Basaltic rocks in the Rensselaer Plateau and Chatham slices of the Taconic allochthon: Chemistry and tectonic setting N. M. RATCLIFFE U.S. Geological Survey, Reston, Virginia 22092 ABSTRACT that contains abundant coarse-grained detri- compared the Nassau-Rensselaer sequence to tus of probable eastern North American Mesozoic rocks of the Newark Supergroup and Tholeiitic to transitional alkalic basalt and Proterozoic basement These relations sug- suggested a similar origin. Until recently, how- basaltic tuff form widely separated but dis- gest a fault-bounded submarine basin (Nassau- ever, the basalts associated with the Rensselaer tinctive units within the Nassau Formation of Rensselaer basin) located near the continental Graywacke Member have been interpreted as late Proterozoic or Early Cambrian age, margin that was fed by submarine fan depos- dikes or sills rather than flows, and the associa- within the Rensselaer Plateau and Chatham its. Submarine morphology of the distributing tion of these basalts in time with the deposition slices of the Taconic allochthon in eastern fan complex may have channelized the ba- of the sedimentary rocks has not been entirely New York State and western Massachusetts. salts. The association of the tholeiitic to tran- clear (Balk, 1953; Prindle and Knopf, 1932), Examination of all known occurrences of sitional alkalic basalts with interpreted except for the exposures of pillow basalt at these basaltic rocks and detailed mapping of marine-fan deposits suggests that the volcan- Banker Pond (loc. 2 in Fig. 1) which were rec- the enclosing strata indicate that these basalts ism occurred after Iapetan rifting and ther- ognized as contemporaneous deposits by Dale are submarine lava flows and water-transported mal subsidence over tectonically thinned (1892, p. 310, 311, and 327), by Balk (1953, tuffaceous deposits restricted to the lower sialic crust, perhaps near the time and place of p. 829), and by Potter (1972, p. 12). Ratcliffe third of the stratigraphic section in both eventual separation. (1969,1974,1978) described the basaltic rocks slices. In each slice, the basalts thicken to the from the Chatham slice and concluded that they west along with thickening and coarsening of INTRODUCTION were basaltic lava flows and/or tuffs inter- enclosing graywacke beds (Rensselaer Gray- calated in the Nassau Formation. As part of the wacke Member of the Nassau Formation) Tholeiitic(?) basalt sills, flows, pillow basalt, reconnaissance geologic investigation for the and thin eastward into pillow basalt and and basaltic tuffs are minor but distinctive rocks new bedrock geologic map of Massachusetts tuffaceous basalt that is associated with fine- in or near the base of the Rensselaer Plateau (Zen, 1983), a systematic examination of the grained graywacke and thinly laminated slice and Chatham slice of the Taconic alloch- major areas of basaltic rocks and tuffaceous vol- purple and green turbidites of the Nassau thon in Rensselaer and Columbia Counties, canics in the Taconic Range was conducted in 1 Formation. New York. Basaltic rocks are absent elsewhere 1978. The results of this study (Table A) and new chemical analyses of the basalts are These high-Ti02, low-MgO basalts resem- within the Cambrian and late Proterozoic strata ble very closely late Proterozoic basalts and of the Taconic slices of Vermont, Massachusetts, presented here along with a brief comparison feeder dikes of the Catoctin Formation of the and southeastern New York State. These basalts with similar Iapetan rift-stage basaltic rocks Blue Ridge in Virginia and late Proterozoic are interlayered with coarse graywacke and from the Blue Ridge of Virginia; Hudson High- metadiabase dikes in the northern Reading graywacke conglomerate of the Rensselaer lands, New York; and Vermont. All of these Prong in New York, both of which intrude Graywacke Member of the Nassau Formation basalts have striking similarities to, and distinc- pre-Iapetan, eastern North American base- or with graywacke beds in the Nassau Forma- tive differences from, eastern North America ment. Major-element and rare-earth chemis- tion that have been correlated with the Rensse- Mesozoic rift basalts. Minor occurrences of try and geology also establish a correlation laer (Ratcliffe, 1974). The age of the enclosing basaltic rocks of probable Early Ordovician age between the Taconic metabasalts and meta- sedimentary rocks is thought to be late Protero- in the Taconic allochthons at Stark's Knob basalts preserved in the Tibbit Hill Volcanic zoic^) or Early Cambrian. Bird and Dewey north of Albany, New York, and in the Ham- Member of the Pinnacle Formation that un- (1970) suggested that the Rensselaer Graywacke burg Klippe of eastern Pennsylvania are not conformably overlies middle Proterozoic Member of the Nassau Formation and basaltic basement, in central Vermont. Palinspastic rocks constitute a rift facies associated with the earliest stages of continental extension leading to 'Tables A and B may be secured free of charge by reconstruction places the Chatham slice requesting Supplementary Data 87-29 from the GSA oceanward of the Rensselaer Plateau slice the development of Iapetus. Bird (1975) later documents Secretary. Additional material for this article (Tables A and B) may be secured free of charge by requesting Supplementary Data 87-29 from the GSA Documents Secretary. Geological Society of America Bulletin, v. 99, p. 511 -528,2 pis., 11 figs., 1 table, October 1987. 511 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/4/511/3998358/i0016-7606-99-4-511.pdf by guest on 25 September 2021 512 N. M. RATCLIFFE associated with rift rocks. The Pennsylvania tinguished: middle Proterozoic gneiss (Y), Chatham slice (II) within the Nassau Forma- pillow basalts differ markedly in tectonic setting, Cambrian and Early Ordovician shelf sequence tion, except for several localities of tuff and age, and chemistry (Lash, 1985) from the basalts carbonates (OC), Middle Ordovician black tuff(?) reported by Balk (1953, PI. 1) which described here. shale (Ow), late Proterozoic and Early Cam- occur in the Giddings Brook slice(?) (I) north of brian Hoosac Formation (€Zh), and late Prot- Petersburg, New York, east of locality 3, and in STRATIGRAPHY AND STRUCTURAL erozoic, Early Cambrian through Middle the Berlin Mountain slice (IV) west of Williams- SETTING OF VOLCANIC ROCKS Ordovician rocks of the Taconic allochthons town, Massachusetts. Several of the latter tuff IN THE TACONIC ALLOCHTHON (OCt, €Znr, €Z, CZnu). The structural level localities mapped by Balk have been examined (lowest to highest) of the Taconic slices is de- by either Ratcliffe (this study) or by Potter dur- Figure 1 shows the distribution and structural noted by Roman numerals I to V on Figure 1. ing reconnaissance mapping for the bedrock position of major slices of the Taconic alloch- Rocks of the Giddings Brook slice (I) range map of Massachusetts and are not included as thon in eastern New York State and adjacent from late Proterozoic (Nassau Formation) to verifiable volcanic deposits. Localities discussed western Massachusetts [based on Potter, 1972, Early Ordovician in age and overlap ages of in this report are identified on Figure 1. All of 1979; Ratcliffe and others, 1975; and the bed- rocks in slices II through V. Basalts and basaltic the volcanic rocks are metamorphosed to the rock geologic map of the State of Massachusetts volcaniclastic rocks (shown in black) are largely lower greenschist facies and consist of well to (from Zen, 1983)]. Five major units are dis- confined to the Rensselaer Plateau slice (III) and poorly foliated greenstone containing varying EXPLANATION W Shows inferred relative west to east depositional positions of units Autochthons -Taconic allochthons - 1 r Eugeosynclinal —. IO rocks of O O e o vWaliom-- Rowe-Hawley Ordovician '.•:>sacwr Giddings Brook Zone SI Formation slice Ow.-.'-' >0- Rensselaer 0€t Ceu s: « Cambrian Plateau Chatham ®o Hoosac slice slice. <%r Berlin Mtn. Formation Everett Mtn. amphibolite slices i€Zrii (CZha) ij V ! fez- €Znr Late Greylock ®Ì2 Proterozoic slice <19 u Nassau Formation (-CZn) i P diabase Rensselaer Graywacke Member (€Znr) dike Volcanics €Znv Green Mountain and Berkshire Massif Middle Roman numerals l-V show present stacking Proterozoic sequence of Taconic allochthons from structurally lowest I to structurally c ® U> highest V, numbers indicate localities discussesd in text. io s2 -A- Syn to post metamorphic thrust m faults (Late Taconic and Younger) Premetamorphic thrust faults bounding Taconic allochthons Figure 1A. Explanation for Figure IB. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/4/511/3998358/i0016-7606-99-4-511.pdf by guest on 25 September 2021 Figure IB. Geologic map showing distribution of volcanic rocks in relationship to major rock units in the Rensselaer Plateau and Chatham slices. Localities 1 through 14 discussed in text. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/4/511/3998358/i0016-7606-99-4-511.pdf by guest on 25 September 2021 Rensselaer Plateau slice Chatham slice Turner Mountain Banker Mercer Mtn.- Queechy Lake The Knob Berkshire Boys Farm (loc. 5, fig. 1) Pond-Ward Hollow Fog Hill (Ioc. 9, fig. 1) (Ioc. 10, fig. 1) East side (ioc. 2, fig. 1) \'oc. 7, fig. 1) Canaan Valley (Ioc. 11, fig. 1) CZnr CZng- top of section MffiöfHj-CZngr not exposed CZnr CZnr. ! I mixed volcanics basalt CZng CZncj ,CZngr basalt base of 'and sediments CZnr illow pillows METERS