VOLCANIC DEBRIS FLOWS IN A TALCOTT-SHUTTLE MEADOW TRANSITION LAKE SEQUENCE, CT

The Hartford Basin is one of 16 Triassic-Jurassic age synrift basins, extending for more than 2000 km from eastern Canada to the southeastern United States, which formed as a result of crustal extension associated with the breakup of Pangaea; continued NW-SE extension produced listric normal faults on the eastern side of a subtropical half-graben at 25 degrees N paleolatitude. The subsiding basin accepted 4-7 km of arkosic fluvial redbeds, alluvial fan conglomerates, black shales and rare carbonates, interbedded with three basalt flows. Marginal environments represented within the basin include 1) fault scarp breccias 2) alluvial fans and river systems 3) swamps and marginal lake environments 4) deltas and shorelines 5) open water shallow lakes and 6) deep water lakes. Volumetrically, the majority of the lacustrine beds are grayish black siltstones; limestones are rare and include a thin bed in the basal East Berlin Formation, a spatially limited spring deposit placed within the Shuttle Meadow Formation and carbonate (also a spring deposit?) entrained within a pillow basalt sequence within the Talcott/Shuttle Meadow transition.

This latter deposit is part of a siliciclastic lake sequence developed within a tertiary order synsedimentary graben in southern Connecticut, preserved within fault blocks and located in close proximity to a Talcott-associated eruption complex. The sequence records the influx into the lake of Talcott volcanic debris in two 2 to 3 foot- thick, hydrothermally- altered, polymodal, arkosic breccias/conglomerates, separated by 16 inches of laminated reddish gray cross-bedded siltstone. These breccia units may be interpreted as debris flows remobilized from the flanks of the eruption complex or eroded from adjacent Talcott basalt flows, and re-sedimented.within the siliciclastic lake sequence.

The overall sequential organization of the basin-fill succession reflects syndepositional tectonics and climate fluctuations on both the large and small scale, whereas the lateral variation in sequence architecture is due to the localized sediment supply, differential subsidence, and to interaction with the volcanic complexes.