sandy debrites & tidalites in upper-slope canyon environments, offshore india - jsr, 2009

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Journal of Sedimentary Research, 2009, v. 79, 736–756

Research Article

DOI: 10.2110/jsr.2009.076

SANDY DEBRITES AND TIDALITES OF PLIOCENE RESERVOIR SANDS IN UPPER-SLOPE CANYONENVIRONMENTS, OFFSHORE KRISHNA–GODAVARI BASIN (INDIA): IMPLICATIONS

G. SHANMUGAM,1 S.K. SHRIVASTAVA,2 AND BHAGABAN DAS2

1Department of Earth and Environmental Sciences, The University of Texas at Arlington, P.O. Box 19049, Arlington, Texas 76019-0049, U.S.A. 2Reliance Industries Limited, E&P Business, Reliance Corporate Park, Thane Belapur Road, Ghansoli, Navi Mumbai-400701, India

e-mail: [email protected]

ABSTRACT: A depositional model is proposed for deep-water petroleum reservoir sands (Pliocene) in the Krishna–Godavari

Basin, Bay of Bengal, India. Based on examination of 313 m of conventional cores from three wells, five depositional facieshave been interpreted: (1) sandy debrite, sandy slump, sandy slide, and sandy cascading flow, (2) muddy slump and debrite, (3)sandy tidalite, (4) muddy tidalite, and (5) hemipelagite. Debrites and slumps constitute up to 99% in one well. Sand injectitesare common. Pliocene environments are interpreted to be comparable to the modern upper continental slope with widespreadmass-transport deposits and submarine canyons in the Krishna–Godavari Basin. Frequent tropical cyclones, tsunamis,earthquakes, shelf-edge canyons with steep-gradient walls of more than 30u, and seafloor fault scarps are considered to befavorable factors for triggering mass movements. Pliocene canyons are sinuous, exhibit 90u deflections, at least 22 km long,relatively narrow (500–1000 m wide), deeply incised (250 m), and asymmetrically walled. Sandy debrites occur as sinuouscanyon-fill massive sands, intercanyon sheet sands (1750 m long or wide and 32 m thick), and canyon-mouth slope-confinedlobate sands (3 km long, 2.5 km wide, and up to 28 m thick). Canyon-fill facies are characterized by the close association of sandy debrites and tidalites. Reservoir sands, composed mostly of amalgamated units of sandy debrites, are thick (up to 32 m),low in mud matrix (less than 1% by volume), and high in measured porosity (35–40%) and permeability (850–18,700 mD).Because upper-slope sandy debrites mimic base-of-slope turbidite channels and lobes in planform geometries, use of conventional submarine fan models as a template to predict the distribution of deep-water sand is tenuous.

INTRODUCTION

The eastern continental margin of India, along the western region of

the Bay of Bengal, is composed of four major sedimentary basins from

north to south: (1) the Bengal, (2) the Mahanadi, (3) the Krishna–

Godavari (KG), and (4) the Cauvery (Subrahmanyam and Chand 2006).

Sediments in these basins have been supplied by the four major river

systems, namely the Ganges–Brahmaputra (two rivers), the Mahanadi,

the Krishna–Godavari (two rivers), and the Cauvery (Fig. 1A), respec-

tively. Operator Reliance Industries Limited and Niko Resources

discovered gas in Pliocene deep-water siliciclastic reservoirs of the

Krishna–Godavari Basin in 2002 (Shirley 2003). These reservoir sandsand the processes that deposited them are the focus of this paper. The

primary objective of this paper is to develop a depositional model to

understand the distribution of Pliocene sand in our study area using

conventional cores from three wells in Block KG-D6 of the offshore

Krishna–Godavari Basin (Fig. 1C). The specific objectives are to: (1)

describe cores for recognizing types of lithofacies present, (2) interpret

depositional processes, (3) construct depositional models through time,

and (4) discuss implications of the model for understanding sand

distribution.

This study is of both regional and universal significance, for the

following reasons:

1. Pliocene reservoir sands represent the first major deep-water

petroleum discovery in India (Bay of Bengal).

2. The Bay of Bengal is an extremely complex oceanographic segmentof the northern Indian Ocean affected by (a) reversal in currentcirculation twice a year due to double monsoon seasons (Gang-adhara Rao and Shree Ram 2005), (b) increasing monsoon intensity

(Goodbred 2003) and related deep-water sedimentation (Weber etal. 1997), (c) tidal currents (Narasimha Rao 2001), (d) internalwaves and tides (LaFond and Rao 1954; Antony et al. 1985), (e)western boundary thermohaline (geostrophic) currents (Sanilkumaret al. 1997), (f) tropical cyclones (Chu et al. 2002), (g) cyclone-related coastal upwelling (Rao et al. 2004), (h) tsunamis (Shanmu-gam 2008a), and (i) earthquakes (NGDC 2007).

3. Bathymetric data of modern upper-slope environments of our studyarea provide an opportunity to understand Pliocene environments.

4. Conventionally, most deep-water reservoir sands have beeninterpreted as turbidites (Shanmugam 2000). However, we offerunconventional process interpretations. This study should be of interest to sedimentologists, oceanographers, and petroleum geo-scientists.

GEOLOGICAL SETTING

Study Area

The Krishna–Godavari Basin is composed of both onshore andoffshore stratigraphic components (Fig. 2). The cored Pliocene intervalsin three wells represent the deep offshore component (Fig. 2). The water

Copyright E 2009, SEPM (Society for Sedimentary Geology) 1527-1404/09/079-736/$03.00



depths in the KG-D6 block range from about 400 m to over 2700 m. The

three cored wells described in this study, labeled 1, 2, and 3, are located onthe present-day upper continental slope in bathyal water depths, ranging

from 688.5 m to 920 m (Fig. 3).

Basin Evolution

Structural and stratigraphic aspects of the Krishna–Godavari Basin

have been reviewed by other workers (Rao 2001; Gupta 2006). The

eastern margin of India came into existence at about 130 Ma (Early

Cretaceous) when India drifted away from East Antarctica (Subrahma-

nyam and Chand 2006). The Bay of Bengal was created by the initial

Paleocene–Eocene collision of India with the subduction zone of the

north side of the Tethys Ocean (Curray et al. 2003). The Krishna–

Godavari Basin evolved as rifted horsts and grabens with nearly vertical

faults (Gupta 2006). Halkett et al. (2001) concluded that subsidence

through the Paleocene and Eocene led to a thick wedge of sediments

developing in front of the Godavari River. A regional Oligoceneunconformity was followed by faulting and deposition of a thick Miocene

succession. The development of submarine canyons occurred during the

latest Miocene and Pliocene. Growth faulting has continued in pulses

through the Pleistocene into the Holocene. The active Godavari graben is

currently affected by moderate-magnitude (M3–6) earthquakes (Sukh-

tankar et al. 1993). A dip-oriented seismic profile, linking shelf to basin

(NW–SE), shows that the present-day shelf edge is underlain by nearly

vertical faults that affect the Pliocene interval (Bastia et al. 2006, their fig.

8).

Modern Estuaries

In interpreting Pliocene environments of cored intervals, an under-

standing of modern environments of our study area is imperative. The

FIG. 1.— A) Index map showing locations of the Krishna–Godavari (KG) Basin and the KG-D6 block (offshore, State of Andhra Pradesh) on the eastern continentalmargin of India. This index map does not represent the precise size of either the KG basin or the D6 block (D 5 Dhirubhai). River mouths are in red dots.GB 5 Ganges–Brahmaputra (State of West Bengal in India and the country of Bangladesh), M 5 Mahanadi (State of Orissa), G 5 Godavari (State of AndhraPradesh), K 5 Krishna (State of Andhra Pradesh), and C 5 Cauvery (State of Tamil Nadu). Map modified after USGS (2006). B) Map showing location of our studyarea in the Block KG-D6. C) RMS (root mean square) seismic amplitude map of our study area showing locations of cored wells 1, 2, and 3. Well 1 is located about55 km southeast of Kakinada. RMS map represents the entire reservoir (400 milliseconds time window). Amplitude color code: bright red 5 high amplitude (gas-charged sandy lithologies), yellow 5 intermediate amplitude (mixed lithologies), blue to dull green 5 low amplitude (non-sandy or muddy lithologies). Sinuous andlobate planform geometries are present. Note position of well 2 in a sinuous form. The seismic profile (see Fig. 12), which passes through well 2, represents an obliquestrike section across a sinuous form (canyon).

SANDY DEBRITES AND TIDALITES IN UPPER-SLOPE CANYON ENVIRONMENTS 737J S R



east coast of India is bordered by a complex coastline with 14 estuaries

(Balasubramanian and Ajmal Khan 2002). The Godavari estuary is the

second largest in India, next to the Ganges–Brahmaputra (Krupadam et

al. 2007). In the northern Godavari region, comprising the Kakinada Bay

and adjacent mangrove forests and tidal flats, the semidiurnal tidal range

is between 2.3 and 4.5 m (Tripathi et al. 2005). Prior to 1889, the

Godavari River discharged a major portion of water directly into the

Kakinada Bay (Reddy and Prasad 1982; Ramasubramanian et al. 2006).Along the shores of the Kakinada Bay, our 2008 field study (unpublished)

has revealed the presence of rhythmic bedding and well developed double

mud layers in recent fine-grained sands. These features suggest tidal

influence on sediment deposition (Visser 1980). The central Godavari

region, composed of the main Gautami–Godavari channel, represents the

seaward portion of a drowned-river-valley system (Narasimha Rao 2001).

Here, the mean semidiurnal tidal range is 1.34 m (Selvam 2003). The tidal

effect is felt up to 48 km upstream (Krupadam et al. 2007). In the

southern Godavari region, comprising the Vasista–Godavari channel, the

tidal effect is felt up to 40 km upstream (Krupadam et al. 2007). The

Krishna River and its three branches exhibit widening of river mouths,

typical of estuaries. The tidal range in the Krishna estuary is between 2

and 3 m (Balasubramanian and Ajmal Khan 2002). In short, the modern

Krishna–Godavari shoreline represents a complex mix of microtidal,

mesotidal, and macrotidal settings. In front of the Godavari estuary,near-bottom ebb-tidal currents were measured to flow at maximum

velocities of 150 cm s21 at 15 m of water depth (Narasimha Rao 2001; hisfig. 14). From the standpoint of seaward sediment transport, these high-velocity ebb-tidal currents are more important than the tidal ranges.

Modern Continental Shelf and Slope

Offshore of the Godavari River, the continental shelf is about 25 kmwide, the shelf break is at about 70 m, and the base of slope is at 2500 mof water depth (Murthy 1999). According to Forsberg et al. (2007, theirfig. 2), the shelf break occurs at a depth of about 50 m. In front of theKrishna River mouth, the continental shelf is about 15 km wide (Hart2001).

The modern shelf edge of the Krishna–Godavari Basin is composed of sedimentary slumps and faults (NIO 1993–1994, its fig. 15; Murthy 1999,his fig. 4). Published seismic profiles from the offshore Godavari Basinreveal that mass-transport deposits are underlain by vertical faults(Solheim et al. 2007, their fig. 6). Some of these faults have generatedstep-like slide deposits on the modern seafloor. Large-scale submarineslides are associated with headwalls of up to 100 m in height and theslides are tens of kilometers in length (Solheim et al. 2007, their fig. 6).Locally steep slopes of over 8u were observed. In interpreting downslope

FIG. 2.— Generalized stratigraphic panel of the Krishna–Godavari Basin from onshore (northwest) to deep offshore (southeast). Stratigraphic position of coredPliocene intervals in Wells 1, 2, and 3 are shown by a vertical bar.

738 G. SHANMUGAM ET AL. J S R



termination of mass movements on seismic profiles from the modern

deep-water Godavari Basin, Forsberg et al. (2007, p. 22) stated that ‘‘We

interpret these features as having been formed by debris flows/slides/

slumps and that can be seen to have originated along faults and other

scarps or steeper areas on the sea floor.’’

Submarine canyons serve as major conduits for downslope sediment

transport from land into the deep-sea environment (Shepard and Dill1966). Synonymous terms, such as gullies, chutes, channels, troughs,

trenches, fault valleys, and sea valleys are sometimes used for submarine

canyons (see Shanmugam 2006a for a review). In this paper, we use the

term ‘‘submarine canyon’’ for linear erosional features that occur on the

continental slope. The present-day shelf edge and slope of our study area

is characterized by numerous submarine canyons (NIO 1993–1994; Bastia

2004, his fig. 7A). In the offshore Godavari basin, three linear erosional

features, labeled as channels A, B, and C, originate near the shelf edge at

about the 50 m isobath (Forsberg et al. 2007; their fig. 2). Channel B is

nearly 7 km wide and at least 35 km long. In addition, three ‘‘paleo

channels’’ (5,000 yr BP) were recognized 40 m beneath the modern inner

shelf, in seismic records north of Kakinada (Murthy 1999). Two major

canyons, namely Nagarajuna Canyon and Machili Canyon, were

recognized off the Krishna River mouth (Hart 2001).

The seafloor bathymetry of our study area reveals that the modern

upper-slope setting is characterized by widespread mass-transport

deposits and submarine canyons (Fig. 3). Most canyons are in their

incipient stages of development. The western shelf edge is characterized

by headwall scarps. Slide blocks have detached from these headwall

scarps (i.e., slide scars) and moved downslope developing chutes. As a

result, slide blocks occur at the mouths of chutes (Fig. 3). Such geneticlinks between chutes and slides on other slope settings have been

documented by Prior et al. (1981). Shelf-indenting submarine canyons

characteristically originate at the shelf edge (Fig. 3). Straight and slightly

sinuous canyons are present (Fig. 3). These canyons are at least 20–25 km

long. The modern canyon walls have steep slopes of more than 8u, and the

canyon floors exhibit slopes of 2–4u. Such steep gradients are conducive

for triggering mass movements along canyon walls (e.g., Lastras et al.

2007).

Transverse ridges seen on the modern Godavari slope (Fig. 3) are

analogous to compressional transverse ridges associated with submarine

slides in British Columbia (Prior et al. 1982). The origin of transverse

ridges was linked to a continued movement of sandy debrites as ‘‘rigid’’

slide blocks after the initial freezing of debrites (Nemec 1990, his fig. 28).

Longitudinal ridges are interpreted to be detached slide blocks from steep

FIG. 3.—Bathymetric image of our study area showing locations of three cored wells (red dots), widespread distribution of mass-transport deposits (i.e., slides, slumps,and debrites), and incipient submarine canyons on the modern upper-slope setting just seaward of the shelf edge (see location map in Fig. 1A). Linked occurrences of headwall scarps (slide scars) near the shelf edge, chutes immediately downslope of slide scars, and slide blocks immediately downslope of chutes are evident. Mass-transport deposits show slope-confined lobate forms in intercanyon areas. Lobate form 2 5 6 km long and 6 km wide. Background scale (0, 500, 1000, 1500, 2000, and2500 m) represents present-day water depths.




sidewalls (Fig. 3). Canyon walls around slide blocks are present (Fig. 3).Curved or crescent-shaped scarps along the walls are interpreted to be dueto slumping (Fig. 3). Mass-transport deposits, composed of debrites,develop lobate forms in intercanyon slope areas (Fig. 3). Debrite lobeshave been documented on the modern Mississippi Delta slopes (Colemanand Prior 1982).

LITHOFACIES AND DEPOSITIONAL PROCESSES

A total of 313 m of conventional cores from three wells were described(Table 1). The slabbed core face of the 1/3 cut section was described forwells 2 and 3, whereas the 2/3 cut section was described for well 1(Table 1). Core description was carried out using a sedimentological logsheet with expanded grain-size scale (Shanmugam 2006a, his fig. 1.8).

Lithofacies were described using the nomenclature of Folk (1968) forclassifying unindurated sediments and indurated sedimentary rocks. We

have recognized five lithofacies.

Lithofacies 1

Description.— Lithofacies 1 is composed of light to medium gray,poorly sorted, amalgamated, unindurated fine- to coarse-grained massivesand. The most diagnostic feature of this lithofacies is floating quartzgranules (Fig. 4A) and floating mudstone clasts (Figs. 5, 6, 7). Mudstoneclasts are up to 25 cm in diameter and exhibit both planar and randomfabric. Deformed and brecciated clasts, inverse grading, and slumpedunits are common (Table 2). Massive sands are clean with low mud

matrix (less than 1% by volume). This lithofacies is common in coredintervals of all three wells (Figs. 4–9). Amalgamated sand units are up to32 m in thickness in well 2.

Interpretation.— Although there is general consensus that deep-watermassive sands are structureless (Stow and Johansson 2000), their origin

has been controversial since the early 1960s. In fact, 14 alternativeinterpretations are possible for deep-water m assive sands: (1) low-densityturbidity currents (Bouma 1962); (2) antidune phase of the upper flowregime (Harms and Fahnestock 1965); (3) bed load (Sanders 1965); (4)grain flows (Stauffer 1967); (5) pseudoplastic quick bed (Middleton 1967);(6) density-modified grain flows (Lowe 1976); (7) high-density turbiditycurrents (Lowe 1982); (8) upper-plane-bed conditions under high rates of sediment feed (Arnott and Hand 1989); (9) sandy debris flows(Shanmugam 1996); (10) slumping (Chang and Grimm 1999); (11)quasi-steady concentrated density currents (Mulder and Alexander 2001);(12) contour currents (Rodriguez and Anderson 2004); (13) sandinjections (Duranti and Hurst 2004); and (14) densewater cascadingflows in submarine canyons (Gaudin et al. 2006).

In discussing the origin of deep-water massive sands, Stow andJohansson (2000) narrowed the possibilities to two processes (1) sandy

debris flows and (2) high-density turbidity currents. However, normal

grading that is considered to be a typical product of turbidity currents is

absent in massive sands in the three cored wells. Shanmugam (1996)

argued that the concept of high-density turbidity currents essentially

represents the physical properties of sandy debris flows. In this regard,

the presence of floating mudstone clasts, floating quartz granules, clasts

displaying planar fabric, and sharp upper contacts in cored sandy

intervals suggests deposition from laminar flows with strength via

freezing of sandy debris flows (Fisher 1971; Hampton 1975; Shanmu-

gam 1996; Marr et al. 2001). Inverse grading of sandy matrix can be

explained by dispersive pressure in grain flows (Bagnold 1954). This is

because larger particles in high-concentration granular flows tend to be

pushed upwards, towards the free upper surface of the flow due to grain

collision (dispersive pressure) (Nemec 1990). For example, sand

avalanches (i.e., grain flows) generated inverse grading on the floor of

Carmel Submarine Canyon in California (Dingler and Anima 1989).

Sand avalanches are akin to densewater cascading flows (Gaudin et al.

2006).

A diagnostic feature of massive sands is their high degree of contortion and brecciation. Layers are commonly at high angles (up

to 60u) that represent synsedimentary slumping. These deformed sands

with slumped mudstone clasts can be classified as a separate

depositional facies, namely sandy slumps (Fig. 6B). However, these

slumped sands are grouped together with sandy debrites because of

practical difficulties in distinguishing their depositional contacts as

separate units. The presence of shear planes and drag folds immediately

beneath the basal contact of some intervals of massive sand suggests

that translational shear-surface movements of massive sands may have

occurred over the primary planar glide plane (Fig. 8A). In this case,

massive sands that were originally emplaced as debrites, were

subsequently remobilized as slides. In other words, thick massive sand

units of lithofacies 1 represent a combination of sandy mass-transport

deposits, which include mostly sandy debrites, some sandy slumps, raresandy slides, and rare cascading flows. These remobilized sands

constitute 65% of cored in well 1 and 56% in well 2 (Table 3). In well

3, this sandy depositional facies constitutes only 1% of the cored

interval because 90% of the cored interval is mudstone with only thin

sand layers.

Thick massive sand units with embayed upper contacts and associated

sand pillars are interpreted as sand injections (Fig. 4A). The origin of

massive sands by injection mechanism has been discussed by Duranti and

Hurst (2004). Although sand injectites are mostly postdepositional

features, their close association with sandy debrites is important from a

reservoir standpoint. Because sand injections and brecciated clasts are

commonly associated with seismic liquefaction (Obermeier 1998), such an

origin is viable in the Krishna–Godavari Basin because of frequent

seismic activity (Sukhtankar et al. 1993).

TABLE 1.— Details of three cored wells in the KG-D6 Block.

Details Well 1 Well 2 Well 3

Spudding seafloor depth 703 m 688.5 m 920 mHole deviation 0u (straight hole) Maximum 17.73u (deviated hole) Maximum 2u (deviated hole)Core type Conventional Conventional ConventionalSlabbed core face examined 2/3 cut 1/3 cut 1/3 cutScale of core description 1:31 1:31 1:31Date of core description 4–5 October 2004 24–29 October 2005 16–21 January 2006Site of core description Kakinada* Kakinada* Kakinada*Cored sand interval 18.5 m (66%) 56 m (65%) 20 m (10%)Cored mudstone interval 9.5 m (34%) 30 m (35%) 179 m (90%)Total cored interval (313 m) 28 m (100%) 86 m (100%) 199 m (100%)

* See Figure 1A for location of Kakinada.




Lithofacies 2

Description.— Lithofacies 2 is composed of medium to dark gray,

indurated mudstone and claystone with slump folds (Fig. 8B). Chaotic

fragments, floating mudstone clasts, and floating sandstone frag-ments are common. Mudstone clasts range in diameter from 3 cm to

14 cm. Internal shear planes (Fig. 8A), drag folds (Fig. 8A), stretched

clasts (Fig. 8B), boudins, sand injections (Fig. 9), and sand offshoots

are present (Table 2). This lithofacies is present in all three wells

(Figs. 8, 9, 10A). The thickness of this lithofacies reaches up to 10 m

in well 2.

Interpretation.— Mudstone intervals with slump folds and contorted

layers are interpreted to be muddy slumps. The origin of chaotic

fragments has been related to slumping (Chang and Grimm 1999). Slump

folds commonly develop as a consequence of downslope mass move-

ments. Floating mudstone clasts suggest deposition from laminar debris

flows (Johnson 1970). Muddy matrix reflects the freezing deposition of

cohesive debris flows (Enos 1977). This muddy slump and debrite facies is

closely associated with sand injection (Fig. 9). This depositional facies

constitutes 34% and 33% in wells 1 and 2, respectively (Table 3). In well 3,

12% of the cored interval is composed of this depositional facies.

Lithofacies 3

Description.— Lithofacies 3 is composed of medium gray, unindurated

fine-grained sand with rhythmic bedding (i.e., rhythmites) and double

mud layers (Fig. 10). Lenticular bedding, wavy bedding, and ripple

laminae with mud drapes are present (Table 2). Intervals of thick–thin

bundles have been observed. The thickness of individual units varies from

a few centimeters to nearly a meter.

Interpretation.— Rhythmic bedding and double mud layers are

diagnostic products of tidal processes. Visser (1980) originally explained

the origin of double mud layers by alternating ebb and flood tidal

currents with extreme time–velocity asymmetry in modern shallow-water

subtidal settings. Similarly, deep-water tidal deposits with double mud

layers have been documented in modern and ancient submarine canyons

FIG. 4.— A) Sedimentological log of core 3 for the interval 2019–2024.5 m in well 2. This massive sand interval of Lithofacies 1, showing amalgamation surfaces,floating quartz granules, and floating mudstone clasts, is from core 3 in Figure 12 near the upper part of canyon-fill facies. This core interval represents the top portion of a 28-m-thick clean sand. Wentworth grain-size class: C 5 clay; S 5 silt; VFS 5 very fine sand; FS 5 fine sand; MS 5 medium sand; CS 5 coarse sand;VCS 5 very coarse sand; G 5 gravel. B) Column showing visual estimation of the amount of sand as a percentage of a given interval. C) Explanation of symbols. Thesesymbols also apply to Figures 5, 7, 9, 10, 13, and 14.




(Shanmugam 2003). Klein (1975), based on studies of DSDP (Leg 30,

Sites 288 and 289) cores, suggested that current ripples, micro-cross

laminae, mud drapes, flaser bedding, lenticular bedding, and parallel

laminae reflect alternate traction and suspension deposition from tidal

bottom currents in deep-marine environments. Traction structures were

used to interpret ancient deep-water tidalites in New Zealand (Laird

1972), New South Wales (Skilbeck 1982), Texas (Mutti 1992), and France(Neumeier 1998). Therefore, we have interpreted double mud layers,

lenticular bedding, wavy bedding, and mud-draped ripples as deep-water

sandy tidalites.

Neap–spring tidal bundles have been documented commonly from

ancient shallow-water estuarine facies (Alexander et al. 1998).

However, Cowan et al. (1998) documented tidal rhythmites of couplets

from modern deep-water estuarine environments at 240 m water depth

in Muir Inlet, Alaska. Muir Inlet is a macrotidal setting with mean

tidal amplitude of 4.2 m. Their documentation was the first to show

neap–spring tidal cyclicity and sediment couplets in cores taken from

modern deep-water environments. In cored intervals (Fig. 10), the

light-colored sand layers represent traction deposition from ebb and

flood tides (daily events), whereas the dark-colored mud layers

represent deposition from suspension during intervening slack-water

periods. The thin sand layers are interpreted to be deposits of neap

tides and the thick sand layers to be deposits of spring tides (Fig. 10).

Sandy and muddy tidalites together constitute 15% of the cored

interval in well 3 (Table 3). This depositional facies is also present in

wells 1 (1%) and 2 (9%).

Alternatively, one might describe these laminated intervals as the Tb

division of the Bouma Sequence and interpret them as turbidites. Butthere are no published experimental, theoretical, or observational

studies for explaining the fluid mechanics behind the origin of double

mud layers by turbidity currents. Turbidity currents are considered to

be sediment gravity flows with Newtonian rheology and turbulent state

in which sediment is supported by fluid turbulence and from which

deposition occurs through suspension settling (Dott 1963; Sanders 1965;

Middleton and Hampton 1973; Shanmugam 2006a; Talling et al. 2007).

Such depositional mechanisms cannot explain the origin of double mud

layers.

Lithofacies 4

Description.— Lithofacies 4 is composed of dark gray, indurated

mudstone with rhythmic bedding (i.e., rhythnites) and double mud layers

FIG. 5.— A) Sedimentological log of core 14 for the interval 2219–2225 m in well 2 showing lithofacies 1 with floating mudstone clasts in amalgamated massive sand.This interval is from core 14 in a seismic unit, showing continuous and parallel reflections, which underlies the Pliocene canyon (Fig. 12). See Figure 4 for explanation of symbols. B) Lithofacies 1 core photograph showing horizontal (planar fabric) and vertical (random fabric) positions of floating mudstone clasts (arrows) in massive sand.




(Fig. 11). Double mud layers in muddy intervals are observable because

of subtle changes in grain size (silt and clay) and related color variations(Fig. 11). Mud offshoots in mud-draped ripples, rhythmites, lenticularbedding, wavy bedding, and burrows are present (Table 2). The thickness

of this lithofacies in close association with lithofacies 5 reaches up to114 m in well 3.

Interpretation.— The units of mudstone with double mud layers areinterpreted as muddy tidalites.

Lithofacies 5

Description.— Lithofacies 5 is composed of dark gray, induratedmudstone and claystone with rare parallel laminae. Burrows are common.The trace fossil Zoophycos is present (Table 2). The thickness of thislithofacies reaches up to 1 m in well 2. Lithofacies 5 is closely interbeddedwith lithofacies 4 in well 3.

Interpretation.— Lithofacies 5 is interpreted to be a deposit of hemipelagic settling (hemipelagite). The presence of rare silt laminaecan be explained by bottom-current reworking (e.g., deep-marine tidal

currents). The hemipelagite facies is closely interbedded with muddy

tidalites (with thin sand layers), and they together constitute 70% of the

cored interval in well 3 (Table 3). Hemipelagites constitute 2% in well 2.

This depositional facies is absent in well 1.

PLIOCENE UPPER-SLOPE CANYON ENVIRONMENTS AND FACIES

In the Krishna–Godavari Basin, the modern environments serve as

analogues for interpreting Pliocene environments. The three cored wells

are located in bathyal water depths (688.5 m to 920 m) (Fig. 3). Seismic-

stratigraphic analysis shows that the Pliocene shelf edge was several

kilometers landward of the modern shelf edge (Bastia et al. 2006, their

figs. 5 and 8). Furthermore, the presence of planktonic forams

throughout the cored intervals of Pliocene age in our study area indicates

open-marine conditions. General dominance of buliminid taxa and the

presence of deep-water calcareous benthonic forams indicate bathyal

conditions. Shallow-water calcareous benthonic forams, neritic dinofla-

gellate cysts, and mangrove palynomorphs all suggest downslope

transport of material from shallow-water into deep-water environments.

The presence of the trace fossil Zoophycos in cored intervals (lithofacies 5)

is also suggestive of bathyal environments.

FIG. 6.— A) Lithofacies 1 core photograph showing mudstone clasts (three arrows) with planar fabric (i.e., long axis aligned parallel to bedding plane) in massive sand.Well 3, 2110.8 m. B) Lithofacies 1 core photograph showing deformed sand with slumped mudstone clasts (sandy slump). Well 1, 2109.4 m.




Canyon-Fill Facies

Sandy Debrites, Sandy Slumps, Sandy Sl ides, and Sandy

Cascading Flows.— In discussing Pliocene upper-slope environments,

we have selected well 2 because cored intervals are interpreted to

contain canyon-fill, intercanyon, and canyon-mouth depositional facies.Importantly, this well is located within a sinuous form (Fig. 1C).

Furthermore, 56 m (65%) of the cored interval (86 m) is sand in well 2

(Table 1). In well 2, maximum (100%) sand content occurs in cores 4, 5,

6, and 7. A seismic-reflection profile across wellsite 2 shows a major

erosional canyon-like feature of Pliocene age (Fig. 12). This canyon is

approximately 1000 m wide, incised more than 250 m into the slope

units, and asymmetrically walled. Both canyon walls are aligned in

trend with underlying normal faults (Fig. 12). The northwest canyon

wall exhibits a slope of more than 30u. Integration of seismic data

(Fig. 12) with core data from well 2 reveals that a canyon apparently

carved into the upper-slope muddy sediments (Fig. 13). For example,

the southeastern canyon wall seen on the seismic profile corresponds to

the contact between canyon-fill sandy lithofacies (core 10) and the

underlying intercanyon muddy lithofacies (core 11) in well 2 (Fig. 13).

Units that occur immediately above and below the canyon wall are

severely deformed. Above the wall, the mudstone unit exhibits

contorted layers and internal shearing. Below the wall, the mudstone

unit shows slump folding, steeply dipping fabric, brecciated mudstone

clasts, floating sandstone fragments, and sandy injectites (Fig. 13).

Similar deformational features have been reported from the Miocene– Pliocene deposits associated with collapsed submarine canyon walls in

north-central Chile (Le Roux et al. 2004). Near the canyon wall

(Fig. 13), the canyon-fill facies is composed of sandy debrites (i.e.,

floating quartz granules and mudstone clasts in massive sand), muddy

slumps (i.e., contorted layers and shearing in mudstone), and sandy

tidalites (i.e., mud-draped ripples in fine sand).

In well 1, which is located in the canyon axis (Figs. 1C, 14B), sandy

debrite and slump (lithofacies 1) is the dominant depositional facies,

constituting 65% of the cores (Table 3). In well 1, sand injectites are

present (Fig. 9). Both sandy and muddy debrites (lithofacies 1 and 2)

together constitute 99% in well 1 (Table 3). The great abundance of

debrites, slumps, and slides during the Pliocene closely resembles the

modern upper-slope setting in our study area where mass-transport

deposits are ubiquitous (Fig. 3).

FIG. 7.— A) Sedimentological log of core 7 for the interval 2115–2121 m in well 3 showing massive sand with floating brecciated mudstone clasts, deformed double mudlayers, and truncated ripples in massive sand (lithofacies 1 and 3). Internal shear plane (Fig. 8A), drag fold (Fig. 8A), sand pinch-out, and sand offshoot in underlyingmudstone (lithofacies 2) are present. See Figure 4 for explanation of symbols. B) Lithofacies 1 core photograph showing brecciated mudstone clasts. Arrow showsstratigraphic position of photograph.




RMS (root mean square) amplitude attribute is used for distinguishing

and delineating areas which are sensitive to the gas-charged sands in our

study area (Fig. 14). Amplitude value is calculated for a defined

stratigraphic window, which may reveal the internal architecture of a

system. Seismic stratal amplitude maps of Pliocene intervals reveal

features with sinuous planform geometry (Figs. 1C, 14B). The entire

length (22 km) of a sinuous canyon appears to be filled with gas-charged

sandy lithofacies (Fig. 14B). Canyon-fill depositional facies, composed of

sandy mass-transport deposits, have been documented in both modern

and ancient submarine canyons worldwide. Examples are: (1) Dill (1964)systematically documented depositional processes in modern submarine

canyons. His underwater photographs showed sheets of moving pebbly

sand (Shepard and Dill 1966, their fig. 139; Stanley 1975, his fig. 26A, B),

which is equivalent to our lithofacies 1. (2) Cores from the modern

Bourcart Canyon in the Gulf of Lions show massive sands that were

interpreted to be products of cascading dense water events (also known as

sand fall, sand avalanche, or grain flow) at the canyon head (Gaudin et al.

2006). (3) The modern Monterey Canyon in offshore northern California

is enriched in massive sands with floating pebbles and clasts (Paull et al.

2005, their fig. 6), which is comparable to our lithofacies 1. (4) Sandy

debrites are common in the Quaternary Bass Canyon in southeastern

Australia (Mitchell et al. 2007). (5) The Pliocene Intra Qua Iboe (IQI)

reservoir in the Edop Field (offshore Nigeria) is composed predominantly

of sandy debrites and slumps (Shanmugam 1997a). (6) In the upperreaches of submarine canyons in southern New Zealand, Oligocene sandy

deposits were interpreted as deposits of ‘‘inertia-flow and slump-creep’’

(Carter and Lindqvist 1975). These processes are akin to sandy debris

flows and slumps. (7) In the French Maritime Alps, massive sand units of

the Annot Sandstone (Eocene–Oligocene) were interpreted to be canyon-

fill facies deposited by creep, sand flow, and debris flow (Stanley 1975).

Cored interval 2045–2060 m in well 3 corresponds to the seismic stratal

amplitude map (time window) that appears to reveal a ‘‘cutoff meander’’-

type feature (Fig. 14B). The cored interval consists mainly of muddy

tidalites, sand laminae formed by bottom-current reworking, and

hemipelagites. It is unclear what type of environment this feature truly

represents in our study area. Cutoff meanders, however, have been

documented from the Cap Timiris Canyon off Mauritania, northwestern

Africa (Krastel et al. 2004). In our study area, sinuosity of canyons varies

TABLE 2.— Descriptions of lithofacies and interpretations of depositional facies.

Lithofacies (Description)Depositional Facies

(Interpretation)

Lithofacies 1Sandy debrite, sandy

slump, sandy slide, andsandy cascading flow(sandy mass-transportdeposits composedmodtly of sandy debrite)

N Unindurated fine- to coarse-grained massivesand (Figures 4, 5, 6, 7, 8 and 9)

N Massive sand intervals with amalgamationsurfaces (Fig. 4A)

N Floating quartz granules common (Fig. 4A)N Floating mudstone clasts abundant (Fig. 5A)N Mudstone clasts with random fabric (Fig. 5B)N Mudstone clasts with planar fabric (Fig. 6A)N Brecciated mudstone clasts (Fig. 7)N Clast-rich intervalsN Inverse grading of floating mudstone clastsN Inverse grading of sand matrixN Poorly sorted matrixN Sharp, irregular, and embayed upper contacts

(Fig. 4A)N Sharp and irregular basal contact (Fig. 8A)N Shear plane immediately beneath the basal contact

(Fig. 8A)N Sand pillars near upper contacts (Fig. 4A)N Slumped mudstone clasts (Fig. 6B)N Contorted layersN Steep layers with dip up to 60uN Dispersed carbonaceous debris in some intervalsN Low mud matrix (, 1% by volume)

Lithofacies 2Muddy slump and debriteN Indurated mudstone and claystone

N Internal shear planes (Fig. 8A)N Drag folds (Fig. 8A)N Slump folds common (Fig. 8B)N Contorted layers commonN Chaotic fragments

N Steeply dipping fabric (Fig. 13)N Stretched clasts, rock fragments, and boudins

(Fig. 8B)N Sandy injectites (Fig. 9)N Sand offshoots (Fig. 7A)N Floating sandstone rock fragments common

(Fig. 10A)N Floating mudstone clasts common (Fig. 10A)N Planar clast fabricN Random clast fabricN Brecciated clasts (Fig. 13)N Fractures filled with quartz granulesN Fossil (calcareous) fragments (bivalves)N Nodules (calcareous)

Lithofacies 3Sandy tidaliteN Unindurated fine-grained sand

N Amalgamated unitsN

Rhythmic bedding (Fig. 10A)N Double mud layers common (Fig. 10B)N Lenticular and wavy beddingN Parallel laminaeN Ripple laminae with mud drapesN Deformed double mud layers and ripples (Fig. 7A)N Flame structuresN Concentration of carbonaceous fragments along

mud layersN Thick-thin bundles (Fig. 10B)N Calcareous nodules

Lithofacies 4Muddy tidaliteN Indurated silty mudstone

N Rhythmic bedding (rhythmites) (Fig. 11A)N Double mud layers common (Fig. 11B)N Mud offshoots in mud-draped ripplesN Lenticular and wavy bedding

Lithofacies (Description)Depositional Facies

(Interpretation)

N Flame structuresN Calcareous nodulesN Burrows

Lithofacies 5HemipelagiteN Indurated mudstone and claystone

N Parallel silt laminaeN Closely interbedded with mudstone with double mud

layersN BurrowsN Trace fossil (Zoophycos)N Calcite-cemented zones

Note: Processes and their products are distinguished using the followingnomenclature:

Debris flow (process): debrite (product).Turbidity current: turbidite.Tidal current: tidalite.Hemipelagic settling: hemipelagite.Injection: injectite.Cascading flow: cascading flow.Slump: slump.Slide: slide.

TABLE 2.— Continued.




from 1.1 to 1.9, width from 200 m to 1900 m, and thalweg depth from 30to 200 m.

Sandy Tidalites.— The sandy tidalites (lithofacies 3) are present

throughout the cored interval in well 2, constituting 9% of the coredinterval (Table 3). Core 8 in well 2 represents canyon-fill facies on seismicprofile (Fig. 12). The core 8 photograph, which shows rhythmic bedding

(rhythmites) and double mud layers in sand (Fig. 10), suggests sandytidalite deposition within a submarine canyon. Core 10, which represents

canyon-fill facies (Fig. 13), contains mud-draped ripples and parallellaminae. We have interpreted these structures as suggestive of tidalitedeposition. Others might interpret such ripples as products of over-banking turbidity currents on levee environments. However, in our casean overbanking scenario is difficult to justify within a canyon setting.Slope canyons provide an ideal setting for amplifying tidal currents due tolateral constriction (Cummings et al. 2006).

Sandy tidalite facies is closely associated with sandy debrites in all threecored wells. Core 10 in well 2, for example, exhibits a close associationbetween sandy tidalites (lithofacies 3) and sandy debrites (lithofacies 1) in

canyon-fill facies (Fig. 13). Similar association of sandy tidalites and sandydebrites, typical of canyon-fill facies, has been documented in the modern La

Jolla Canyon box cores, southern California (Shanmugam 2003). Shepard etal. (1979, their Appendix Table 1) documented velocities of deep-marine

tidal bottom currents in the La Jolla Canyon, where the up-canyon currentvelocity was 27 cm s21 and the down-canyon current velocity was 25 cm s21

at 375 m water depth. Here, the meantidal range was 2.5 m (mesotidal). Theassociation of sandy tidalites and sandy debrites has also been documentedin the ancient Qua Iboe Canyon conventional cores (Pliocene, Edop Field),offshore Nigeria, and in the ancient Annot Sandstone outcrops (Eocene– Oligocene), onshore SE France (see Shanmugam 2003).

Origin of Sinuous Canyons.— We consider that sinuous forms on upper-slope settings (Figs. 1C, 14B) represent erosional submarine canyons, notaggradational channels. This is because sinuous aggradational channelsare unlikely to develop on upper-slope settings with steep gradients.

Sinuous submarine canyons have been documented on the world’scontinental margins. An example is the Cap Timiris Canyon on thenorthwestern African margin (Krastel et al. 2004).

FIG. 8.— A) Core photograph showing shear plane with drag fold in mudstone (lithofacies 2) in well 3 (see Fig. 7 for location of this photograph). The shear plane at2119 m and the irregular basal contact of sand is interpreted to be the primary planar glide plane over which the overlying massive sand unit with brecciated clasts(lithofacies 1) moved as a slide block. Basal contact of lithofacies 1 represents the primary glide plane. B) Lithofacies 2 core photograph showing slump fold (dashed line)and stretched clasts in mudstone. Well 2, 2083.2 m.




Although the popular tendency is to interpret the origin of deep-water

sinuous forms seen on seismic images as aggradational ‘‘channel–levee’’

complexes by turbidity currents using fluvial-point-bar analogy (Abreu et

al. 2003; see Wynn et al. 2007 for a review), the absence of turbidites in

the cored intervals (Table 3) makes it unrealistic to justify such an

explanation. Unlike fluvial-point-bar sands that deposit preferentially at

inner bends of sinuous channels (Allen 1965), sands within sinuous

canyons deposit regardless of inner bends (see Fig. 14B, sinuous form 3).

From the standpoint of fluid dynamics, fluvial currents are not equivalent

to deep-water turbidity currents. Subaerial fluvial currents are fluid

gravity flows in which the fluid is moved by gravity and drives the

sediment along, but deep-water turbidity currents are sediment gravity

FIG. 9.— A) Sedimentological log of core 2 for the interval 2113.75–2119.25 m in well 1 showing sand injection in a mudstone unit (lithofacies 2) that is sandwichedbetween massive sand units (lithofacies 1). See Figure 4 for explanation of symbols. B) Lithofacies 2 core photograph showing injection of sand into host mudstone.Truncation of horizontal laminae in mudstone by injected sand is evident. Arrow shows stratigraphic position of photograph.

TABLE 3.— Distribution of lithofacies and depositional facies.

Depositional Facies

Well 1 Well 2 Well 3

m % m % m %

Lithofacies 1 (Sandy debrite, slump, slide, and cascading flow) 18.2 65 48 56 1.7 1Lithofacies 2(Muddy slump and debrite) 9.5 34 28.5 33 23.5 12Lithofacies 3(Sandy tidalite) 0.3 1 8 9 0 0Mixed lithofacies 3 and 4 (Sandy and muddy tidalite) 0 0 0 0 29.3 15Mixed lithofacies 5 and 4 (Hemipelagite and muddy tidalite) 0 0 0 0 140 70Lithofacies 5 (Hemipelagite) 0 0 1.5 2 0 0Sandy injectite * present present present present 4.9 2Total 28 100 86 100 199 100

* Sandy injectite is a postdepositional feature, but it is difficult to quantify independently of sandy debrite, slump, and slide (lithofacies 1) in wells 1 and 2.




flows in which the sediment is moved by gravity, and the sediment motion

moves the interstitial fluid along (Middleton and Hampton 1973).

A sinuous canyon with 90u deflections is at least 22 km long along its

thalweg (Fig. 14B). Its downslope reach along this segment is 16 km,

giving a sinuosity value of 1.37. The origin of similar sinuous canyons

with 90u

deflections, reported from the Baltimore and Wilmingtoncanyons on the U.S. Atlantic Margin, has been linked to faults (Twichell

and Roberts 1982). The tight meander of the Monterey Canyon was

explained by structural (fault) patterns (Martin and Emery 1967). Small-

scale faulting has been considered to exert control over channel bends of

sinuous submarine channels (Mayall and Stewart 2000). Alternatively,

canyon deflections were related to slumping (Gardner et al. 1991) and

diapiric structures (Butman et al. 2006).

In the Krishna–Godavari Basin, some submarine canyons may have

been initiated by mass-transport processes. This is based on modern

bathymetric data that links slide blocks to the formation of chutes

(Fig. 3). On the modern Godavari slope near the shelf edge, sediment

bodies from steep walls due to sliding and slumping are considered to be

associated with the development of incipient canyons. During the

Pliocene, shear-surface movements of sandy slides occurred over the

primary glide plane (Fig. 8A). Furthermore, deformational features

observed in the mudstone unit that underlies the Pliocene canyon wall

can be explained by major mass movements during the initiation phase of

this canyon (Fig. 13). The initiation of submarine canyons by mass

movements has been discussed and documented by other researchers

(Shepard 1981; Moore et al. 1989; Ridente et al. 2007). Both massmovements and faulting are important mechanisms in our area.

Intercanyon Facies

Immediately beneath the Pliocene canyon, a seismic unit shows

continuous and parallel reflections (Fig. 12). In well 2, cores 12, 13, and

14 were recovered from this seismic unit. In wireline logs, these three cores

constitute the lower part of a 32-m-thick sand package. These cores consist

mainly of massive sand units of lithofacies 1 with floating mudstone clasts,

suggesting deposition from sandy debris flows (Fig. 5). We have interpreted

this seismic unit as sheet sands, which are at least 1750 m long or wide,

composed of sandy debrites deposited on intercanyon environments

(Fig. 12). Core 11 in well 2 is interpreted to represent intercanyon mudstone

facies with slump folds and injectites (Fig. 13).

FIG. 10.— A) Sedimentological log of core 8 for the interval 2072–2077.5 m in well 2 showing alternation of sand (lithofacies 3) and mudstone (lithofacies 4) intervalswith continuous presence of double mud layers (DML). Note floating sandstone rock fragments and mudstone clasts in a basal mudstone interval (lithofacies 2). Thecored interval represents core 8 of canyon-fill deposits in seismic profile (Fig. 12). See Figure 4 for explanation of symbols. B) Lithofacies 3 core photograph showingrhythmic bedding (rhythmites) and double mud layers (DML, arrows) in sand. N 5 Neap (thin) bundle; S 5 Spring (thick) bundle.




Intercanyon sandy debrites have been reported from the Ocean Drilling

Program Leg 150, New Jersey continental margin. At Site 903A, forexample, a 20-m-thick massive sand unit (Cenozoic) with floating clasts

has been interpreted to be sandy debrites (McHugh et al. 2002).

Canyon-Mouth Facies

The seismic stratal amplitude maps of Pliocene intervals reveal slope-

confined lobate planform geometries at the mouths of submarine canyons(Figs. 1C, 14B). The lobate form 1 (bright red amplitude), which is 3 km

long and 2.5 km wide, corresponds to the cored interval of amalgamated

sandy debrites (up to 28 m) of lithofacies 1 in well 2 (Fig. 14A).

Therefore, the bright red areas on amplitude maps are inferred to be gas-charged sand-rich slope elements (e.g., sinuous and lobate forms) in our

study area. Core 6, recovered from the uppermost part of canyon-fill

deposits on seismic profile (Fig. 12), is interpreted to represent canyon-mouth (lobe) environment. Cores 4, 5, 6, and 7, which contain the highest

amount (100%) of sand, represent the upper part of a 28-m-thick

amalgamated sand package. Pliocene lobate form 4 is 6 km long and5 km wide (Fig. 14B). Lobate form 1 is composed of sandy lithofacies,

whereas lobate form 4 is composed of muddy lithofacies (Fig. 14B). In

lobate forms 2 and 3, the proximal area is sandy but the distal area ismuddy. In other words, not all Pliocene lobate forms are composed

entirely of sand.

Late Pleistocene canyon-mouth lobes have been reported from thenorthern margin of East Corsica (Deptuck et al. 2008). In this area, piston

cores from the proximal lobes are composed of massive sands withfloating mud clasts that have been interpreted to be deposits of

hyperconcentrated flow with frictional freezing (Gervais et al. 2006).

Analogous to our lithofacies 1, canyon-mouth lobes on the western

margin of Corsica and Sardinia in the Mediterranean Sea are composedof massive sands with low mud matrix (2–3.4%) and floating mud clasts,

which have been ascribed to deposition from hyperconcentrated density

flows (Kenyon et al. 2002). Rheologically, hyperconcentrated flows (e.g.,Mulder and Alexander 2001) are similar to sandy debris flows

(Shanmugam 2000, his fig. 4). On the NE Faeroe margin, upper-slope

mass-transport complex consists of slides and debrite lobes (Van Weeringet al. 1998). Upper-slope gullies and down-slope debrite lobes have beenrecognized on the West Shetland margin (Leslie et al. 2003). Clearly, notall deep-water lobate forms are composed of sandy turbidites.

PLIOCENE DEPOSITIONAL MODEL AND IMPLICATIONS

In understanding Pliocene sand distribution, we have developed adepositional model with four evolutionary stages using cored intervals inwell 2 (Fig. 15).

Stage 1

This is the oldest stage, and it represents open intercanyon environmentwith mass-transport processes. This Pliocene setting is very similar to thedistribution of mass-transport deposits seen on the modern intercanyonslope environment (Fig. 3). Cores 11, 12, 13, and 14, which comprisesheet-like intercanyon facies beneath a canyon (Fig. 12), are dominatedby sandy mass-transport deposits and muddy hemipelagites. Sand bodiesof intercanyon environments show a thickness-to-width ratio of 1:55 (i.e.,32 m thick: 1750 m wide, discussed earlier). Outcrop studies of debriteshave shown ratios of 1:30 to 1:50 (Cook 1979).

Stage 2

This stage represents confined canyon-cutting and canyon-fillingenvironments. Cores 7, 8, 9, and 10, which contain canyon-fill facies(Fig. 12), are characterized by sandy mass-transport deposits and sandytidalites (Fig. 13). Unlike open environments, estimation of thickness-to-width ratio in this confined environment is impractical because the lateralsand distribution is controlled ultimately by the canyon width.

FIG. 11.— A) Lithofacies 4 core photograph showing rhythmic bedding (rhythmites) in mudstone with double mud layers (DML). Silty layers are light in color. Well 2,2074.98 m. B) Lithofacies 4 core photograph showing double mud layers (DML) in mudstone. Silty layers are light in color. Well 3, 2083 m.




Documentation of deep-water tidalites is rare in the geologic record (e.g.,Klein 1975). There are no depositional models for sand distribution by tidal

currents on upper-slope environments (e.g., Southard and Stanley 1976;Shanmugam 2008b). Indications are that tidal currents may develop barsthat are aligned parallel to canyon (or channel) axis, whereas turbidity

currents would develop channel-mouth lobes that are aligned perpendicularto the channel axis (Shanmugam 2003, his fig. 15). The implication is that

the wrong use of a turbidite lobe model with sheet geometry in lieu of a tidalbar model with bar geometry will result in an unrealistic overestimation of

sandstone reservoirs in deep-water exploration. Only through systematicdocumentation of tidalite facies can we develop an understanding of sand

distribution by tidal currents in deep-water environments. Despite theprevailing turbidite mindset (Shanmugam 1997b), it is imperative that we

not miss an opportunity to document deep-water tidalites.

Stage 3

This stage represents an open canyon-mouth environment. Cores 3, 4,5, and 6, which contain canyon-mouth facies (Fig. 14), are dominated by

sandy debrites. These cores possibly reflect a retreating successionfollowing the canyon-fill stage. Sand bodies of canyon-mouth lobe

environments show a thickness-to-width ratio of 1:90 (i.e., 28 m thick:2.5 km wide, discussed earlier). In comparison to intercanyon sandy

debrites, canyon-mouth sandy debrites are much wider. This could beexplained by the highly amalgamated sandy units without interveningmudstone units that characterize canyon-mouth sandy debrites. Similar

relationships have been reported for the upper-slope sandy debrites on theNorwegian continental margin (Shanmugam et al. 1994).

On the Pliocene upper-slope environments (Fig. 14), sinuous canyonsand canyon-mouth lobate forms resemble those of channels and lobes of submarine fans that commonly develop at the base-of-slope settings by

turbidity currents (e.g., Mutti 1977). However, sinuous and lobate formsin our study area on the upper-slope settings are composed mostly of sandy debrites. In comparison to upper-slope debrites, base-of-slope

turbidite lobes exhibit a higher thickness-to-width ratio of 1:1200 (25 mthick: 30 km wide) (Casnedi 1983; Shanmugam 2006a, his Table 12.1).

Unlike base-of-slope turbidite fans, which show thinning-upwards(channel) and thickening-upwards (lobe) cycles (Mutti 1977), upper-slopedebrites in our study area show random trends. This is perhaps due to the

highly complex oceanographic setting of our study area, which has beenaffected frequently by random earthquakes, tsunamis, and cyclones

(Shanmugam 2008a). Such events are likely to trigger sediment failures,irrespective of sea-level changes (Shanmugam 2007).

Stage 4

This youngest stage, which is similar to stage 1, represents the slopeenvironment without numerous canyons. Core 1, which represents the

slope stage (Fig. 12), is composed of sandy mass-transport deposits andmuddy hemipelagites.

A benefit of our process-based interpretation is that it allows one toappreciate the complexities of sand distribution by mass-transportprocesses. Because upper-slope sandy debrites mimic base-of-slope

turbidite channels and lobes in planform geometries (Fig. 14B), the use of a conventional fan model as a template to predict the distribution of deep-

water sand can be misleading in the absence of rock-based process models.

FIG. 12.—Seismic profile showing boundaries of a major erosional feature of Pliocene age, which we have interpreted as a submarine canyon on the upper-slopeenvironment. Cored intervals are shown by yellow boxes on the wireline log of well 2. The southeast canyon wall, which corresponds to the contact between cores 10 and11 (rectangle box, see Fig. 13 for details), is characterized by slump folds, sand injections, and other sediment deformation in core. Both walls of the canyon are aligned intrend with underlying normal faults. Immediately beneath the canyon, a seismic unit (with cores 12, 13, and 14) exhibits continuous and parallel reflections. This seismicunit, which is 1750 m long or wide, is composed primarily of sandy debrites in core 14 (Fig. 5) in the intercanyon environments. This NW–SE seismic profile represents an

oblique strike section across a sinuous canyon with well 2 (see Fig. 1C).




Sedimentological process-based concepts, derived fromthis core study, wereused for constructing a geocellular reservoir model for the Pliocene deep-

water sands in the Krishna–Godavari Basin (Shrivasatava et al. 2008).

CONTROLLING FACTORS OF UPPER-SLOPE RESERVOIR SANDS

The development of upper-slope reservoir sands during the Pliocenecan be attributed to the following favorable factors using modern and

ancient analogs and experimental observations.

1. Frequent tropical cyclones (Chu et al. 2002), frequent tsunamis(Shanmugam 2006b, 2008a), monsoon-related rapid sedimentation(Solheim et al. 2007), earthquakes (Sukhtankar et al. 1993), shelf-

edge canyons with steep-gradient walls, seafloor fault scarps(Forsberg et al. 2007), and gas hydrates (Ramana et al. 2006),which are important factors for triggering mass movements in the

present-day Krishna–Godavari Basin, are also considered to havebeen viable mechanisms during the Pliocene. These mechanismswould operate during periods of both highstands (e.g., Dennielou etal. 2009) and lowstands of sea levels.

2. Freezing deposition from plastic debris flows (Fisher 1971), slumping

that could facilitate ponding of sand in submarine canyons (Cronin et

al. 2005), emplacement of slide blocks (Fig. 3), and free falling of sand

(i.e., grain flows) from canyon heads (Shepard and Dill 1966, their fig.

55) are viable mechanisms for explaining the emplacement of massivesands. Multiple triggering and emplacement mechanisms would

explain the development of thick intervals of amalgamated sandy

debrite units (lithofacies 1).

3. Mudstone permeability barriers are absent in Pliocene amalgamat-

ed sand units. This can be attributed to high frequency of sandy

depositional events triggered by multiple mechanisms that could

operate concurrently. Such conditions do not allow sufficient time

to accumulate muddy units in between sandy events. Thick

reservoir sands are expected to behave as a single sandbody during

petroleum production.

4. Sandy debrites are low in mud content. This paradox can be

explained by experimental results (Marr et al. 2001), which have

shown that sandy debris flows can deposit sands with only a minute

amount of clay (0.7% by weight). Low mud matrix can be explained

FIG. 13.—Integration of seismic data (Fig. 12) and core data for the interval 2107–2113 m in well 2 showing the position of southeast canyon wall, which intersects thecore interval near the contact between core 10 and 11 at 2111 m. Severe sediment deformation is evident both below and above the canyon wall. The lack of core recoveryat the canyon wall may be due to extreme sediment deformation. The canyon-fill facies is composed of sandy debrites (lithofacies 1), sandy tidalites (lithofacies 3), andmuddy slumps (lithofacies 2). The intercanyon facies is composed of muddy slumps and debrites with sand injectites (lithofacies 2) in the upper part of core 11. SeeFigure 4 for explanation of symbols.




by elutriation of mud during flow transformation (Fisher 1983;Shanmugam 2006a, his fig. 3.23).

5. Tidal estuaries, narrow shelf widths, and upper-slope canyons,

which characterize the present-day Krishna–Godavari Basin, are

considered to be viable factors for focusing tidal currents in

submarine canyons during the Pliocene. At high velocities of ebb-

tidal currents (150 cm s21) in front of the Godavari estuary

(Narasimha Rao 2001), even gravel-size grains would be eroded

and transported seaward. Because of the narrow shelf widths

(, 15–25 km), these high-velocity bottom flows would require only

three to five hours to transport sediments to the submarine canyon

heads. Transport of coastal sand into the deep ocean by ebb tidal

currents during the present highstand has been documented in the

offshore areas of Hervey Bay and Fraser Island, southeast

Australia (Boyd et al. 2008). We consider that the Hervey Bay

area in Australia, with (a) mesotidal range (2–4 m) (Boyd andLeckie 2004), (b) high-velocity (150 cm s21) ebb-tidal currents, (c)northward-flowing longshore currents, (d) narrow (5–25 km) shelf widths, and (e) numerous slope gullies, is analogous to theKakinada Bay area in the Krishna–Godavari Basin. Furthermore,ebb-tidal currents have been ascribed to triggering sediment

gravity flows in submarine canyons (Kottke et al. 2003; Boyd etal. 2008).

RESERVOIR QUALITY

The best reservoir facies is composed of sandy debrites (lithofacies 1).This facies exhibits high values of measured porosities (35–40%) andpermeabilities (850–18,700 mD) (Table 4). Sandy tidalites (lithofacies 3)and related bottom-current reworked (BCR) facies exhibit moderate

FIG. 14.— A) Sedimentological log of core 6 for the interval 2033.5–2039.5 m in well 2 showing an amalgamated massive sand interval with floating quartz granules andfloating mudstone clasts indicating deposition from sandy debris flows (lithofacies 1). This sandy interval corresponds to the lobate form 1 (Fig. 14B), which is bright redin seismic amplitude map. Hence, bright red amplitude areas are inferred to be gas-charged sand in our study area. See Figure 4 for explanation of symbols. B) RMS (rootmean square) seismic amplitude map (25 milliseconds time window) showing sinuous planform geometries and canyon-mouth lobate forms. SF 3 5 well developedsinuous form, with 90u deflections (deflected arrow), is at least 22 km long along its thalweg. The downslope reach along this segment is 16 km. The entire length (22 km)of the sinuous canyon appears to be filled with gas-charged sandy lithofacies (i.e., bright red amplitude). Note the location of well 3 close to a ‘‘cutoff meander’’-likefeature (dotted line). The lobate form 1, which is 3 km long and 2.5 km wide, corresponds to the cored interval of amalgamated sandy debrites (more than 10 m thick) incore 6 of well 2 (see arrow towards Fig. 14A). The sandy debrite interval from core 6 is recovered from the uppermost part of canyon-fill facies in seismic profile (Fig. 12).Lobate form 4 5 6 km long and 5 km wide. Lobate form 4 is composed of non-sandy or muddy? (dull green) lithofacies. In lobate forms 2 and 3, the proximal areas aregas-charged sandy (bright red), but the distal areas are non-sandy (dull green) in lithology. See Figure 1 for amplitude color code.




porosity (31–40%) and permeability (525–6930 mD). Muddy tidalites

(lithofacies 4) are poor reservoirs (Table 4). Muddy slumps and debrites

(lithofacies 2) and hemipelagites (lithofacies 5) are considered to be

nonreservoirs. Postdepositional sandy injectites, closely associated with

lithofacies 1, also exhibit high values of porosity (34–35%) and

permeability (8,680–11,760 mD) (Table 4). Sandy debrites on the

upper-slope settings develop not only sheet-like geometries but also

exhibit high porosities and permeabilities because of low mud matrix.

FIG. 15.—Pliocene depositional model showing four evolutionary stages based on cored intervals in well 2 and on RMS maps that correspond to cored intervals in eachstage. Stage 1 (oldest) represents intercanyon environment with mass-transport processes. Cores 11, 12, 13, and 14 are dominated by sandy mass-transport deposits(lithofacies 1) and muddy hemipelagites (lithofacies 5). Stage 2 represents canyon-cutting and canyon-filling environments. Cores 7, 8, 9, and 10 are characterized bysandy mass-transport deposits (lithofacies 1), muddy slumps (lithofacies 2), and sandy tidalites (lithofacies 3) (Fig. 13). Stage 3 represents canyon-mouth environment.Cores 3, 4, 5, and 6 are dominated by sandy mass-transport deposits (lithofacies 1). Stage 4 (youngest) represents slope environment. Core 1, which contains slope facies(Fig. 12), is composed of sandy mass-transport deposits (lithofacies 1) and muddy hemipelagites (lithofacies 5). Although grouped under stage 4, core 2 was not describedand interpreted because the cored interval (1 m thick) is rubble.

TABLE 4.— Representative porosity (%) and permeability (mD) values from various facies. All measurements were made at 300 psi.

Facies

Well 1 Well 2 Well 3

Depth m Poros. % Perm. mD Depth m Poros. % Perm. mD Depth m Poros. % Perm. mD

Sandy debrite 2129.68 35.00 2703 2224.76 30.50 7555 2166.60 39.0 10,0182114.50 31.00 3072 2046.42 34.10 1788 2109.90 34.2 18691

Sandy tidalite 1887.25 40.39 6930 2072.22 40.90 586 2114.37 38.7 5477- - - 2047.03 34.00 525 2106.33 37.9 5977

Muddy tidalite - - - 2074.11 39.30 3375 2150.20 29.2 2.76Sandy injectite* - - - 2108.76 35.50 11,760 2161.76 34.0 8681

- - - 1958.18 34.90 11,205 2126.47 34.7 11,212

*Postdepositional facies.




CONCLUSIONS

1. Pliocene upper-slope environments in the Krishna–Godavari Basinare characterized by sand-prone mass-transport deposits (mostly

sandy debrites) that occur as canyon-fill, intercanyon, and canyon-mouth facies.

2. Upper-slope sinuous canyons and canyon-mouth lobate forms arecomposed of remobilized sands (i.e., sandy debrites, sandy slumps,sandy slides, sandy cascading flows, and injectites). Tidalites arecommon, but turbidites are absent. The association of sandydebrites and tidalites is typical of canyon-fill facies.

3. Amalgamated units of sandy debrites, which constitute the primaryreservoir facies, are thick (up to 32 m), clean with low mud matrix(less than 1% by volume), and high in measured porosity (35–40%)and permeability (850–18,700 mD).

4. The routine interpretation of sinuous and lobate planform seismic

geometries as the products of turbidity currents in a base-of-slopefan setting, without core calibration, is dubious.

ACKNOWLEDGMENTS

We thank Reliance Industries Limited (RIL) in Mumbai (India) forgranting permission to publish this paper. We wish to thank G. Srikanth forRIL internal review. We also thank Anil Kumar (RIL) for initiating this corestudy. We are grateful to M. Acharya, M. Chowdhury, M. Santra, S.S. Roy,S. Gupta, A. Soman, S. Sharma, R. Das, S. Mushnuri, A. Kumar, and V.Yesudian for their assistance during core description (2004–2008). Our fieldinvestigation of Godavari estuary near Yanam and Antarvedi (AndhraPradesh) in August 2007 was assisted by S. Sharma and S.I. Arsalan, and of the Kakinada Bay in January 2008 was assisted by Sandeep Sharma,Chakradhar Rao Basa, Jyoti Rout, Amit Sinha, Sandeep Rawat, HemaSharma, and Mahendra Thame. Sandeep Sharma also assisted in our study of RMS amplitude maps. This paper is based on the first author’s consultingwork for RIL (2004–2009). We are grateful to JSR reviewers George DevriesKlein and Rick Beaubouef, associate editor Stacy Atchley, and the editor

Paul McCarthy for their valuable time spent on providing thorough, critical,and caring reviews. We wish to thank JSR corresponding editor JohnSouthard for improving sentence clarity and managing editor Melissa Lesterfor handling of the logistics involved in moving the manuscript through thefinal stages of preparation. Naresh Kumar offered constructive comments onan earlier version of this manuscript, which have improved the organization.Jean Shanmugam is thanked for her general comments. Melodies of LataMangeshkar provided the inspiration during midnight hours when this paperwas written and revised in Texas.

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Received 14 September 2008; accepted 27 March 2009.


sandy debrites & tidalites in upper-slope canyon environments, offshore india - jsr, 2009

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