Transition From Storm Wave-Dominated Outer Shelf to Gullied Upper Slope: The Paleo-Orinoco Shelf Margin (Moruga Formation), South Trinidad
Shelf-edge deltas are the key agents for accreting sediment onto clinothems of shelf-margins though the mechanism by which this happens remains understudied. The Moruga Formation (the St. Hilaire Siltstone and Trinity Hill Sandstone members), south Trinidad, reveals growth across the earliest Pliocene shelf-break area in front of the paleo-Orinoco Delta and records a transition from the gullied uppermost slope to the storm wave-dominated delta front and outermost shelf. Analysis of the measured outcrops (240 m thick) demonstrates that: (1) In the outer-shelf areas, multiple coarsening-upward parasequences show increased wave influence upwards. The prodelta is dominated by up to 2 m thick, laminated mudstones with interbedded siltstone (< 10 cm thick). The delta-front deposits are also dominated by laminated mudstones (units up to 1 m) with interbeds of parallel-laminated/wave-ripple laminated, very-fine sandstones (< 50 cm thick) and abruptly changing up to sandstones with amalgamated hummocky and swaley stratification (up to 3 m thick). (2) On the underlying muddy upper slope, sets of gullies (4 to >10 m deep) filled with interbedded mudstones and gravity flow deposits that include debris-flow conglomerates and sandy turbidte beds. Two types of slope gully underlain by distinctive erosion surface are documented here. Steeply-dipping (up to 50°) deposits are interpreted to represent gully-head morphology and the other, less steeply-dipping (up to 30°) gully-fill strata as the distal parts of a gully. (3) Further down the slope, the turbiditic deposits are dominantly channels (up to 2.2 m deep) filled with structureless to rippled sandy beds and sheetlike very-fine sandstone units (up to 1.3 m thick) associated with an abundance of soft-sediment-deformed strata. The observations suggest a setting where upper-slope gullies formed from slump scars near the shelf edge as gradients were locally increased. The key question then is whether much sand bypassed through these gullies to generate sandy submarine fans downslope? Our initial answer on this is NO! Experience from elsewhere suggests that storm-wave dominance at the shelf edge is negative for sand delivery immediately across that boundary. However, if the gullies amalgamate to form a larger conduit, a canyon that extended back onto the shelf and could capture longshore drifting sand, delivery to form fans is possible. In the Moruga gullied slope there was little sign of this.
AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015