CHARACTERIZATION OF A VOLCANIC BRECCIA (JURASSIC), HARTFORD BASIN, CT

Field mapping in the North Guilford area identified an isolated previously undescribed volcanic breccia outcrop in close proximity to the Eastern Border Fault. The breccia is a matrix-supported, poorly sorted polymict rubble breccia (Norton, 1932) and may be volcaniclastic or epiclastic according to Fisher’s 1960 classification. The outcrop is approximately 90 feet long and 15 to 20 feet high, and incorporates clasts from 1/2 inch to 1.5 feet long, including altered diabase clasts, angular and rounded pebble arkose clasts, vesicular basalt, massive basalt, and detached or brecciated pillow fragments, and fist sized volcanic bombs. The matrix is, in part, a tuff to lapilli size ash of possible hyaloclastite origin.

The area is structurally complex, highly faulted and of uncertain stratigraphic affinity. William Morris Davis mapped the area as belonging to the Talcott Basalt in the 1880s, an interpretation repeated in Rodgers’ 1985 Bedrock Geological Map of Connecticut. However, there are also adjacent small blocks mapped as Hampden Basalt and a potential eruptive complex identified by Foye in 1930, which has been suggested to be Hampden in age and may be related to this outcrop. This would suggest a similar origin and stratigraphic equivalency to the Granby Tuff, a volcaniclastic unit of the Hampden Basalt in the Deerfield Basin of Massachusetts. Alternatively, the volcanic breccia may be a resedimented flank bed of the vent structure, spatially separated or fault-separated from the center of the complex.

Detailed petrography and chemical analysis of the included basalt clasts will be used to constrain stratigraphic age and position.