Middle Miocene oceanographic event in the eastern Gulf of Mexico: Implications for seismic stratigraphic succession and Loop Current/Gulf Stream circulation

摘要

Analysis of 1,500 km of high-resolution, single-channel air-gun seismic-reflection profiles, correlated with two drill cores, demonstrates that the central west Florida carbonate-ramp slope experienced an oceanographic event in the middle Miocene that resulted in a major stratigraphic break or succession of depositional sequences. During the long-term eustatic sea-level rise in the early to middle Miocene, carbonate shelf-margin deposition was characterized by seaward-prograding, sigmoidal clinoforms that built both laterally and vertically. Shelf-margin sediments were relatively fine grained and rich in dolomite, total organics, and clay minerals. Approximately 12–15 m.y. ago, tectonically controlled oceanographic closure of the Isthmus of Panama, and/or increased climatic gradients, resulted in an intensification of oceanic circulation. Increased flow velocity of the Loop Current truncated antecedant clinoforms along the mid-Miocene shelf margin as the Loop Current, for the first time, became a dynamic oceanographic barrier to off-shelf sediment transport. Carbonate deposition along the west Florida slope changed dramatically from a series of prograding clinoforms to a pelagic slope-front-fill system bounded on its landward margin by contour-current flow of an invigorated Loop Current. Shelf-margin sediments, presently represented as a condensed sequence, became more carbonate rich, coarser grained, lower in organic content, and depleted in dolomite. Our results suggest that oceanographic events can generate major seismic stratigraphic breaks that result in the succession of depositional sequences and systems along continental margins and that they may be as important as eustatic oscillations. Our data also suggest that "modern" Loop Current/Gulf Stream circulation was initiated in the middle Miocene, controlled by tectono-oceanographic and/or climatic processes. The challenge now is to identify seismic stratigraphic-scale oceanographic events in the rock record, differentiate them from eustatic responses, and evaluate their paleoenvironmental significance.