27-4 WHERE ARE THE LAVA FLOWS OF THE TALCOTT BASALT IN PORTIONS OF THE SOUTHERN HARTFORD BASIN, CONNECTICUT?

Session: Recent Work in Mesozoic East Coast Rift Basins: Structure, Sedimentology, Paleontology, Mapping, and More!

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Abstract:

The Talcott Basalt, oldest of three basalt lava flows in the Mesozoic Hartford Basin in Connecticut, was erupted onto the southern three-quarters of the basin floor due to antecedent topography and proximity to the dikes from which the lavas effused.  Lava flows of the Talcott Basalt have, however, not been recognized in most of the Wallingford Quadrangle in the southern basin.  Rather, the stratigraphic position of the Talcott Basalt is occupied by valleys with water reservoirs or by poorly exposed pepperite, agglomerate, and arkose with basalt clasts.  These rocks have been variably altered to ferroan carbonate and iron oxides.  Hydrothermal fluids associated with a hot-spring deposit overlying the Talcott have been suggested as the cause of the alteration.  The absence of the Talcott lava flows has been explained due to an unusual array of faults and possibly because of hydrothermal alteration to carbonate that simply is poorly exposed in a deeply weathered topography.

            One locality in the southern part of the quadrangle near the intersection of CT-Route 17 with Village Street in North Haven offers another possible explanation.  Here, the Talcott basalt crops out, in contact with as much as 10 m relief, cut into the underlying New Haven arkose.  The contacts with the basalt and arkose appear erosional rather than faulted which suggests that the basalt was erupted into a channel in the arkose deposit.  In the channel, pillow basalt is overlain massive basalt which in turn is overlain by sandy agglomerate. The outcrop is interpreted as an incised-channel fill.  This implies important topographic relief existed of the floor of the rift basin during the eruption of the Talcott Basalt.  The hummocky topography may have restricted the flowing lava to valleys and swales, leaving the higher topography uncovered by lava where only pyroclastic material could have been deposited.

            An interesting coincidence is that in the same area where the Talcott flows are absent, the Fair Haven dikes, which supplied magma for the Talcott flows, tend to intrude into the sedimentary layers forming local sills.  These sills may have inflated the local topography amplifying relief, leading to channel incision, and diversion of the lava flows.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2, 2026

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