JOHN RODGERS Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520 The Taconic Orogeny Address as Retiring President of The Geological Society of America, Milwaukee, Wiscon- sin, 11 November 1970 ABSTRACT difficulty with this definition, for like a once The Taconic orogeny of eastern North popular definition of graywacke—the sedimen- America was not, as traditionally defined, a sin- tary analogue of gabbro as arkose is of granite gle erogenic event that occurred at the end of —it practically defines the thing out of exist- the Ordovician period, but rather a complex ence. Just as resedimented gabbro is rare and series of erogenic episodes or climaxes spread the original Saxon graywacke is nothing of the over the larger part of that period. In most sort, so the orogenic episode in the Appalachi- sectors of the northern Appalachians it in- ans exactly at the end of the Ordovician, if any, cluded at least three of the following: discon- was quite minor, and the early Paleozoic oro- formity in an external belt where carbonate was genic activity in the area of the Taconic Moun- accumulating; severe early deformation in an tains was all over by that time. Thus, the internal volcanic belt; gravity slides from inter- textbook definition I have cited reduces one to nal uplifts into the external belt; and wide- the state of the schoolboy who, having some spread deformation, especially in the more vague idea of the Baconian controversy, wrote external belts. In general, these events did not on an examination that the plays of William occur at the same time in the various sectors; Shakespeare were not written by William each took a considerable time, and they over- Shakespeare but by another man of the same lapped to some extent. The Taconic orogeny name. also affected the southern Appalachians and Now it happens that we have recently ac- may have been the most important one there, quired a great deal of reasonably accurate infor- but evidence for this assertion is meager and mation about the extent and timing of orogeny inconclusive. Detailed analysis of the "fine in the Taconic region (for example, Zen, 1968; structure" of the Taconic orogeny combats the Bird and Dewey, 1970; Berry, 1970) and of dogma that orogenies are sharp, discrete events roughly contemporaneous orogeny in other punctuating the geologic record (separating parts of the northern Appalachians (Poole, periods and abruptly terminating geosynclinal 1967; Pavlides, and others, 1968; Williams, sedimentation) and suggests instead that they 1969; Hall, 1969). The Taconic orogeny has reflect "random-walk" processes within the always fascinated me, for the Appalachians and Earth, in all likelihood the same as those re- specifically the Taconic region are home base sponsible for sea-floor spreading and the pre- for me. I was born and brought up in Albany, sent tectonic state of the Earth. New York, within sight of the western Taconic foothills, and my first independent field work brought me face to face with their geological INTRODUCTION problems. Hence, at the risk of being provin- The Taconic orogeny (for some the cial, I would like in this address to analyze the Taconian) is normally defined, in textbooks or information we now have to see if, in addition the AGI glossary, as the orogenic disturbance to telling us what and when the Taconic that occurred in eastern North America at the orogeny was, it can point us to any more gen- end of the Ordovician period. Indeed, Stille eral conclusions about orogeny and its time re- (1924) included it in his original list of world- lations. wide orogenic episodes at that point in the geo- The Taconic orogeny is of further interest as logic column. To be sure, there is a certain a mildly uncomfortable exception to the corn- Geological Society of America Bulletin, v. 82, p. 1141-1178, 13 figs., May 1971 1141 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/5/1141/3432678/i0016-7606-82-5-1141.pdf by guest on 02 October 2021 1142 JOHN RODGERS—THE TACONIC OROGENY mon doctrine that orogeny closes geosynclinal HUDSON VALLEY AND TACONIC history. According to this doctrine, the history MOUNTAINS of the Appalachian geosyncline (the type geo- As the orogeny was first recognized because syncline, be it remembered) ought properly to of the unconformity in the Hudson Valley, we consist (and does in some accounts) of a long, may start there. The unconformity is well dis- uninterrupted period of sedimentation con- played in a number of places, from Becraft cluded by a single climactic Appalachian revo- Mountain and Mount Ida, which lie near the lution marking the end of the Paleozoic era, city of Hudson on the east side of the Hudson during which all or most of the Appalachian River, south past the Kingston area (Rondout structures were produced. Preliminary skir- and Rosendale) on the west side, and along the mishes, like the Taconic orogeny (or for that southeast front of the Shawangunk Mountain. matter the Acadian), disturb this straightfor- In some places the discordance is nearly a right ward picture, so easy to incorporate into ele- angle, and in all it is appreciable (Fig. 2, A and mentary lectures. Perhaps this is why there B). At Becraft Mountain, the older rocks show have been persistent efforts to derogate the im- not only intense folding but also cleavage; the portance of the Taconic orogeny and to rele- younger rocks there and throughout much of gate it to the status of a minor and quite local the region are also severely deformed but not episode or "disturbance." Yet the existence of enough to obliterate the unconformity. North a period of deformation older than the classical of Kingston, the oldest strata above the uncon- Appalachian folding of Pennsylvania and the formity belong to the topmost Silurian, if not Virginias was recognized as early as that folding the basal Devonian, and are dominantly car- itself, and by the same people, specifically bonate. To the southwest, older detrital strata Henry D. Rogers (no relation to the present come in, especially a unit of quartz-pebble con- speaker—note the spelling), who saw and un- glomerate and quartzite (Shawangunk con- derstood the resulting unconformity in the glomerate). Fossils are too few, however, to Hudson Valley in 1837,* and not much later indicate exactly how low in the Silurian these by Sir William Logan in the Gaspe Peninsula strata descend. Among the folded rocks and farther southwest in Quebec (Fig. 1). As beneath (which include beds as old as Early soon as correlations with Europe were estab- Cambrian), the youngest dated by fossils are lished, the unconformity was assigned a posi- Middle Ordovician,2 of Trenton age (medial tion between the Lower and Upper Silurian of Trenton of Kay, 1937; Barneveld of Fisher, Murchison (what we now call the Ordovician 1962; upper subzone of Zone 13 of Berry, and Silurian), and James D. Dana incorporated 1970. Thus, the span of time within which the the idea of such an epoch of deformation mark- deformation recorded by the unconformity ing the end of a period into the account of might have taken place is considerable. geological history in his Manual of Geology Farther south westward, however, the uncon- (1863, p. 226-229). In the second edition formity acquires narrower limits (Fig. 3). It can (1875, p. 212) he named it the Green Moun- be followed at least to the Lehigh River in east- tain Revolution, but in the fourth (1895, p. ern Pennsylvania and, in my opinion, consider- 386, 532) he renamed it Taconic, for the Ta- ably farther, probably to the Swatara River conic Mountains along the border between (Fig. 4A). Fossils reported to be lower Upper New York and Massachusetts, and that name Ordovician (assigned to the Eden—although has become the accepted one. still within Zone 13 of Berry—near the Dela- 1 In his 1838 report, Rogers mentioned only a locality "on the Delaware and Hudson canal....near Rondout" (p. 37), 2 Throughout this paper, I have followed the classical but in his final report (1858, v. 2, p. 785—actually written North American usage for the terms Lower, Middle, and about a decade earlier) he stated that the unconformity "was Upper Ordovician, which is based on the generally accepted discovered by me near the city of Hudson" at "Becraft's interpretation of the shelly faunas in the carbonate sequence Mount." W.W. Mather, within whose field area both these of the western Appalachians and the central platform; that is, localities lay, also saw it at "Becraft Mountain and Mount Lower Ordovician includes the Beekmantown; Middle Or- Bob" (now Mt. Ida; Mather, 1838, p. 165-166), presumably dovician the Chazy, Black River, and Trenton; and Upper at the same time but, if one may judge by the discussion in Ordovician the Eden, Maysville, and Richmond. As the Cara- his final report (1843, p. 368), his understanding of the doc of Great Britain is roughly equivalent to the Trenton, the unconformity and its implications was considerably less clear term Upper Ordovician here refers mainly to Ashgill equiva- than Rogers'. Perhaps Mather showed Rogers the localities, lents. If the classical North American subdivision is upset, the but Rogers first grasped the significance of what they saw. assignments in this paper will need revision. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/5/1141/3432678/i0016-7606-82-5-1141.pdf by guest on 02 October 2021 Figure 1. Map of Appalachian Mountains showing lo- cation of Taconic unconformities, Taconic-type klippen, and places mentioned in text. Line with dots to one side: Taconic unconformity (dots on side of younger strata); dashed line: margin of Taconic-type klippe or slide mass; cgl in circle: coarse polymict conglomerate in Middle Ordovician strata (Kellberg and Grant, 1956).