Does Tempestite Proximality Explain Mudstone-Limestone Cycles? A Case Study in the Upper Ordovician of the Cincinnati Arch
The limestones of the Cincinnati Ordovician interbedded with mudstones have been interpreted as “tempestites”, created by storm winnowing suspension and redeposition from a mixed shell-mud precursor, and interbedded with “background” muds that accumulated in fair weather. In the tempestite proximality model finer siliciclastics were transported distally, leaving amalgamated shell-bed grainstones in proximal environments, and mud-rich sediments in distal environments. In this model, water depth exerted ultimate control over limestone deposition, because the sea-floor had to shallow above storm wave base to be winnowed. Cycles were controlled by fluctuations in water depth. This model does not fit observations. While there is evidence that high-energy events (e.g. storms) reworked most shell-bed limestones, there are several lines of evidence that limestones were not created by these events, but grew in place and moreover that storm effects were common in the mudstone intervals. Some limestones, lack clear evidence of reworking. Micritic matrix of many shelly limestones highlights the improbability of a shelly siliciclastic mud precursor. Mixed taphonomy of fossils, growth and reworking of carbonate concretions, and concentrations of diagenetic phosphates in shelly limestones all point to long periods of gradual, repeatedly reworked skeletal accumulation. Conversely, cm-scale depositional units with ‘lam-scram’ structures suggest that mudstones were deposited rapidly as events. The typically low fossil content of muds would require reworking at least 10 times the compacted thickness of mudstone to generate any given accumulation of shells, for which there is no evidence. The faunal content of limestones is not significantly different from that of the mudstones, indicating that limestones were not consistently deposited in shallower water than mudstones. While the tempestite proximality model predicts that packages of interbedded limestones and mudstones should grade and condense proximally into fewer more amalgamated grainstones, Cincinnatian limestones show the opposite pattern. A single thin, deep-water grainstone “fans out” proximally, upramp into multiple component muddy limestone beds. This distribution of facies suggests that bedding and cyclicity was controlled by siliciclastic sediment supply rather than by the ubiquitous storms or other high-energy events, a conclusion that may apply to many other mixed clastic-carbonate systems.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014