AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Stratigraphic Architecture of Shallow-Marine Siliciclastic Sequences in an Updip Passive-Margin Setting: Insights into the Miocene Aquifers of the Central Delmarva Peninsula
(1) Delaware Geological Survey, University of Delaware, Newark, DE.
(2) Department of Geological Sciences, University of Delaware, Newark, DE.
(3) Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ.
Shallow-marine aquifer sands are major sources of groundwater in the coastal plain of the central Delmarva Peninsula (Delaware and Maryland, USA). The lower to middle Miocene strata are characterized by alternating shelly sands and muds predominantly deposited in shallow-marine, wave-dominated shoreline environments. We assess the stratigraphic distribution and continuity of aquifer sands in this interval in a sequence-stratigraphic framework. The analysis utilizes wireline core records from two sites, one basinward (SE) at Bethany Beach and one shoreward (NW) at Marshy Hope, as well as a network of geophysical logs obtained from water wells and test holes.
The lower to middle Miocene section was deposited landward of the depositional shelf break in a passive-margin setting. It is progradational in the lower part (Calvert Formation), transitions upward to more aggradational (Choptank Formation), and is capped by the muddy St. Marys Formation, which serves as a regional confining layer. Repeated cycles of shallowing- and coarsening-upward deposition in the Calvert and Choptank produced a stack of unconformity-bound sequences composed predominantly of prograding highstand systems tract packages, with thin transgressive systems tracts at the base locally. Subsidiary parasequences are rare. Strontium isotope chronostratigraphy indicates the sequences are mostly 0.5 to 1.5 Myr in duration and commonly separated by resolvable hiatuses. The section increases in thickness basinward, much of it from thickening of individual sequences, particularly in the Calvert Formation. The added thickness is primarily in the muddy, lower highstand portions of the sequences; these muds are commonly thin or even absent in updip locations.
Aquifer sands are typically developed in upper shoreface deposits at the tops of the sequences. The sand bodies are laterally extensive, in many cases extending >100 km in an updip-downdip direction. Sequence boundaries at the tops of these sand bodies are relatively planar, with little significant incision documented. Thus, defining the sequence-stratigraphic character of Miocene, wave-dominated shoreline deposits in this updip, passive margin setting allows us to better understand the distribution and significant continuity of laterally extensive shoreface sands that serve as important aquifers (and useful reservoir analogues) in central Delmarva.