El modelo de diamante de Porter

Facies F9 consists of dark grey, structureless/graded to laminated siltstone and mudstone beds (forming units 0.5 cm to 9.0 m thick) and occurs throughout the study succession (Fig. 3.11). In the upper third of the measured section (Fig. 3.3), inversely to normally graded beds of siltstones and mudstones are observed in places, and they commonly overlie rippled sandstones with sharp erosional bases (Fig. 3.11B). The cross laminae of the ripples are tangential towards the lower set boundary and dip at variable angles in opposite directions (Fig. 3.11B). The bioturbation is rare with an assemblage of Chondrites and Planolites (BI = 0 to 1) in the siltstones and mudstones (Fig. 3.11C). Siltstone and mudstone beds in other parts of the section described here are faintly laminated to structureless and lack bioturbation (BI = 0).

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Figure 3.11: Characteristics of siltstones and mudstones (F9). (A) Thick dark-grey laminated mudstones. (B) Inversely to normally graded beds of siltstones and mudstones overlying the lenticular and wave-rippled sandstones with sharp wave-scoured bases. (C) Bioturbated mudstone beds showing Chondrites (Ch) and Planolites (P) trace fossils.

The siltstone and mudstone beds in the upper third of the measured section (Fig. 3.3) were deposited in a delta front and prodelta setting. The oppositely dipping ripple laminae with variable dip angles are interpreted as wave ripples (de Raaf et al., 1977). The sharp erosional bases to the rippled sets represent storm-wave-generated scoured surfaces on which the wave ripples were subsequently deposited. The overlying inversely to normally graded siltstones and mudstones are interpreted as the products of storm- wave-enhanced muddy sediment gravity flows given the fact that the muddy beds are

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commonly associated with wave ripples (Bhattacharya and MacEachern, 2009; Macquaker et al., 2010; Plint, 2014). Alternatively, the inversely to normally graded siltstones and mudstones can also be turbidites (mud density flows) (McCave and Jones, 1988; Talling et al., 2012b) or hyperpycnal flow deposits (Mulder et al., 2003). The low- diversity and low abundance trace fossil assemblage in the laminated siltstone and mudstone beds suggests a rapidly deposited muddy deposit (Bhattacharya and MacEachern, 2009) and/or a stressed-shelf environment indicating the proximity to delta distributary mouths (MacEachern et al., 2005; MacEachern and Bann, 2008). The structureless and unburrowed mudstones in the lower section (i.e. 0 to 140 m in Fig. 3.3) suggest rapid deposition of cohesive muddy debris flow (Mulder and Alexander, 2001). The parallel-laminated mudstones in the upper slope environment indicate deposition under low-energy conditions.

Outer Shelf Facies Summary

The outer shelf environment exhibits muddy prodelta deposits grading upward into sandy delta-front deposits dominated by hummocky cross-stratified and swaley cross-stratified sandstones. The coarsening-upward succession, coupled with rare bioturbation represents the progradation of storm wave-dominated delta front on individual deltaic lobes. The isolated hummocky cross-stratified sandstone beds gradually changing upward to amalgamated swaley cross-stratified sandstone beds indicate increased storm-wave influence upward in the delta front. Despite the common absence of distributary channels cutting down into delta front strata, the interpretation of storm wave-dominated shelf-edge delta rather than a simple, non-deltaic outer shelf is also supported by the sand-prone character of the succession and the clear upward-coarsening

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character of the 200 m thick units, implying marked progradation of the shelf edge (Fig. 3.1B and C) (Sydow et al., 2003; Dixon, 2005).

DEPOSITIONAL ENVIRONMENT AND PALEOGEOGRAPHY

The deposits of the mid-Pliocene Moruga Formation as described above exhibit a range of depositional systems from sand-rich, outer-shelf and shelf-edge wave-dominated delta lobes of the Trinity Hills Sandstone (Facies F7 to F9) transitioning downward to the St. Hilaire Siltstone that is dominated by very large slide blocks and intervening irregular gullies filled with mudstone and sandstone sediment-gravity flow deposits. At the base of the study succession there is a large, chaotically infilled area with mass transport deposits, all within an upper slope setting (Facies F1 to F6) (Fig. 3.12). A great thickness of muddy St. Hilaire Siltstone (>200 m thick), which represents deepwater slope deposits, occurs below the study area but is only rarely exposed.

Although the wave-dominated deltas on the outer shelf are likely to have had a lower number of coeval distributary channels than those in river-dominated deltas (Bhattacharya and Giosan, 2003), the high sediment supply of the paleo-Orinoco River would have enabled the delivery of large volumes of sand to the delta front. The open Atlantic storm waves and swells subsequently reworked sands into laterally extensive delta lobes and strandplains close to the shelf edge (Fig. 3.12). Evidence from nearby well data (Sydow et al., 2003) suggests that the outer shelf sandstone belts commonly accumulated thicknesses up to 200 m on the outermost and deepest parts of the shelf. The location of the study site was probably adjacent to one of the delta distributary mouths because marine bioturbation is not well-developed (Facies F7, F8 and F9).

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Figure 3.12: (A) Schematic three-dimensional block diagram of the interpreted paleo- geomorphology and depositional environments in the mid-Pliocene Orinoco Delta (Moruga Formation). Slope gullies extended back onto storm wave- dominated delta front on the outer shelf, captured longshore-drifted sands, and transported sands to the deep water. Turbidites in the amalgamated gullies are shown by a range of orange color to illustrate that they are derived from different episodes of gully infilling processes. (B) Cross- section view along the depositional strike illustrating stacked gullies filled with a variety of mass-transport and sediment gravity flow deposits. The dashed line indicates a possible master collapsed surface which constrains the development of amalgamated gullies. (C) Cross-section view along the depositional dip illustrating the collapsed scar of shelf margin with the overlying sets of slope gullies and renewed delta front deposits. Note that the rotational slides in the gullies are preserved near the base of each slump scar.

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As the basinward-dipping gradient of the shelf-margin clinoform increased, slumping and mass wasting processes became dominant, leading to the generation of turbidity currents and debris flows on the slope (Mayall et al., 1992; Porębski and Steel, 2003). High sediment supply, together with storms and river floods, caused sets of gullies to be initiated on the upper slope by slope failure processes and eventually to be filled with debris-flow and sandy turbidite deposits (Fig. 3.12). Sandy sediment may also have been captured from the shelf-edge sandbelts through sets of gullies and delivered directly to the deepwater settings (Puig et al., 2003; Marchès et al., 2007). The stacked character of the gullies and common paleocurrent direction (north-east) measured from gully infill deposits indicate the gullies were probably constrained by an underlying larger conduit such as a master erosion/collapsed surface, fault, or canyon (Coleman et al., 1983; Surlyk, 1987; Sylvester et al., 2012). However, it is not possible to draw further conclusions about any such larger conduit due to limited data (see also Dasgupta and Buatois, 2015).

MODEL FOR STRATIGRAPHIC EVOLUTION OF AN UNSTABLE,WAVE-DOMINATED

SHELF-MARGIN