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Landing , Ed,
The Great American Carbonate Bank in Eastern Laurentia: Its Births, Deaths, and Linkage to Paleooceanic Oxygenation (Early Cambrian–Late Ordovician)
1New York State Museum, Albany, New York, U.S.A.
Early research on the lower Paleozoic of eastern New York and Vermont was completed by the author under National Science Foundation grant EAR76-10601. More recent study was supported by the New York State Museum. R. B. Koepnick and A. R. Palmer (reviewers) and J. Derby (volume editor) provided useful suggestions on the manuscript.
The Cambrian–Ordovician carbonate platform units on the New York promontory of eastern Laurentia reflect the south tropical location of the area. The slow subsidence of the region through much of the Cambrian–Ordovician meant that strong eustatic rises and falls defined unconformity-bound carbonate formations. These depositional sequences aid in paleooceanographic reconstruction as they correlate with organic-rich dysoxic–anoxic mudstones on the Laurentian continental slope. Eustatic rise increased insolation as epeiric seas covered the platform and produced climate maximums with reduced deep-water circulation. The oldest carbonate platform unit (Forestdale Marble and equivalents, upper Lower Cambrian) overlies rift facies deposited with the Rodinia breakup and origin of the Iapetus Ocean and marks the transition to a passive margin. Drowning of the Forestdale platform occurred, and the overlying anoxic black mudstone (Moosalamoo Phyllite) abruptly shoals up into tidalite sandstone (Cheshire Formation). This depositional history records a decreased rate of sea level rise as the Cheshire Formation continued to onlap middle Proterozoic basement. Super-Cheshire Cambrian carbonate platform units in the northern Appalachian are mostly hydrothermally dolomitized, record eustatic highs (Dunham, Winooski, and Little Falls Formations), and correlate with black mudstone macroscale units on the slope (Browns Pond and Hatch Hill dysoxic–anoxic intervals). The latest Early Cambrian Hawke Bay regression ended carbonate platform deposition of the Dunham Formation, led to quartz arenite or red shaly dolostone offlap or shoaling deposits on the platform, and was coeval with oxic green mudstone on the continental slope (Hawke Bay oxic interval in Taconian allochthons). Subsequent Middle Cambrian eustatic rise is recorded by dolostone (Winooski and upper Stissing), but carbonate deposition was again suppressed as quartz sand swept toward the shelf margin (Danby Formation) coincident with cratonic transgression by the upper Potsdam Formation (uppermost Middle Cambrian–lower Upper Cambrian). Post-Potsdam deposition was carbonate dominated through the middle Late Ordovician and included the Beekmantown, Chazy, Black River, and Trenton Groups. The Cambrian-Ordovician boundary is an unconformity between platform carbonates (Little Falls and Tribes Hill Formations of the Beekmantown Group). The Lower Ordovician–lower Upper Ordovician is a series of unconformity-bound platform depositional sequences (Tribes Hill, Rochdale, Fort Cassin, and Providence Island Formations of the upper Sauk megasequence and Chazy Group of the lower Tippecanoe megasequence). The Ordovician depositional sequences coincide with eustatic highs and show a repeated depositional motif (lower transgressive sandstone, upper highstand carbonate). The Ordovician eustatic highs also correlate with thin (as much as 10 m [33 ft] thick) macroscale dysoxic–anoxic black mudstones on the slope. The black mudstones alternate with oxic greenish mudstones, locally with debris flows with giant carbonate blocks on the upper slope (Levis conglomerates), which indicate platform-margin caving during eustatic falls. Ordovician green mudstones are composed of mesoscale redox-carbonate mudstone cycles (Logan cycles) on the upper slope. A major development was the abrupt formation of the latest Early Cambrian–Early Ordovician Franklin Basin in northwestern Vermont. The dysoxic–anoxic Franklin Basin resulted from fault-driven foundering of part of the carbonate platform that overlay the failed arm of the Ediacaran triple junction. This faulting is coeval with the oldest (late Early Cambrian) onlap in the Ottawa-Bonnechere aulocogen. Late Ordovician collision with the Ammonusuc arc ended carbonate platform deposition in the New York promontory region, as sands and muds eroded from the Taconic orogen filled a fore-arc basin and extinguished carbonate deposition across eastern Laurentia.
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