MAPPING THE GETTYSBURG TRIASSIC-JURASSIC RIFT BASIN (PENNSYLVANIA, USA): IMPLICATIONS OF PRELIMINARY OBSERVATIONS ON STRATIGRAPHY, FACIES, AND CYCLICITY TO TECTONIC DEVELOPMENT OF EASTERN NORTH AMERICAN RIFTS

Early 20^th century pioneers of Eastern North American Geology such as Florence Bascom, George Stose, and Anna Jonas, laid the basic groundwork for the geology of the Gettysburg Basin. Funded by, and working with, the PA Department of Conservation and Natural Resources, we are building on their foundation using LIDAR and field testing of stratigraphic and structural hypotheses with paleomagnetics and biostratigraphy. Although only in its initial stages, our project has already yielded observations with significant implications. LIDAR reveals the stratigraphy of the of the basin in stunning detail including: 1) a hierarchy of laterally persistent sedimentary cycles with the same kind of large-scale temporal variations as seen in the Newark Basin (e.g., damped cyclicity in Rhaetian strata compared to Norian); 2) presence of a mappable, lacustrine, gray and black shale and carbonate sequence at the base of the New Oxford Fm, not seen in the Newark Basin; 3) clear boundaries of intrusive and rare extrusive basaltic rocks, including cross cutting relationships; 4) hitherto unmapped, relatively high-amplitude transverse folds, like those in the Newark Basin; and 5) easily mapped faults, although large ones are surprisingly rare. Field checks reveal that the most obvious strike ridges reflect 405 kyr cycles, and these are bundled into sets of ridges reflecting the ~1.8 Myr cycle in the Newark Basin suggesting paleomagnetically-testable, detailed correlations at the 405 kyr-level. Fourier analysis and TimeOpt accumulation rate models indicate very high accumulation rates compared to contemporaneous Newark Basin strata. Lacustrine facies and stacking patterns within the middle Gettysburg Fm are virtually identical to those of the middle Passaic Fm, although considerably more fossiliferous. From these preliminary observations and analyses, pending planned tests, we hypothesize that the tectonic and stratigraphic development of the Gettysburg Basin is similar to the Newark Basin with lakes that during their deepest phases may have extended continuously from at least the Newark into the Culpeper Basins. However, the basal stratigraphy is more like that in the Taylorsville and Richmond Basins, including the growth from smaller subbasins in the early Late Triassic to a very wide basin in the later Late Triassic.