STRATIGRAPHIC OBSERVATIONS ON CORED BOREHOLES IN THE MESOZOIC HARTFORD BASIN, HARTFORD, CONNECTICUT

Stratigraphic observations were made on cored boreholes collected in south Hartford. Boreholes penetrated the upper 50m of the early Jurassic Holyoke Basalt, all of the East Berlin Formation and Hampden Basalt, and the lower 700m of the Portland Formation. The Holyoke Basalt consists of two flows, herein designated the upper and lower Holyoke Basalt, separated by a 10m thick sedimentary section, herein designated the South Hartford Formation. The base of the lower Holyoke Basalt was not penetrated. The upper 30m consists of vesicular/amygdaloidal basalt with much of the upper 10m brecciated with a volcaniclastic scoriaceous fabric. The 10 m (26-32’) thick South Hartford Formation consists of a lower section of alluvial and playa facies, overlain by 1-2m of lacustrine facies. The upper Holyoke Basalt has 30m (84’-95.5’) of non-vesicular basalt with a sharp erosional upper contact. Coarsely crystalline basalt occurs within 2m of the contact, suggesting that erosion occurred prior to deposition of the overlying East Berlin. The East Berlin Formation is almost 186m (610’) thick with seven saline lacustrine cycles. Biomarker analysis is ongoing. Profundal facies of many cycles contain calcite-replaced evaporite(?) crystals, perhaps magnesite, as has been reported in the type section to the southeast. The Hampden Basalt is 35m (115’) thick; the upper 10+ m is vesicular and brecciated and the lower 20+ m is massive and more coarsely crystalline. Boreholes penetrated more than 650m (~2200’) of the Portland Formation, including eleven lake cycles that correlate well with the section recovered from the Park River Auxiliary Tunnel a few kilometers to the north. Interpreted Portland paleosols contain desiccation cracks, slickensides, and nodules, similar to those in the East Berlin.

Numerous faults cut the section including a major fault, herein called the Cedar Mountain Fault, which is mapped from north of Trinity College to New Britain. This fault has approximately 1000m of displacement, some of which occurred during the Holyoke events (interpreted from these cores) and some of which occurred during the Hampden event (documented in the Park River cores). Movement of this fault may have continued into the Cretaceous.