A MODERN ANALOG FOR SHALE RIP-UP CLAST FORMATION: CASE STUDIES OF MULTI-GENERATIONAL MUD CLASTS FROM THE EASTERN SHORE, VIRGINIA, USA

One of the grand challenges for stratigraphers is to make concrete connections between the rock record and modern analogs. Here I report on a modern lithofacies/biofacies from the Eastern Shore of Virginia that reflects not only the impacts of storms, but also the outcomes of the unprecedented rate of modern sea level rise (nearly 7 mm/year). Landward migration of barrier islands and shoal complexes, and increased headland bluff erosion are all yielding a time-transgressive facies that has the potential to inform interpretation of enigmatic associations in deeper time. Shale rip-ups clasts, often found in cross-bedded sandstones or in slightly conglomeratic beds and are often interpreted as deltaic or as fluvial channel facies. In other cases, mud lithoclasts are found embedded in fossiliferous coarse-grained sediments and are considered evidence of storm/tempestite deposition in shelf environments. So how are these formed and under what conditions/processes?

Recent erosion of Assateague and Wallops Islands and Chesapeake Bay dune-headlands at Savage Neck have yielded pavements of mud rip-ups, which may be an excellent analog for interpretation of ancient rocks. After storms, clasts are strewn across the beachface. They appear in overwash, and offshore. Mud clasts originate as low energy, sound-deposited sediments (sparsely to occasionally fossiliferous) which pass as a stratal layer under the salt marsh/barrier island. Layers can be broken up by inlet formation or in the surf of the seaside shoreface. In some cases, these underclay horizons are colonized in the surf by mud piddock clams (Pholadidae) or other fauna yielding a range of small to large borings that deteriorate the integrity of the indurated clays. Some clasts have been found with byssate threads of mussels, and occasionally algal holdfasts. Subsequent storms produce yet more mud-clast pavements that show evidence of the biological communities (often found with storm-winnowed heavy mineral assemblages). Once on the beach, they are subject to solar baking which helps to further harden the clasts making them more durable and subject to further remobilization and additional rounds of colonization.

Given these observations, it is important to re-think shale rip-up clasts in the context of what is suggested here to be a multi-step process that isn’t just a single rip-up event, but a more protracted series of events that collectively produce a time-averaged assemblage.