2016 Spring Field Trip - Geologic Evolution of the Lower Connecticut River Valley - June 11, 2016
Trip Leaders: Janet Radway Stone, Ralph S. Lewis, and Phil Resor
Geologic evolution of the Lower Connecticut River Valley: Character and age of the bedrock valley, Glacial and glaciodeltaic deposition, Postglacial erosion, and Relative Sea-level fluctuations
Leaders: Janet Radway Stone, Ralph S. Lewis, and Phil Resor
Description: At Middletown, the long tidal Connecticut River leaves the broad Mesozoic lowland in central Connecticut and flows easterly through The Straits gorge into a much narrower bedrock valley that cuts across Paleozoic and Proterozoic metamorphic rocks in the eastern highlands. During Late-Wisconsinan deglaciation, the upper Connecticut River valley contained the extensive glacial Lake Hitchcock, but the lower valley also contained a series of smaller glacial lakes. As a result of meltwater deposition in these lakes, the narrow valley was clogged with meltwater sediment that filled the valley from side-to- side at altitudes as high as 165 ft (50 m) above today’s sea level. This sediment provided an initial barrier that had to be deeply incised before Lake Hitchcock could come into existence. Sediment eroded from the lower river valley was carried along a channel system that cuts drained lakebeds in Long Island Sound and Block Island Sound, through a notch in the terminal moraine at Block Channel, and continues about 100 km south to Block Delta. Here deltaic sediment prograded into the LGM sea which stood 125 m below today’s level. By 20 cal ka, eustatic sea level had begun to rise and because the land remained glacio-isostatically depressed, sea level progressed up the paleo-channel system and occupied Long Island Sound by 17 cal ka. At 16 cal ka, eustatic sea level stood at -102 m, and associated relative positions rise up the River valley from -20 m altitude at the mouth to about +20 m at Middletown due to the higher amounts of depression to the north. So marine waters occupied the lower Connecticut River estuary through Middletown and up the Mattabesset River valley at relative levels higher than today. When glacio-isostatic rebound began at ~15.7 cal ka, sea level fell again and remained at a -40- m relative level in Long Island Sound for several thousand years as the rate of eustatic sea-level rise was nearly matched to the rate of glacio-isostatic uplift. During this time of lower sea level, the Connecticut River was able to incise deeper into the glacial lake deposits due to the differentially northward increase in the amount of uplift of the land. Sea level rose again during the last 10 ka as the rate of eustatic rise overtook the rate of land uplift. Deeper channels beneath today’s river are filled with estuarine sediment, and salt-marsh and freshwater tidal-marsh deposits bury former floodplains along the lower valley. These marsh deposits have accumulated during the last 4-5 ka. The fieldtrip illustrated the character of the lower Connecticut River bedrock valley, the lithology and structure of bedrock units it cross-cuts, and present evidence that the River valley is at least pre-Late Cretaceous in age.
Description: At Middletown, the long tidal Connecticut River leaves the broad Mesozoic lowland in central Connecticut and flows easterly through The Straits gorge into a much narrower bedrock valley that cuts across Paleozoic and Proterozoic metamorphic rocks in the eastern highlands. During Late-Wisconsinan deglaciation, the upper Connecticut River valley contained the extensive glacial Lake Hitchcock, but the lower valley also contained a series of smaller glacial lakes. As a result of meltwater deposition in these lakes, the narrow valley was clogged with meltwater sediment that filled the valley from side-to- side at altitudes as high as 165 ft (50 m) above today’s sea level. This sediment provided an initial barrier that had to be deeply incised before Lake Hitchcock could come into existence. Sediment eroded from the lower river valley was carried along a channel system that cuts drained lakebeds in Long Island Sound and Block Island Sound, through a notch in the terminal moraine at Block Channel, and continues about 100 km south to Block Delta. Here deltaic sediment prograded into the LGM sea which stood 125 m below today’s level. By 20 cal ka, eustatic sea level had begun to rise and because the land remained glacio-isostatically depressed, sea level progressed up the paleo-channel system and occupied Long Island Sound by 17 cal ka. At 16 cal ka, eustatic sea level stood at -102 m, and associated relative positions rise up the River valley from -20 m altitude at the mouth to about +20 m at Middletown due to the higher amounts of depression to the north. So marine waters occupied the lower Connecticut River estuary through Middletown and up the Mattabesset River valley at relative levels higher than today. When glacio-isostatic rebound began at ~15.7 cal ka, sea level fell again and remained at a -40- m relative level in Long Island Sound for several thousand years as the rate of eustatic sea-level rise was nearly matched to the rate of glacio-isostatic uplift. During this time of lower sea level, the Connecticut River was able to incise deeper into the glacial lake deposits due to the differentially northward increase in the amount of uplift of the land. Sea level rose again during the last 10 ka as the rate of eustatic rise overtook the rate of land uplift. Deeper channels beneath today’s river are filled with estuarine sediment, and salt-marsh and freshwater tidal-marsh deposits bury former floodplains along the lower valley. These marsh deposits have accumulated during the last 4-5 ka. The fieldtrip illustrated the character of the lower Connecticut River bedrock valley, the lithology and structure of bedrock units it cross-cuts, and present evidence that the River valley is at least pre-Late Cretaceous in age.