LITHOSTRATIGRAPHY OF TRIASSIC-JURASSIC BOUNDARY AND LOWERMOST JURASSIC ROCKS, HARTFORD BASIN, SOUTHERN NEW ENGLAND

The Hartford basin (central CT-MA) contains 8 km of sedimentary and volcanic rocks that span the Norian-Hettangian and preserve the Triassic-Jurassic (Tr-J) boundary. The boundary is located within the upper 15 m of the 3 km thick New Haven Fm (Trnh). This horizon coincides with the base of a prominent interval that records a basin-wide switch from fluvial to lacustrine deposition. Near the eastern border fault (EBF), uppermost Trnh beds are 3 m of black and red shale/siltstone capped by 12 m of fluvial sandstone/ conglomerate. Correlation with other Newark Supergroup basins, places the Tr-J boundary within the basal 3-m black shale. These strata thin toward the basin hinge zone to 1 m or less of lacustrine red/purple shale, siltstone and red/ buff sandstone. The Trnh is overlain by basalt and volcaniclastics of the Talcott Fm (Jta). Jta basalts are 20-70 m thick across much of the Hartford basin, and were extruded from eruptive fissure complexes where feeder dikes intersected the basin floor. Within ~5 km of these sites, the Jta consists of basalt and volcaniclastics that thicken toward the fissure complex to as much as 300 m. Where basalts are totally lacking, the Jta consists of altered and reworked ash beds (~25 thick). Elsewhere, Jta basalts display well developed pillowed horizons that intertongue with uppermost Trnh beds, indicating no depositional hiatus. The overlying Shuttle Meadow Fm (Jsm) consists of ~20 m of coarse clastics followed by 80 m of lacustrine deposits. Above the lakebeds are ~150 m of red, gray and buff clastics and limestone deposited in lacustrine, fluvial, floodplain and fan environments. The lower lacustrine member is restricted to areas within ~20 km of the EBF. Its distribution was controlled by topography resulting from interbasinal block faulting of the basin floor contemporaneous with Jta extrusion. The upper, mostly red portion of the Jsm onlapped and buried interbasinal highs and thins to as little as 15 m in northern CT. Milankovitch cyclicity of the Trnh and Jsm indicate deposition from the Tr-J boundary to the top of the Jsm occurred within 300-320 ky, thus providing a rare opportunity to document basal Jurassic biotic recovery in the aftermath of the Tr-J mass-extinction.