DETRITAL ZIRCON U-PB GEOCHRONOLOGY OF SANDS FROM AND NEAR THE CALVERT CLIFFS, MARYLAND

The Calvert Cliffs (CC) along the western coast of the Chesapeake Bay (CB) expose fossil-rich, Miocene (ca. 18 to 8 Ma) shallow marine strata, overlain inland by the ca. 8-2.5 Ma fluvial Upland Deposits (UD). Erosion of both feed modern beaches (MB) along cliffs. Zircon ages were measured from seven sand samples (two CC Miocene, three UD, of which two reworked, and two MB). The Susquehanna River might have contributed some glacial sands to the MB during sea level lowstands, when CB did not exist. Samples vary in grain size and texture, reflecting varied modern and ancient depositional environments. 54-58% of zircon ages from each sample correspond to the Grenville Orogenic Cycle (1245-980 Ma), while only 0.7-3.6% corresponded to Superior Province ages (2800-2600 Ma). Combining all 1613 analyses reveals peaks at 126 Ma (0.25%), 367 Ma (0.56%), 453 Ma (1.30%), and 723 Ma (5.83%), likely material derived from the White Mtn. plutonic series, Alleghanian and Taconic tectonism, and Neoproterozoic rifting, respectively. Grenville-aged zircons include Rigolet ~1010-980 Ma (15.81%), Ottawan ~1090-1020 Ma (32.18%), Shawinigan (12.40%), and Elzevirian (0.74%) components. Ages of ~1321, 1368, 1469, 1622, and 1730 Ma, and the lack of ages from the interval 2500-2000 Ma, indicate the influence of pre-Grenvillian source terranes located west and north of the Miocene and younger CB watersheds. None of the zircon ages correspond to Triassic rifting and subsequent opening of the North Atlantic ca. 220-175 Ma. We found subtle stratigraphic changes (Miocene to modern stream deposits) in detrital zircon age distributions: 1) a younging of the Ottawan “peak” age; 2) reduced abundance of Shawinigan-aged zircons; 3) a younging of Neoproterozoic “peak” age; and 4) a reduction in the Paleozoic/Neoproterozoic component. The limited exposure of Grenville rocks in the watershed, and current isolation of Superior Province and other older Proterozoic sources from the Chesapeake Watershed, suggest recycling of Precambrian zircons from younger Paleozoic sedimentary rocks, also dominated by Grenville-aged detrital zircons, into Miocene to recent sands. Comparison of age distributions suggests modern beach sands derive largely from the Upland Deposits, not the Calvert Cliffs.