Stratigraphy and Primary Sedimentary Structures of Fine-grained, Well-bedded Strata, Inferred Lake Deposits, Upper Triassic, Central and Southern Connecticut
Published:January 01, 1968
John E. Sanders, 1968. "Stratigraphy and Primary Sedimentary Structures of Fine-grained, Well-bedded Strata, Inferred Lake Deposits, Upper Triassic, Central and Southern Connecticut", Late Paleozoic and Mesozoic Continental Sedimentation, Northeastern North America, George deVries Klein
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The Upper Triassic rocks of Connecticut occupy part of a fault trough that was kept filled as it subsided by deposition of strata consisting of sediment derived from the uplifted block at the eastern margin of the basin, and by extrusion of lava that solidified on the basin floor to make sheets of basalt of uniform thickness and widespread distribution. The Triassic strata have been tilted toward the east and folded and faulted during complex structural activities that occurred after the Triassic and prior to Late Cretaceous time.
Upper Triassic sedimentary strata of central and southern Connecticut consist of two suites of materials: (1) coarse-grained, irregularly bedded deposits, and (2) fine-grained, well-bedded deposits.
For the most part, coarse-grained materials are concentrated along the eastern margin of the basin, which is formed by a steep fault. Tongues of coarse material project variable distances toward the center of the basin. In the New Haven area, some reach as far as the western edge of the present outcrop belt, 11.5 km northwest of the fault. The coarse-grained, irregularly bedded strata are believed to be deposits made by streams and by mudflows on alluvial fans and adjacent areas.
Typically, the fine-grained, well-bedded strata occur at localities well away from the fault. In southern Connecticut, exposed fine-grained strata compose 10 per cent of the aggregate thickness of sedimentary strata younger than the most ancient extrusive sheet in localities ranging from 0.1 to 9.6 km distant from the fault. In central Connecticut, exposed fine-grained strata compose 32 per cent of the aggregate thickness of strata interbedded with the extrusive sheets at localities ranging from 11 to 19 km distant from the fault.
Most fine-grained, well-bedded strata are red beds and have been interpreted as deposits of flood plains. Some of the gray calcareous interbeds have been considered to be lake deposits, and most of the black strata, swamp deposits. Evidence presented here suggests that most fine-grained, well-bedded strata were deposited in lakes of varying size. Two critical localities found during detailed mapping in the Branford area show finegrained, well-bedded strata in two formations at localities only 0.3 to 0.4 km from the border of the basin, where younger and older strata all consist of coarse conglomerates.
Fine-grained, well-bedded strata at one locality in Branford consist of interbedded maroon and gray claystones, shales, siltstones, and sandstones ranging from very fine grained to coarse grained, and dark gray siltstones and interbedded gray sandstones showing grading of the kind generally considered to have been made by turbidity currents, features made by subaqueous slumping, and syndepositional faults. Fine-grained, well-bedded strata at the other Branford locality consist of maroon siltstones that are overlain by altered volcanic rock; detached spheroids of altered basalt occur in the siltstones as much as 0.5 m below the base of the overlying volcanic rock. These fine-grained, well-bedded strata are inferred to have originated in two separate large, deep lakes whose waters submerged former alluvial fans and whose shore lines stood high up on the steep marginal escarpment of the basin. At other times, lakes did not reach the margin of the basin and deposition on alluvial fans continued without interruption.
Maroon strata in the inferred lake deposits at these two localities near the margin of the basin closely resemble maroon strata interpreted previously as fluvial deposits that are exposed at localities well away from the margin of the basin. This similarity and absence of interbedded stream channel deposits raises the alternative, preferred here, that many of these fine-grained strata are lake deposits rather than fluvial deposits.
Some inferred lakes are considered to have been only a few meters deep; in them only maroon siltstones, claystones, and fine-grained sandstones were deposited. Mud cracks are common. Other lakes are inferred to have been a few tens of meters deep; in them gray carbonate rocks containing algal bioherms were deposited. Still other lakes are inferred to have been large and many tens or some hundreds of meters deep. Bottom waters of these deep lakes are thought to have been oxidizing at some times and reducing at other times. Sediments of inferred large lakes include interbedded maroon and gray strata of various grain sizes and black laminated, slumped, and graded sediments.
The source of Triassic sediment probably lay to the east in Connecticut, but paleocurrent indicators measured in the fine-grained, well-bedded strata show many directions of last transport, including toward the east and southeast. This diversity is thought to have resulted from the influence of morphology on turbidity currents flowing across the bottoms of lakes.
Pillows and various volcanic breccias are interpreted to have resulted from interaction of hot lava and cold lake water.
Lake deposits having characteristics similar to the fine-grained, well-bedded strata of the Upper Triassic formations in Connecticut, as well as numerous other characteristics not seen in Connecticut, occur in the Triassic strata of the Newark and Nova Scotia basins; Pleistocene and Recent strata of the Dead Sea graben, Israel; Cenozoic strata of the Rocky Mountain region; and Carboniferous strata of Nova Scotia, to name only a few examples.
The physical characteristics of fillings of nonmarine fault-trough basins are held to be due to the environments that prevailed within the basin, not simply because the tectonic setting was a fault trough. Environmentally controlled depositional features have been preserved clearly, even though rates of subsidence were extremely rapid.