SEDIMENTOLOGICAL AND STRUCTURAL INTERPRETATION OF THE GEOMETRY AND TIMING OF THE EASTERN BORDER FAULT SYSTEM (CONNECTICUT AND MASSACHUSETTS): NEW METHODS TO EXAMINE OLD PROBLEMS

The Hartford Basin is an asymmetric Mesozoic rift basin that formed during the opening of the Atlantic Ocean. Although is has been the subject of research for about 200 years, many fundamental questions regarding its tectonic and sedimentological history still exist. The basin is bounded on the east by a series of normal faults collectively known as the Eastern Border Fault (EBF). Since the EBF was only exposed in one location (now covered) interpretations of fault geometry and timing of displacement rely on examination of sedimentary facies adjacent to the fault, field and microscopic analysis of fault-related deformation, and lateral thickness variations of stratal units. Initial results are used to examine a number of basin evolution issues.

First, fault geometry is established by examining the correlation between coarse sedimentary facies and deformational features in rocks adjacent to the EBF. Conglomerate makes up 44% of the stratigraphic section where the EBF dips 55° to 80°, but only 12% where it dips 30° to 40°. Furthermore, the EBF is probably not a single fault as traditionally mapped. It is mapped as a number of curved segments each of which is most likely a separate fault. Concentration of coarse facies at the junctions of these curved segments is interpreted to represent point sources of sediment input associated with relay ramps between two fault segments.

Second, facies patterns suggest that Harford Basin sediment fill is contemporaneous with fault movement, and is not, as recently postulated, an erosional remnant of horizontal Triassic/Jurassic strata later affected by Cretaceous normal faulting. Basin sediment initially fines-upward, then coarsens-upward, reflecting a gradual increase then decrease in the ratio of fault-generated accommodation to sediment supply. Thicknesses of lava flows and intervening sedimentary units increase toward the basin center, indicating synchronicity between sedimentation and fault displacement.

Finally, distribution of coarse facies suggests the EBF is the depositional edge of the Hartford Basin, and that sedimentary strata and lava flows did not extend further eastward in the past as is currently debated. Cobble conglomerate is confined to within 1.5 km from the EBF where observed today, and was most likely restricted to the basin edge in the past.