Switching off the carbonate factory: A-tidality, stratification and brackish wedges in epeiric seas

Carbonate productivity in tropical seas is depth, nutrient, salinity and temperature dependent. The tides in modern coastal settings are a powerful mixing force, controlling salinity and temperature. Tidality in epi-continental seas is therefore a powerful but under appreciated, influence on carbonate productivity. Many seas of this sort, in the geological past, would have been isolated from the open ocean tidal bulge, rendering them prone to micro-tidality and possibly a-tidality. The absence of tidal mixing would have promoted stratification due to temperature and or salinity differences and influenced the output of the carbonate factory. We illustrate this phenomenon with reference to the late Triassic-early Jurassic transgression in southern Britain where, despite a tropical climate, classical shallow-water carbonates are absent. Instead the transgressive deposits are characterized by reduced salinity and schizohaline biotas, shallow-water anoxia, and a lack of tidal influences. Criteria are provided for the recognition of similar reduced salinity shorelines in other epi-continental sea successions.