The Recognition of a Marine Flooding Surface Using Trace Fossil Assemblages
Increased rates of sediment input diminish the resolution and utility of microforaminifera for stratigraphic studies in the Pliocene of Trinidad, relative to the earlier Cenozoic. Other techniques, such as trace fossil analysis, must be employed to delimit significant stratigraphic horizons. The 1600 m thick Morne L'Enfer Formation represents a late stage of foreland basin fill and is characterized by two major highstand events separated by a sequence boundary, lowstand and transgressive fills. The maximum flooding event is represented by the Lot 7 Silt Member of the Morne L’Enfer Formation, which is also a prominent regional marker in the subsurface. Outcrops of the Lot 7 Silt are rarely exposed but trace fossil assemblages provide a very good measure to delimit both the position and relative proximity to this major flooding event.
Throughout the Morne L'Enfer Formation, wave-, tide- and fluvial-dominated sucessions and their deeper-water density and gravity-driven sediment equivalents are differentiated by lithofacies and ichnofacies. Wave-dominated sequences comprise swaley cross stratified sands with a characteristic Skolithos ichnofacies while tide-dominated sequences display lenticular-wavy-flaser bedding and aspects of the Skolithos and Psilonichnus ichnofacies. Fluvial-dominated sequences comprise trough cross-bedded sands of lenticular geometry that are typically not bioturbated. Deeper-water sediments are either extensively bioturbated lower shoreface silts (Cruziana ichnofacies) or rarely bioturbated pro-delta silts and clays.
Ichnofacies and lithofacies models were applied to pinpoint the Lot 7 Silt maximum flooding event along Puerto Grande Bay on the southwest coast of Trinidad. The Lot 7 Silt beds are characterized mainly by Scolicia and Thalassinoides traces that overlie variably bioturbated shoreface parasequences typified by Skolithos and Cruziana ichnofacies. The flooding event is subsequently overlain by a tide-influenced parasequence with common Psilonichnus borrows, and capped by root traces signifying a return to coastal depositional environments. The beds identified using this technique calibrate with the surface geological map and agrees with the original description of the Lot 7 Silt Member. The successful application using trace fossils demonstrates their utility as a direct, field-based tool for stratigraphic studies.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009