Appalachian Orogen Structure and Stratigraphy
2015. "Appalachian Orogen Structure and Stratigraphy", Tripping from the Fall Line: Field Excursions for the GSA Annual Meeting, Baltimore, 2015, David K. Brezinski, Jeffrey P. Halka, Richard A. Ortt, Jr.
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Measuring the rate at which rivers cut into rock and determining the timing of incision are prerequisite to understanding their response to changes in climate and base level. Field mapping and measurement of cosmogenic 10Be in 106 rock samples collected from the Great Falls area of the Potomac River show that the river has cyclically incised into rock and that the position of the knickzone, now at Great Falls, has shifted upstream over the later Pleistocene. Exposure ages increase downstream and with distance above the modern channel. The latest incision began after 37 ka, abandoning and exposing a strath terrace (the old river channel) hundreds of meters wide beginning at Great Falls and ending at Black Pond, 3 km downstream. This incision was coincident with expansion of the Laurentide ice sheet. Exposure ages of samples collected down the walls of Mather Gorge downstream of Great Falls indicate incision, at rates between 0.4 and 0.75 m/k.y., continued into the Holocene. The 10Be data are more consistent with continued channel lowering through this 3 km reach than the steady retreat of a single knickpoint. Prior to 37 ka, the primary falls of the Potomac River were likely at Black Pond. Ongoing incision siphoned water away from these paleofalls, leaving them high and dry by 11 ka. Downstream of Black Pond, the strath terrace surface is covered with fine-grained sediment, and the few exposed bedrock outcrops are weathered and frost-shattered from periglacial processes active during the Last Glacial Maximum.
Figures & Tables
Prepared in conjunction with the 2015 GSA Annual Meeting in Baltimore, Maryland, this volume contains guides to field trips in this historic region. Emanating from the Fall Line city of Baltimore, these trips reflect the diversity of geological features in the mid-Atlantic region including the Piedmont, Appalachian Mountains, and Coastal Plain, and the importance of geology on the development and construction of the Baltimore-Washington, D.C., metropolitan area. Trips to the core of the Appalachian orogen concern themselves with the tectonic and metamorphic history, early Paleozoic carbonate platform development, Devonian paleoclimate, and coal-mine fire hazards. Excursions to the Coastal Plain examine various aspects of Cenozoic stratigraphy, structure, barrier island formation, and wetland and ecosystem development. A variety of trips also explore urban geology, including building and monument stones of Baltimore and Washington, D.C., urban hydrogeology, and Civil War battlefield geology.