folded, east-verging, late acadian woodstock nappe ... · verging minor folds, from the overturned...

FOLDED, EAST-VERGING, LATE ACADIAN WOODSTOCK NAPPE, DEFORMED BY DOMES,

EAST-CENTRAL VERMONT

ByPeter J. Thompson

Earth Sciences Department, University of New Hampshire

Research supported by Vermont Geological Survey, Department of Environmental Conservation and the U.S. Geological Survey, National Cooperative Mapping Program,

Award No. EC27-05Special thanks to Marjorie Gale, Vermont Geological Survey,

for digitizing Figures 5 and 6.

Woodstock is situated between the north-plunging Chester dome and south-plunging Pomfret dome.

The saddle between the domes was referred to by Chang and others (1965) as the South Woodstock syncline.

Domain 1n=12

Domains 2, 3 & 6n=59Domains 4 & 5

n=61

ALL LINEATIONS

The plunge of lineations changes from north to south across the syncline.

The Waits River Formation consists of garnet schist and punky brown-weathering, sandy meta-limestone beds. An upper facies, which contains fewer punky brown beds, was mapped out to help define the map pattern.

Ob

SnSw

Ssp

Dg

Graded beds in the schist-rich “Mount Tom member”(blue) confirm that the sequence tops to the east across the Standing Pond Volcanics (green).

A zone of steep dips in bedding (green) separates the relatively flat-lying upright limb to the west, with east-verging minor folds, from the overturned limb, with west-verging minor folds, in a large east-verging nappe-like anticlinal fold.

The overturned limb is in turn folded by a Z-shaped fold in the northeast corner of Woodstock.

B

B’

folded inverted

limb

The cross-section in the final slide portrays the refolded nappe, draped over the domes. It follows that the stratigraphy north of the Ascutney pluton is inverted, and the Gile Mountain (yellow) in the narrow antiform south of the pluton is in a stratigraphic syncline.

On the basis of his work in Woodstock, the author recommends that the recent reassignment of many rocks in Hartland and elsewhere in southeastern Vermont to members of the Waits River Formation (e.g. Walsh, 1996 and Armstrong, 1997) should be reconsidered.

Cross-section from SW to NE across Woodstock into Hartland, Vermont.

B B’

The change in thickness of the Waits River Formation between theupright limb on top of the domes to the overturned limb suggests that the contact of Waits River and Northfield with the pre-Silurian rocks

might represent a décollement.