Abstract The Taconic sequence in the northern Shenandoah Valley is located on the Strasburg and Toms Brook, 7½-minute Quadrangles, or the Strasburg, 15-minute Quadrangle. In addition, geologic maps are available for both 7½-minute quadrangles (Rader and Biggs, 1976). All stops are readily accessible on paved heavy duty and medium duty roads and are numbered on the index map (Fig. 1) in stratigraphic order. All stops are on public land or right-of-way and require no special permission for access.

An intensive search for fossils in Taconic sequence strata in the region of Columbia County, New York, produced three Lower Cambrian, two Middle Cambrian, and two Upper Cambrian trilobite faunas, of which only the oldest Early Cambrian fauna was previously known from the area. All these faunas contain diagnostic genera and species, allowing precise correlation with Cambrian strata of other regions of North America and Europe. The trilobites occur in limestone units which are generally lenticular and interstratified with a much greater thickness of grey, green, or black unfossiliferous shales. These findings indicate that there is no unconformity between the Lower Cambrian and the Lower Ordovician strata of the Taconic sequence in the southern portion of the Taconic allochthon. It is likely, instead, that an essentially complete sedimentary sequence extends from the Cambrian system upward into the Lower Ordovician. Several sections were measured, and are described in detail, with lists of the fossils observed at various stratigraphic positions. For the first time in the study of the Cambrian of the Taconic sequence, several diagnostic faunas in unquestionable stratigraphic order have been collected from a single section. Aspects of the stratigraphy of the Columbia County region are compared with those determined by recent investigations of the northern portion of the Taconic allochthon, and sedimentary aspects are briefly described.

The Taconic rock sequence extends from near Sudbury, Vermont, to near Poughkeepsie, New York, a length of about 150 miles; and from just west of the Green Mountain Range and Berkshire Highlands to the valleys of the Hudson River and Lake Champlain, a width of about 20 miles. The Taconic rocks are now in the axial region of the Middlebury synclinorium and its southward extension. The Taconic sequence consists of about 2000 feet of slate, with subsidiary gray-wacke, quartzite, and limestone; many of the rock units are turbidites. Fossils from the rocks include forms of Early, Middle, and Late Cambrian and Early and Middle Ordovician age. Evidence of stratigraphic tops derived from the fossils and from primary sedimentary features agrees with structural data and demonstrates that the Taconic rocks are geometrically the highest strata within the Middlebury synclinorium. The underlying rocks of the synclinorium (the synclinorium sequence) are right side up; they also range in age from Early Cambrian to Middle Ordovician but belong to a different sedimentary lithofacies (dominantly carbonate and orthoquartzite). The relation between the Taconic sequence and the synclinorium sequence, therefore, is a baffling problem. Lithostratigraphically, the Taconic sequence falls into three groups: (1) the pre-Normanskill “low Taconic” sequence, occurring in the area between the main Taconic Range and the Hudson River, as far south as Rhinebeck, New York; (2) the Normanskill Shale in the same area, as well as in the area south of Rhinebeck at least as far as Poughkeepsie, New York, and also west of the Hudson River; and (3) the “high Taconic” sequence, occupying the main Taconic Range from Dorset Mountain, Vermont, south to Indian Mountain in Sharon, Connecticut, as well as Mount Greylock in Massachusetts. Rocks of (1) and (2) are fossiliferous, but to date no fossil has been found in (3). Rocks of (1) and that part of (2) areally coextensive with (1) thus are of known age but uncertain three-dimensional geometric configuration, whereas rocks of (3) are of known configuration (in the centers of open synclinoria) but unknown age. Only that part of (2) beyond the areal confines of (1) is both of known age and known configuration; these rocks are in sedimentary contact above the older rocks of the synclinorium sequence and are autochthonous. At the north end of the Taconic sequence in western Vermont, rocks of group (1) are beyond reasonable doubt allochthonous. Because of the southward geometric continuity of the structural elements, all the Taconic rocks of group (1), and that part of group (2) areally coextensive with it, are interpreted as allochthonous. The structure of group (3), the high Taconic sequence, is inferred by topography and by detailed lithostratigraphic matching with rocks of the east Vermont sequence; on this basis, as well as on the basis of the broad lithic similarity with rocks of group (1), rocks of group (3) are concluded to be also allochthonous. A discontinuous polymict conglomerate underlies and surrounds the allochthon on all sides and is interpreted here as a record that dates the imminent arrival of the allochthon at each locality. The conglomerate contains unsorted blocks of rocks of both the Taconic sequence and the synclinorium sequence; the matrix is the autochthonous upper Normanskill Shale or its equivalent. Fossils from the matrix shale date the event as Trenton, probably Sherman Fall in age. The geologic history of the area is reconstructed as follows: The pre-Normanskill Taconic rocks were deposited in the area of the present Precambrian massifs of the Green Mountains–Berkshire Highlands belt between the clastic, eugeosynclinal east Vermont sequence to the east and the miogeosynclinal synclinorium sequence to the west; they constitute the transitional facies between these two belts. Conditions were relatively stable until early Middle Ordovician time, when the Green Mountain–Berkshire Highlands area began to rise and the area of the present Middlebury synclinorium began to subside. Subsidence took place largely by a series of high-angle longitudinal faults that, as a whole, step down to the west. Argillaceous sediments (the Normanskill Shale) began to inundate the former miogeosynclinal area; because the conditions of sedimentation had become similar, the sediments resembled, in facies, the synchronous Taconic rocks that were being deposited to the east. Continued rise of the Green Mountains–Berkshire Highlands area led in middle Trenton time to the décollement of the Cambrian and Ordovician sediments into the area of the present Middlebury synclinorium in a series of giant submarine slides. Sedimentation continued at the receiving site throughout the event; sedimentation may also have persisted on the moving slides. The record is found today in the turbidite-laden shale and graywacke in the upper part of the Normanskill Shale of both the allochthon and the autochthon. Restoration of the allochthonous rocks to the original site of deposition leads to correlations between rocks of the Taconic sequence and of the largely autochthonous east Vermont sequence. The lithic correlation can be carried to the level of individual formations and is confirmed by a few known ages in the east Vermont sequence. Several lines of reasoning lead to a plausible correlation of part of the Cavendish Formation of southeastern Vermont with the oldest part of the Taconic sequence. This correlation leads further to the conclusion that in this area the contacts between the Green Mountain massif and the Cavendish Formation and between the Cavendish and the overlying east Vermont sequence must both be thrust faults of large displacements. This conclusion is in fact inevitable because one of the Taconic thrust slices that extends without interruption between the latitudes corresponding to the gap in the Precambrian massifs has been shown by local structural evidence to be allochthonous; an outside original depositional site must be found for it. The present Taconic allochthon is coextensive with an area of marked negative Bouguer gravity anomaly; the Green Mountains–Berkshire massifs constitute a belt of positive anomaly. It is here proposed that these anomalies resulted from a deep-seated transfer of material; subcrustal addition of material caused the rise of the Green Mountains–Berkshire Highlands area, and the concurrent subtraction of material caused subsidence in the Middlebury synclinorium area through a series of faults which were the near-surface expression of an episode of crustal collapse. If this interpretation is correct, then the regional gravity anomaly represents an uncompensated feature that has persisted since Middle Ordovician time.

Abstrack The Taconic sequence, predominantly argillaceous-arenaceous strata, is both autochthonous and allochthonous. The allochthonous rocks constitute the Taconic klippe, which extends from southeastern New York to west-central Vermont. They are of Cambrian(?), Cambrian, and early and middle Ordovician ages, as are the rocks of the autochthonous carbonate belt that surrounds the Taconic region. Autochthonous rocks of the Taconic sequence are middle Ordovician shale and graywacke that extensively overlie the carbonate rocks. Complex geometric and spatial relations within the Taconic allochthon, and between it and the autochthonous rocks, involve regionally extensive, successive thrust slices, Wildflysch -type conglomerate, and cryptic sedimentary boundaries between the allochthon and autochthon. In 1961, Zen proposed that the allochthon was emplaced by westward submarine gravity sliding of Taconic- sequence rocks into mud (now Normanskill shale) contemporaneously accumulating in a basin that developed within the region of the previous carbonate accumulation (now the autochthonous carbonate rocks). Fossiliferous, allochthonous, Taconic-sequence rocks, originally deposited on the east, represent deeper water, lateral facies correlatives of the carbonate rocks. Stratigraphic- structural relations between the allochthon and autochthon, and extensive Wildflysch -type conglomerate and associated flysch within Normanskill shale of Trentonian age, are direct evidence for the proposed Taconic submarine gravity sliding.