18 Geologic Studies of the U.S. Atlantic Coastal Plain Province

Friday, 31 March 2017
8:00 AM - 12:00 PM
Shenandoah H

POLLEN EVIDENCE FOR THE AGE OF THE LILESVILLE GRAVELS: A NEOGENE STRATH TERRACE DEPOSIT FROM THE FALL ZONE OF NORTH CAROLINA, USA

Paper number: 18-4
Program: Theme Sessions
Time: 9:00 AM – 9:20 AM

Authors

Abstract

In the vicinity of Lilesville, NC, there are extensive, previously undated, quartz-rich, coarse-grained gravel deposits found capping interfluves in the dissected Piedmont landscape of the Fall Zone. These deposits unconformably overlie the Late Paleozoic Lilesville pluton and its saprolite. They are interpreted as the highest surviving strath terrace from the paleo-Pee Dee River which currently flows at an elevation about 100 m below the Lilesville gravels. The Lilesville gravels occur as channel fills, channel bars, pebble lags, trough cross-stratified sands, reworked mud clasts and low-flow deposits composed of fine sands. Paleoflow indicators suggest southerly transport, parallel to the modern Pee Dee River. The age of the gravels has been estimated at about 7–12 Ma based solely on the height of this terrace above current river level using the approach of Mills (2000). A recently discovered organic-rich mud layer below the gravels contains abundant, well-preserved plant macrofossils and microfossils. The organic-rich mud layer is sandwiched between dark gray to black clay layers which together are ~4 meters thick. The clay layers likely protected the plant material from oxidizing and thereby resulted in this unusually well-preserved deposit. Pollen from the black, organic-rich mud layer support a Neogene age for the gravels and show that the sediment was derived from an upland mixed hardwood/pine environment. Based on analysis of three samples, the most common pollen are (mean %): Taxodium (18.2), Pinus (17.1), Quercus (15.4), Carya (13.5), Tricolpites (5.7), and Alnus (5.3). Also noteworthy is the presence of the Tertiary taxon Nothofagidites (0.95) but the absence of grasses or composites. The macroflora has not yet been examined. It is likely that the organic-rich mud layer was deposited in an abandoned channel of the ancestral Pee Dee River when it was a steeper-gradient, coarser-bedload, higher-discharge, aggrading, braided stream as compared to the current incising, single-thread river. These observations are consistent with a wetter paleoclimate in the Neogene than at present, and/or topographic rejuvenation in the source area via either stream network reorganization or regional upwarping.

Geological Society of America Abstracts with Programs. Vol. 49, No. 3

doi: 10.1130/abs/2017SE-291305

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