Hickey, L. J. , Taylor, David W. , Hu, Shusheng .
Early Angiosperm Penetration into a Gymnosperm Dominated Ecosystem: The Dutch Gap Locality, Virginia USA.
Aptian-age sediments (Lower Zone I, Potomac Group) at Dutch Gap, east of Richmond, Virginia contain the oldest megafossil angiosperms in North America. Sediments at this locality probably represent the meander belt of a fluvial setting, with from four to five cycles of evulsion. Multistoried channels at the site are filled with arkosic sand, with pebble to boulder-sized clasts of gneiss, and pinkish grey claystone and mudstone at channel bases and on reactivation surfaces. These channels are cut into, and grade laterally into inferred levee and splay deposits that consist of thin interbeds of sand and silt alternating with medium grey, micaceous siltstone and mudstone. This sequence becomes finer and slopes away from the channels, gradually grading into relatively thick, dark grey beds of mudstone, representing deposition in a flood-basin setting, with only thin laminae of fine-grained sand and silt.
Megafossil plants at Dutch Gap exhibit a clear pattern of association with these lithofacies. Five species of very rare angiosperm leaves, including the putative earliest megafossil monocotlydon, Acaciaephyllum, the dicotyledons Ficophyllum, Quercophyllum, Proteaephyllum, and an unnamed serrate-chloranthoid occur with ferns in silty interbeds in what appear to be distal-levee settings, while the inferred floodbasin mudstone was dominated by Dioonites, with associated taxodioid conifers and ferns. A diverse suite of conifers, including Elatocladus and Geinitzia, and ferns, together with cycadophytes, are dominant in pinkish, rhizomorph bearing clasts of homogeneous mudstone, which are inferred to have been transported from higher, better-drained, floodplain terraces. In contrast, the palynomorphs show no clear relationship to lithofacies, possibly due to the more ready transport of pollen-size grains over a relatively small area. Thus the rare megafossil angiosperms at Dutch Gap are associated with sediments of the distal levee environment, which we infer to have been an open habitat due to the preservation of thin alluviation event-beds and the paucity of rooting or other evidence of bioturbation.
Log in to add this item to your schedule
1 - Yale University, Department of Geology & Geophysics, Yale Station, P.O. Box 208109, New Haven, Connecticut, 06520-8109, USA
2 - Indiana University Southeast, Department of Biology, 4201 Grant Line Road, New Albany, IN, 47150
3 - Yale University, Peabody Museum of Natural History, P.O. Box 208118, New Haven, CT, 06520
Presentation Type: Oral Paper:Papers for Sections
Location: 555A/Convention Center
Date: Tuesday, August 3rd, 2010
Time: 9:15 AM