INFLUENCE OF FAULTING ON STRATIGRAPHIC RELATIONSHIPS WITHIN LOWER JURASSIC BASALT AND SILICICLASTIC FORMATIONS IN THE HARTFORD BASIN, CONNECTICUT

An examination of lower Jurassic rocks from a core in the Hartford Basin (in South Hartford) revealed enigmatic relationships between basalt and clastic alluvial/lacustrine strata. The core consists of, from bottom to top: (1) a lower basalt unit (47m) the base of which was not penetrated, (2) reddish-brown mudstone and sandstone (34m) with one black shale interval 7m from the top, (3) an upper basalt unit (15m), and (4) reddish-brown mudstone and sandstone (12m). These strata differ significantly from the regional stratigraphy in that the upper basalt is much thinner than any regional basalt layer, and the sedimentary unit between the two basalt layers is thinner than either the Shuttle Meadow or the East Berlin Formations exposed locally.

The core is located within 100 meters of an intrabasinal normal fault system referred to as the Cedar Mountain Fault system. We have interpreted the unusually thin basalt and siliciclastic units as being truncated by several faults, with the best evidence for faults occurring within the 34m thick sedimentary unit and at the base of the upper basalt unit. This evidence includes post-depositional brecciation, shearing, mineralized fractures, and highly disturbed strata. The top of the lower basalt is a flow-top breccia, and is interpreted as a stratigraphic contact with the base of the overlying sedimentary unit. Dendritic magnetite crystals and the occurrence of iron-rich drops of immiscible liquid in the upper basalt and most of the lower basalt are similar to the entablature portion of the Holyoke. However, the upper six meters of the lower basalt exhibit octahedral magnetite crystals typically seen in the Hamden Basalt.

Tentatively, the upper basalt and most of the lower basalt are assigned to the Holyoke Formation with a possible sliver of Hamden Formation in normal fault contact with the lower basalt. This suggests that the intrabasalt sedimentary unit is Portland Formation. In addition to normal faulting, reverse faulting associated with basin inversion led to the juxtaposition of these units.