Lateral and Vertical Facies Relations of Transgressive Barrier

Abstract

Identification of transgressive barrier sequences in the stratigraphic record is important because preserved barriers are potential reservoirs of petroleum. The transgressive barrier complex on the Atlantic Coast of Delaware lies at the edge of the ongoing Holocene relative rise in sea level. The Delaware coastal zone is on the northwest flank of the subsiding Baltimore Canyon Trough geosyncline of the Atlantic continental shelf. Four major variants of a transgressive barrier complex in this area are (1) a spit-beachdune complex, (2) barrier overriding a coastal marsh, (3) beach against pre-Holocene highland, and (4) a barrier-tidal delta-lagoon system which includes a linear baymouth barrier and a lobate tidal delta. Characteristic vertical sequences in each of these var ants indicate that this barrier originated farther seaward and migrated landward to its present position. Coarse sediments overlying fine sediments, generally believed to be characteristic of regressive barriers, also are present in this transgressive example. Time lines clearly diverge and cross lithologic boundaries in both transgressive and regressive barrier sedimentary units. Hence, the transgressive or regressive nature of a preserved barrier sequence cannot be positively identified on the basis of characteristic shapes of geophysical logs or sedimentary sequences. Rather, projection of barrier trends must be based on a synthesis of barrier morphology, precise identification of elements of barrier stratigraphy, knowledge of time versus stratigraphic units and lateral and vertical s quences, and an understanding of sedimentary basin tectonics.