A HYPOTHESIS FOR THE DISAPPEARENCE OF THE TALCOTT BASALT IN THE VICINITY OF WALLINGFORD, CT

Talcott Basalt crops out continuously from northern Connecticut to Long Island Sound, except for a local area north and west of Totoket Mountain, and west of both Pistagpaug Mountain and Fowler Mountain in the towns of North Branford, Durham, and Wallingford. Previous workers mapped a set of faults to explain the disappearance of outcropping Talcott Basalt, an interpretation accepted during compilation of the State map.

Researchers at the University of Connecticut in the early 1980’s sampled an orange dolostone north of Totoket Mountain and about 2 km east of the western Durham quadrangle boundary. The dolostone contained olivine psuedomorphs, leading to the interpretation that the rock was hydrothermally altered Talcott Basalt. We hypothesize that the alteration was related to the nearby hot-spring deposit at Coe’s Quarry, located 0.5 km north of Totoket Mountain and about 1.5 km east of the Durham quadrangle boundary.

Recently, as part of a Statemap-National Cooperative Geologic Mapping Project, we mapped several outcrops of a volcaniclastic sheet interpreted to be hydrothermally altered Talcott Basalt. The degree of hydrothermal alteration and abundance of carbonate is proportional to the proximity of the hot spring. Samples collected closer to the hot-spring deposit (1.4 km east) are more altered, mainly to hydrated iron oxides and ferroan-calcite, than samples collected at a greater distance (~2 km) from the hot-spring deposit. Approximately 4 km west of the hot-spring deposit the volcaniclastic sheet is cemented by and partially altered to calcite.

Talcott outcrops end about 7 km north of the hot-spring deposits. Instead deep valleys have been eroded at the stratigraphic position that should be occupied by the Talcott Basalt and overlying Shuttle Meadow Formation. We hypothesize that the disappearance of the Talcott is not because of faulting but rather because of hydrothermal alteration associated with the hot spring that began soon after the volcanic activity. The altered basalt, easily eroded by a combination of dissolution and glacial abrasion, now forms low areas as karst features which are occupied today by reservoirs.