stratigraphy have emphasized the importance of
or local erosion surfaces bounding the channel lowstand basin floor fans as major petroleum
complexes. exploration targets (Van Wagoner et al., 1990;
8 Overall, the three LP channel complexes form a Posamentier et al., 1991). However, this study
succession characterized by channel-fill deposits shows that coarse-grained canyon-filling tur-
that become narrower, thinner and finer grained bidites may comprise thick successions of poten-
upward. This succession is associated with pro- tially high-quality reservoirs. It also shows that
gressively finer grained, less frequent and less thick successions of coarse-grained turbidites can
erosive turbidity currents. These trends were accumulate in passive margin basins even during
induced mostly by a longer-term (>400 000 years) long-term trends of sea-level rise, as long as large
decrease in sediment supply that resulted from amounts of sediment are available at the basin
the combined effects of a long-term (second- margin and adjacent hinterland.
f 1997 International Association of Sedimentologists, Sedimentology, 44, 17-46
44 C. H. L. Bruhn and R. G. Walker
ACKNOWLEDGMENTS Carter, L. and Carter, R.M. (1988) Late Quaternary
development of left-bank-dominant levees in the
C.H.L.B. is grateful to PetrobrAs for sponsoring his Bounty trough, New Zealand. Mar. Geol., 78, 185-
197.
doctoral programme at McMaster University, and
Chang, H.K., Kowsmann, R.O. and Figueiredo, A.M.F.
for releasing most of the data used in this (1988) New concepts on the development of East
research. R.G.W. thanks the Natural Sciences and Brazilian marginal basins. Episodes, 11, 194-202.
Engineering Research Council of Canada for Chang, H.K., Kowsmann, R.O., Figueiredo, A.M.F. and
continuing support of his research. Bender, A.A. (1992) Tectonics and stratigraphy of the
east Brazil rift system: an overview. Tectonophysics,
213, 97-138.
Chough, S. and Hesse, R. (1976) Submarine meandering
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