Spencer's Cave: an Adirondack Anomaly
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• Spencer's Cave: An Adirondack Anomaly by Alice Waldhauer A senior thesis submitted to fulfill the requirements for the degree of B.S. in Geology, 1986 The Ohio State University Advisor Acknowledgments During the preparation of this report innumerable people have offered their support and encouragement. In particular I would like to thank Howard and Elizabeth Jaffe, two University of Massachusetts professors who first told me of Spencer's Cave, and introduced me to the man who discovered the cave. This man, Spencer Cram, a Department of Environmental Conservation Ranger, guided me to the cave in September 1985. I would also like to extend thanks to my advisor Professor R. Tettenhorst, and Professor C. Corbato of the Ohio State Geology Department for their guidance, and to my sister Ann for the photographs that appear in this paper. Many others including my family, classmates, and professors have given generous aid in the preparation of this report. TABLE OF CONTENTS Abstract ..... i Introduction. 1 Regional Adirondack Geology. 5 Spencer's Cave ......................... 8 Cave Morphology ........................ 10 Cave Mineralogy ........................ 18 Calcite . .......................... 20 Pyroxenes . 21 Garnet .... 26 Magnetite. 28 Discussion of Results. .31 Conclusion . .35 Footnotes .. ....................... .38 Bibliography .. 40 i Abstract This Report is a morphological and mineralogical survey of Spencer's Cave in Essex County, New York. Discovered in 1978 by Ranger Spencer Cram of the New York State Department of Environmental Conservation (DEC), this small cave is situated in a geologically complex area of anorthosites, syenites, gneisses, calc-silicates, and metasediments. Spencer's Cave is a water carved feature in a body of white, green, or blue marble containing small amounts of augite, magnetite, and many diverse members of the diopside-hedenbergite solid solution series. The rocks surrounding the marble body are garnetiferous and represent the effects of regional and contact meta morphism, assimilation, and metasomatism. Formation of the cave was probably controlled by differential dissolution along a zone of fracture in Grenville metasediments. 1 Introduction The Adirondack Mountains of northern New York are a southern extension of the Grenville Province of Canada. Precambrian igneous rocks within the Adirondack mass (Figure 1) are among the oldest in the United States with an age calculated to be lJOO mybp (Silver, 1969). These crystalline rocks have been subjected to numerous cycles of deformation and intrusion, and their history is deduced from the structures and minerals present. This report is concerned with a small area located at the northern edge of the Mount Marcy quadrangle (Figure 2) ·along State Route 73 in a vall-ey that contains the Cascade Lakes. Figure 1 The Grenville Province of Canada and New York. Anorthosites (black) intruded and metamorphosed Grenville Series sediments (white). (reprinted from Petrology: Igneous, Sedimentary, and Metamorphic (Ehlers and Blatt,1980) after J. Martignole and K. Schrijver, 1970). e e Port Kent 44°30 c -a /.... ~ 0 < Q;:' ~ ~ TUPPER LAKE 44°00' ,,, • ,,; , ,..~/"'\ / I' ....... -,,1- I-'° / -1() - I' I <i:r I' I ...... 0 ,,,0 \ fr) ,.._ ,..,0 ,.._ I RAOUETTELAKE I 4/t;-~·:-11,3. ...- \(.I --···- ( ((( v--• « ....( ( ;-:·/((\'-14711~1·:-rtl] 43•45' 0 KM 50 Figure 2 Spencer's Cave is located in the valley of the Cascade Lakes near .the center of the northern edge of the Mount Marcy quadrangle close to location 8. (reprinted from Jaffe et al (1983) Bedrock Geology of the High Peaks Region, Marcy Massif, Adirondacks, N New York) 3 Three major families of rock- anorthosite, syenite, and Grenville metasediments-comprise the vast majority of the Adirondack high peak region. Of these three, anorthosite is the most abundant with exposures on many of the highest peaks. The Marcy anorthosite is composed of blue lathlike phenocrysts of plagioclase imbedded in a crushed white matrix of similar composition. The phenocrysts (up to 3 cm in length) commonly exhibit Carlsbad and albite twinning and lie in the range of andesine-labradorite (An 40-60). 1 2 By definition anorthosite contains over 90% plagioclase. In this region minor constituents include lime garnets and magnetite which are visible macroscopically, and hornblende and augite which are seen with a microscope. An interesting feature of this anorthosite is schiller iridescence, a blue-green play of color seen on some labradorite crystals when wet. The Opalescent River takes its name from this property of the feldspars. The syenites of the Adirondacks are a variable group of highly deformed intrusive rocks that represent a period of intrusion later than that of the anorthosites. The variability of this group is evidenced by the wide range of names. Baillieul (1976) used the term pyroxene-microperthite gneiss, deWaard (1968) employed the names mangerite, jotundite, farsundite, and charnockite, Jaffe (1983) used the name Pitchoff gneiss, and Buddington (1939) called it pyroxene- quartz syenite. Early geologists believed that the syenite gneiss suites represented rocks formed by the intruding 4 anorthosites.3 Present thinking suggests they are the result of an independent period of intrusion complexed with the effects of the assimilation of country rocks, ion exchange between magma and the country rocks, and contact metamorphism. Samples of this group, especially those taken from the slopes of Pitchoff Mountain, contain pink feldspar (K feldspar and plagioclase), clear to gray quartz, and bands of dark amphibole. The presence of quartz in this group indicates a change of the amount of free silica in the parent magma. The anorthosites are notably quartz poor. Studies by Jaffe4 indicate that members of the Pitchoff gneiss which include syenitic-monzonitic syenodioritic facies are a result of separate batches of melt and do not represent a true magmatic fractionation process. The third and oldest family of rocks in the Adirondacks are the metamorphosed Grenville metasediments. Deposited in a marine environment, the Grenville metasediments (quartzites, marbles, shists, and gneisses) have also been interpreted in a number of different ways because of their complex relations with adjacent facies. Fragments of the Grenville Series in anorthositic and syenitic rocks and evidence of repeated partial melting and recrystallization suggest several periods of intrusion and deformation. Unusual calc-silicate and microperthite gneiss sequences are the product of metamorphic alteration, assimilation, and metasomatism of the Grenville Series during the emplacement of the Marcy anorthosites. 5 Regional Adirondack Geology Rocks of the Adirondacks show evidence of numerous geologic events superimposed on previous structures. The Grenville sediments were deposited lJ00-1350 mybp over the course of 12-67 my.5 In this time interval the Grenville Series sediments had also been metamorphosed and subjected to two periods of folding. Intrusion of anorthosites, syenites, and melagabbro dikes occurred in pulses during 300 million years of the Precambrian. Metamorphism of these facies is believed to have taken place at a depth of 35 km (21 mi). 6 These pulses of activity and the long time interval account for the wide composition range of the anorthosites. Andesitic anorthosite composes the core facies of the Adirondack mass, and gabbroic anorthosite and syenitic rocks represent the type of intrusion occurring near the perimeter of the mass. Still later in the Precambrian the andesitic anorthosite core facies were thrust over gabbroic and syenitic rocks. Remnants above the thrust fault comprise many of the summits in the high peak region, especially in the Great Range(Fig. 3). An interesting contact between the core facies and the overthrusted autochthonous mass is located on Basin Mountain along the trail east of the summit.7 Figure J, reprinted from Jaffe et al. (1983) shows the generalized relations of different anorthosites and syenitic gneisses throughout the Great Range. The Cambrian period brought another interval of intrusion, this time Adirondack rocks were injected with camptonite and diabase (both are very fine-grained black rocks) dikes. Since the Cambrian, many kilometers of material have been e e SW' NE The. G rea.t Ra.n ge.. N 4.,., ---- - < r-Pe ' ,_, .... /' ~ \ -- ' ....... ./ I / / / A '/ / - I "' / /' ' ------ / /i- I A ,,,. A ..., d es i. n e A ,,, Q./r- t /,.} o s \:. tt:. / ......_ / , / / ,,,- --- ?> / ' I / ' ' - '\. A ,.,.. / . ' I ~ / /\ B""l.S1 ....:' ........ ' - A ' ---- ---N lf-io I-...... - '- - -- ·1U,s7 ,.,.. M" v-cy-- ' B . ' s:t'acl<. ; :"' 5~~ Saal~ d le b~ ac.r--:-<"S_:?J _ ./t-t~~ Goth 1c.s Ar.,.,.,st,..o,..,g ~,~,/ lr-eSertt Wol~ Ja.o..)s ...... , ...... , - ...... ' ~- '"',,.....'-.::"'---:...'-.::.......:._ -foM,..arh;i x )(' ~ )\ -+- - - ;-t:,~ gA x ~- - • ~·c; - x gA x x.. )( ..-r )(" >< >( x 9A x ')(. x 1. )( -t Figure 3 This diagram is a generalized cross section of the Great Range showing the andesitic anorthosite core facies (A) thrust over intensely deformed gabbroic anorthosite gneiss (VHg) and ferromonzonitic gneiss (POg) and (monz gn). Cascade Valley is approximately 10 km north of the northeast end of the cross section. °' 7 eroded and uplifted to expose these deep seated rocks. Geologic interpretation of the Adirondacks is constantly being revised. Informati6n from individual studies serve to augment preexisting knowledge. The rock here still eludes an explanation that encompasses