3 The New Bedrock Geologic Map of Vermont: New Answers, New Problems, and New Uses of Bedrock Geologic Data

Sunday, 14 March 2010
7:50 AM - 12:05 PM
Liberty A

ALONG-STRIKE CHANGES IN DEPOSITIONAL FACIES AND STRUCTURAL STYLE AS PORTRAYED ON THE NEW BEDROCK MAP OF VERMONT

Paper number: 3-5
Program: Symposia Sessions
Time: 9:05 AM – 9:25 AM

Authors

Abstract

The position of Vermont between the New York promontory and Quebec re-entrant has important consequences both for depositional facies changes and for variations in the intensity and style of structures, which developed as Laurentia collided with a series of tectonic elements in the Taconian and Acadian Orogenies. The new Vermont bedrock map illustrates key along-strike facies changes. For example, in the north, finer-grained sedimentary rocks and more volcanics are found in the Late Proterozoic rift basin than in the south. Deeper water facies occur to the north within the allochthonous Lower Paleozoic carbonate bank, whose edge lies oblique to the orogen (first noted by Rodgers, 1968), and whose basal Cheshire Quartzite becomes more pelitic to the north. The former Camels Hump Group has been subdivided in different ways on the new map, in part reflecting W to E and S to N facies changes. Conglomerate and metawacke horizons extend away from Precambrian massifs at several levels in the stratigraphy. Problems remain interpreting the Pinney Hollow Fm, with implications for identifying the root zone of Taconic allochthons.

The Champlain thrust (CT) displaced the bank edge by as much as 80 km westward from its autochthonous counterpart, which in northern Vermont lies beneath the Green Mountain anticlinorial axis. We interpret the GMA as a fault-bend fold above a basement slice that rode up over the bank edge. En echelon anticlines along the GMA axis suggest sinistral transpression, as expected north of a promontory. To the south, east of the Lincoln massif, depth to autochthonous basement decreases by more than 1000 m and the fault-bend fold gives way to imbricated E-dipping faults. Throw on the CT decreases at about the same latitude and displacement is taken up by subsidiary faults, which propagate in Ordovician carbonates and shales beneath the main CT. The great breadth of fault-bounded lithotectonic packages in the north diminishes southward as faults converge and strain increases toward the promontory, where some packages are cut out entirely. A sinistral rhombic pattern appears within several packages; horizons shift toward the east northward between faults. Ultramafics, sheared out along faults in Vermont, are locally preserved as complete ophiolite sequences in the Quebec re-entrant (Doolan et al, 1982).