SEQUENCE STRATIGRAPHY OF THE YORKTOWN FORMATION AND RELATED PLIO-PLEISTOCENE UNITS, LIZZIE, NC

A three-dimensional analysis of geomorphology, facies, and bounding surfaces in the post-Cretaceous section between the Surry and Suffolk Scarps was conducted at a small site (km x km) called Lizzie from 30 cores and associated gamma logs. The site straddles the coastwise Wicomico Plain (24 m above mean sea level (MSL)) and several younger fluvial/estuarine terraces that step down to a Holocene drainage with wetland flats at 12 m. The deposits are late Pliocene to Holocene in age. These unconformably overlie a Late Cretaceous marine unit.

This poster will focus on the sequence stratigraphy of the late Pliocene Yorktown Formation, the oldest post-Cretaceous unit beneath the Wicomico Plain. A series of structure contour and isopach maps show that the post-Cretaceous section (<25 m) evolved as a series of nested, sand-rich, complex incised valley fills separated by interfluves with distinct marine signatures. Detailed mapping of facies sequences, facies geometries, and subtle fabrics that mark sequence boundaries/flooding surfaces (SB/SF) suggests that two or three late Pliocene to early Pleistocene sequences underlie the Wicomico Plain.

The oldest sequence (1) includes three major facies. A quartz sand with lignite occurs as incised valley fill. Above this is a phosphatic shelly sand/coquina (Rushmere Member/Yorktown Formation) that sits as a lag on the transgressive surface (SB/FS). This facies thickens towards incised valleys, and locally into barforms on the drowned paleoshelf. Above this is an areally extensive sheet of clayey silt with Mulinia beds (Morgarts Beach Member/Yorktown Formation). This unit shoals upward into a conformable, phosphatic shell bed associated with the base of sequence 2.

Sequence 1 represents a major marine transgression that extended west of the Surry Scarp. Sequences 2 and 3 are tidally bedded units, deposited during a progressive seaward shift of shallower water facies during the subsequent Plio-Pleistocene regression. The regression included sea level fluctuations, a highstand at 30 m (Surry paleoshoreline), and the formation of the Wicomico Plain.