EOLIAN SANDSTONES FROM THE POMPERAUG AND HARTFORD RIFTS, CONNECTICUT: INDICATORS OF EARLY JURASSIC PALEOCLIMATE GRADIENTS?

The recent recognition of eolian beds in Early Jurassic age rocks of the Pomperaug and Hartford rifts is noteworthy because, with the exception of a single eolian bed in the Late Triassic rocks of the Hartford basin (Smoot, 1991), eolian deposits in the Newark Supergroup are mainly found in the Fundy basin, Canada at about 14° -15° paleolatitude. The Pomperaug sandstone contains sedimentary features, including grainfall and grainflow layers, inverse-graded pinstripe laminae, and high sorting, that compare favorably with those observed in modern eolian dune sands. The eolian beds overlie caliche horizons in red, rooted and mud-cracked siltstone indicative of arid- to semi-arid depositional environments. The beds are part of a basin-wide eolian dune field traceable for at least 5km along the axis of the basin. The eolian beds, including the surface morphology of the dunes, were preserved by the overlying basalt flow.

Re-examination of sedimentary features reveal that eolian deposits are a significant component of the classic Portland brownstone quarries in Connecticut. These rocks contain sedimentary features attributable to sand sheets, low angle dunes, and linear "coppice" dunes. The eolian beds were apparently preferred for building stone because of their grain size and texture. The eolian beds alternate with fluvial beds in intervals about 15m thick indicating possible cyclic climatic control on deposition.

Eolian sedimentation in the Pomperaug and Hartford rifts was promoted by both favorable paleolatitudinal position and deposition within relatively dry climatic intervals. These deposits formed at about 11° paleolatitude, on the arid side of the estimated 10° latitude arid-humid climate boundary based on the evaporation-minus-precipitation models of Crowley and North (1991). The presence of eolian sand suggests that the Early Jurassic humid equatorial climatic zone may have been constrained to a narrow zone less than 10 degrees north and south of the paleoequator, contra Parrish (1993). High-resolution correlations with arid to semi-arid intervals in the nearby Newark basin support the hypothesis that the eolian sandstones are indicators of regional paleoclimate conditions, rather than just local depositional environments.