M. J. RICKARD Deft. Geology, Australian National University, Box 4 P
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M. J. RICKARD Deft. Geology, Australian National University, Box 4 P. O., Canberra City, A. C. T., Australia Taconic Orogeny in the Western Appalachians: Experimental Application of Microtextural Studies to Isotopic Dating Abstract: Study of the texture of mica flakes in of the method, however, was not adequately tested microflexures associated with crenulation cleavage because the micas from early and late cleavage suggested criteria for estimating the extent of mica planes gave the same K-Ar age, but the results per- recrystallization. These criteria were applied to a mitted the interesting conclusion that the Taconic sample case, the dating of cleavage-plane micas orogeny probably occurred in three distinct phases from folds developed during two deformation epi- of markedly contrasting structural styles. sodes in the Canadian Appalachians. The validity CONTENTS Introduction 523 Figure Acknowledgments 523 1. Generalized structural map and section across Western Appalachian section in southern Quebec 524 the western edge of the Appalachians in Problem of the age of folding 524 southern Quebec 525 Microtextural evidence for state of mica recrystal- lization 526 2. Relationship of minor folds and cleavages Samples selected for dating 527 across the Sutton anticline, western Ap- A, Pinnacle graywacke 527 palachians, southern Quebec 526 B, Sutton schist 529 3. Early slaty cleavage in Pinnacle graywacke Discussion of results 530 specimen A 527 Geological factors governing possible errors ... 531 4. Textural details of Sutton schist 528 Three phases of the Taconic orogeny 531 5. Bending and recrystallization of mica during Local considerations of the Acadian orogeny . 533 development of crenulation cleavage . 529 Conclusions 533 6. Three phases of the Taconic orogeny .... 532 References cited 534 ing that they belong to different orogenies— INTRODUCTION either Taconic and Acadian, or Acadian and A study of the development of microflexures Appalachian. associated with crenulation cleavage (Rickard, The two fold systems were dated by de- 1961) suggested a means of determining the termining the K-Ar ages of muscovites recrys- ages of different deformation episodes. The tallized during the development of axial-plane method was tested in the Appalachian Moun- cleavages. Surprisingly, the results indicated a tains of southern Quebec where detailed map- Taconic age for both (Lowdon and others, ping by the writer between 1957 and 1960 in- 1963, p. 104). The writer, herein, carefully con- dicated two periods of deformation. The writer siders the method, the validity of the results, chose this test section to help resolve the dis- and their regional significance in this test case agreement over the age of the folding at the because he feels that this approach could have western margin of the Appalachians; workers in wide application to the dating of multiple-fold Quebec believed this folding was Taconic sequences in orogenic belts. (Clark, 1921; 1934; Neale and others, 1961), and workers in Vermont believed it was ACKNOWLEDGMENTS Acadian (Cady, 1945; 1960). Additionally, the I extend my grateful thanks to the many section seemed ideal because the early and late people who assisted with this project; to the structures differ so markedly in style, suggest- various members of the Geological Survey of Geological Society of America Bulletin, v. 76, p. 523-536, 6 figs., May 1965 523 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/5/523/3442381/i0016-7606-76-5-523.pdf by guest on 28 September 2021 524 M. J. RICKARD—TACONIC OROGENY IN THE WESTERN APPALACHIANS Canada who carried out the mineral separations tern of the Pinnacle graywacke (Fig. 1), but a and age determinations and kindly gave advice; few minor folds in the incompetent slate beds to Mr. H. Steinbach who helped collect sam- are reclined (Fig. 2A). ples in mid-winter; and to Professor D. A. There is a pronounced bedding schistosity Brown, Drs. W. H. Poole, H. A. K. Charles- (SI) in the Sutton schist, and this is folded into worth, W. M. Cady, and A. J. R. White who minor reclined folds in the hinge zones of which critically read the manuscript. I especially a crenulation cleavage (S2) is sometimes de- thank W. M. Cady for lengthy discussions and veloped (Fig. 2B). This cleavage is, for the for permission to use unpublished age deter- most part, parallel to the bedding schistosity minations from Vermont. Acknowledgment is and is folded with it across the Sutton anticline not intended to imply their acceptance of the (Figs. 1; 2B, 2E). The slates and graywackes ideas set forth by the writer. The work was overlying the Sutton schists to the east of the carried out at McGill University under a Na- anticline have a strong slaty cleavage developed tional Research Council Post-Doctoral Fellow- parallel to the axial-planes of folds with a steep ship. plunge (Fig. 2C, 2F). The folds are mostly small, but the outcrop of competent beds like WESTERN APPALACHIAN SECTION the Bolton volcanics shows that larger ones also IN SOUTHERN QUEBEC exist (Fig. 1). Figure 1 shows a simplified stratigraphic and In each of these three districts, the folds just structural map and section across the western described are the earliest observable fold struc- margin of the Appalachians. To the west of Sut- tures, and although their plunge direction ton anticline, which consists of a zone of albitic, varies from N.-S. to E.-W. (Fig. 2D, 2E, 2F), pelitic and graphitic schists, is the Lower to they are thought to be equivalents'; the only Middle Cambrian Oak Hill Group. This com- earlier structure is the bedding schistosity (SI) prises distinct formations of chlorite schist in the Sutton anticline. (Tibbit Hill Formation), slate, graywacke A later system of minor folds with an associ- (Pinnacle Formation), phyllite, and dolomite ated axial-plane cleavage is clearly superimposed (Clark, 1934; Eakins, 1964). The upper part of on these structures. From the Oak Hill district the Sutton schists are equivalent to Oak Hill to the Sutton anticline, the early cleavage (S2) formations (Clark, 1934; Cady, 1960). is cut by a crenulation cleavage (S3); in the Tib- On the eastern flank of Sutton anticline, a bit Hill anticline, this crenulation cleavage series of Upper Cambrian to Lower Ordovician crosses both limbs of the minor folds outlined (?) graywackes and slates (Ottauquechee and by the Pinnacle graywacke with a consistent Stowe Formations of Cady and others, 1963; 5°-10° difference of strike (Figs. 1; 2A, 2D). Mansonville formation of Clark, 1934) has been In the Sutton anticline, these structures are correlated across the anticline with the upper better developed; the crenulation cleavage (S3) formations of the Oak Hill Group (Osberg, is accompanied by gently plunging chevron- 1956). The Middle Ordovician Beauceville type folds with a vergence sense consistent with slates and Bolton volcanics then follow, ap- their being "drag folds" on the limbs of the parently conformably. A thin conglomerate anticline (Fig. 2B, 2E). Metamorphism, which (the Peasley Pond conglomerate) separates accompanies "drag folding," increases in in- these rocks from the Silurian-Devonian cal- tensity into the core of the fold where albite careous slates of the Glenbrooke formation that schists are developed. Thus presumably, the occupy the tight Sargent Bay syncline (Fig. 1). Sutton anticline and its associated cleavage de- The earliest observable fold structures in the veloped into its present form later than the Oak Hill formations have a strong axial-plane folds to the west of it (Fig. 1, cross section). slaty cleavage (S2) that cuts bedding (Fig. 2A). The superimposed late cleavages (S3) die out The axial-plane slaty cleavage is vertical in the rapidly to the east and are represented in the Tibbit Hill anticline, but progressively, it dips Beauceville slates only by a weak crenulation. less steeply eastward toward the west where the Paleozoic sediments have been thrust west- PROBLEM OF THE AGE OF FOLDING ward in a series of slices over the foreland (Fig. The Sutton anticline is a continuation of the 1). The most westerly thrust (Logan's Line) Green Mountain anticline of Vermont; Cady marks the edge of the Appalachian mountain belt. The folds generally have a variable gentle 1 The structural details on which this relationship is plunge that markedly affects the outcrop pat- established are to be published elsewhere. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/5/523/3442381/i0016-7606-76-5-523.pdf by guest on 28 September 2021 I I I OAK HlLL TlBBlT HlLL SUTTON ANTICLINE SARGENT BAY ANTICLINE SYNCLINE Figure 1. Generalized structural rnap and section across western edge of Appalachians in southern Quebec, showing superpositio~lof late clcnvages (Si) rclated to formation of Suttorl anticline. Scale is 1 inch: approximately 8 milcs. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/5/523/3442381/i0016-7606-76-5-523.pdf by guest on 28 September 2021 526 M. J. R1CKARD—TACONIC OROGENY IN THE WESTERN APPALACHIANS (1960, p. 564) believed that the Sargent Bay folds was found to be similar to that of minor syncline and other folds involving the Silurian- folds in the Tibbit Hill anticline (Figs. 1; 2A, Devonian were an integral part of the Green 2C) supported an Acadian age for the folding. Mountain anticlinorium, and on this basis, he The late structures do not affect the Glen- postulated an Acadian (mid-Devonian) age for brooke slates, but because of the crosscutting the folding in Vermont. In Quebec, Ambrose relationships and marked difference of style, the Glenbrooke 51 Syncline 51- Figure 2. Relationship of minor folds and cleavages across Sutton anticline, western Appalachians, southern Quebec: (A) late cleavage (S3) crosscutting early axial-plane slaty cleavage (S2) in the Tibbit Hill anticline. The Pinnacle graywacke forms variable but gently plunging folds (LI) on all scales.