TIDAL CAPS AND CYCLE ASYMMETRY IN PLATFORM CARBONATES

Peritidal carbonates seem often to be organized in asymmetric, shallowing-upward cycles in which muddy supratidal deposits form the cycle top. Robert Ginsburg proposed an autocyclic model (assuming a lag time, steady subsidence, and depth dependent sediment production) that elegantly explains the shallowing-upward succession in platform carbonates. Unfortunately his elegant model has often been misinterpreted to perpetuate the notion that tidal flat facies always form the top of shallowing-upward cycles, covering the entire platform at the end of an accommodation cycle. The Pleistocene/Holocene facies succession on Great Bahama Bank (GBB) illustrates the boundary conditions for Ginsburg's model and questions some assumptions for cycle asymmetry.

The predominant facies on the modern bank is a blanket of subtidal facies consisting of bioturbated peloidal sand in water depths from 3- 7 m. In a few places sedimentation filled the accommodation space to build inter- and supratidal facies successions. In high–energy grainstone belts, stabilized bioturbated sand flats occur in the intertidal zone, (e.g. Joulters Cay), and channelized sandy tidal flats form the transition to the supratidal zone, while amalgamated beach and storm ridges form the high supratidal. In these grainstone areas muddy tidal flats are absent, but grainy tidal flats develop in protected pockets. In contrast, sediments in all three tidal zones of the Andros tidal flats are predominantly muddy while coarse-grained sediments are restricted to the tidal channels and beach ridges. Water depths in both the grainy and muddy tidal environments are similar; indicating that shoaling alone does not generate a mud tidal flat. The Pleistocene bedrock underneath the Andros tidal flats, however, is a grainstone shoal whose antecedent topography provides the low-energy conditions for the modern mud flat. In this case the Pleistocene grainstone is overlain by a Holocene mud tidal flat which forms the base of a new shallowing-upward cycle. Progradation of the Andros tidal flat will bring the muddy facies across part of the bioturbated wacke-packstone of the platform interior but not the high-energy grainstone shoals. Based on facies distribution on GBB it is highly unlikely that muddy tidal flat caps develop on top of tidally-influenced grainstone shoals.