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.
Callan Bentley is an assistant professor of geology at Northern Virginia Community College in Annandale, Virginia. He is particularly interested in structural geology and the evolution of the Appalachian mountain belt. Callan draws cartoons and writes for EARTH magazine. He lives in the Fort Valley of Virginia.
Thomas R. Holtz, Jr. said on 8 December 2011
These look great! Any hints on getting to that outcrop? Have been thinking about updating the Historical Geology field trip, and this would be a more-than-mere-replacement for the Martinsburg turbidites near Hagerstown on 70 N.
Callan said on 8 December 2011
Easy to get to — drive to the very end of the publicly open stretch of new Route 55 (west of Moorefield). It’s the last outcrop before the bulldozers. These were both seen on the south side of the road, but there’s equivalent exposures on the north side.
Steve Gough said on 8 December 2011
Cool! I love the last photo.
Tim said on 12 December 2011
Hmmm, deep water, anerobic, Dev???
Any soft tissue preservation in the formation?
Callan said on 12 December 2011
There certainly could be – I’ve seen itty bitty brachiopods from elsewhere in the Brallier, but we didn’t see any fossils at all at this particular outcrop.
Brian Romans said on 13 December 2011
very nice, looking forward to looking at these strata
Robin R. said on 27 January 2012
Getting caught up on the blog. Those flute casts look awfully familiar.
Callan said on 27 January 2012
Post a link to the Gigapan you shot of it, Robin!