8 December 2011

Currents in the Devonian deep

Posted by Callan

This blog has been noticeably photo deficient lately! Time to remedy that. Today, I offer you a couple of shots of the Brallier Formation (shale / fine-grained sandstone) in West Virginia’s Valley & Ridge province, a few miles northwest of Moorefield, on the newly-opened section of New Route 55. The Brallier was deposited in a deep, low-oxygen portion of the epeiric Kaskaskia Sea.

Flute casts:

Cross-bedding:

These sedimentary rocks were deposited by turbidity currents, high-energy roiling bundles of water, sand, and mud that avalanched down into the deep part of an inland sea during the Devonian period of geologic time. Back then, mountains were being built on the east coast (the Acadian Orogeny) and as those mountains weathered and eroded, they generated lots of sediment. Some of that sediment was deposited on the land, or at the coast, and sometimes high-energy events (perhaps submarine landslides initiated by Acadian earthquakes) took it much further out into the depositional system than “normal.” There, the turbid clouds settled out, producing graded beds and other primary sedimentary structures. The two sedimentary structures shown here are parts of turbidite deposits, collectively called the Bouma sequences. They record currents that had the capacity to erode (gouging out the flutes) and move sediment in a directional current (laying down the cross-beds).

If we listen to them, these rocks speak to us of ancient currents, flowing strongly at times, deep in a sea long deceased.