Eustatic and Tectonic Controls on Deposition of Hybrid Siliciclastic/Carbonate Basinal Cycles: Discussion with Examples

Abstract

Changes in sea level represent one of the dominant controls on basinal sedimentation adjacent to mixed carbonate siliciclastic sediment-source areas. Sedimentary responses to sea level change of the siliciclastic and carbonate components of these hybrid systems commonly result in deposition of alternating siliciclastic and shelf-derived carbonate basinal deposits. Such deposition is particularly pronounced adjacent to carbonate platforms, where alternate immersion/exposure of the flat platform top results in carbonate highstand deposition and siliciclastic lowstand deposition. In contrast, small to moderate-amplitude sea level drops in distally steepened ramp settings may expose only the inner part of the gently sloping shelf, resulting in lowstand deposition of mixed sil ciclastic/allodapic (FOOTNOTE 3) carbonate sediments. Along active margins, tectonic uplift and subsidence of sediment-source areas act as additional major controls on deposition of siliciclastic/carbonate basinal cycles. Two of the most important tectonic effects are (1) uplift of siliciclastic sediment-source areas, resulting in increased erosion and swamping of carbonate shelf sources and (2) exposure or immersion of shelf-carbonate sediment-source areas due to vertical tectonic movements. Most published seismic-stratigraphic studies of sea level control on deep-sea sedimentation focus on lowstand deposition of siliciclastic sediment and commonly neglect the contribution of allodapic carbonate sediment deposited during sea level highstands. However, the examples discussed in this paper indicate that highstand carbonate deposits may make up a significant proportion of many hybrid carbonate/siliciclastic basinal sequences.