ABSTRACT

The Rensselaer Graywacke near Troy, New York, has been investigated and described by numerous geologists. Previous investigations were concerned primarily with structural and stratigraphic relations, fossils, and qualitative descriptive petrology. Prior to the present investigation, thoughts on source-area direction and mode of emplacement of the Rensselaer were suggested from nonsystematically sampled and qualitatively described rock specimens.

The objectives of the present investigation were directed toward detecting any discernible trends in petrographic and geochemical variables; therefore a sampling design in which sampling sites were equally spaced from (in other words, orthogonal to) one another was constructed to obtain optimum geographic control. In addition to eight sampling sites and analyses of 33 samples from a preliminary investigation, 58 localities were visited and 119 samples collected. The following 38 variables were investigated: the proportion of the basic compositional variables (grains, rock fragments, matrix, and cement); the proportion of the types of grains (quartz, K-feldspar, plagioclase, epidote, sphene, opaques, and others); "a" and "b" axes of quartz grains; sorting of "a" and "b" axes; grain sha e; perfection of grain orientation; bulk density; percent concentration of SiO₂, Al₂O₃, total iron, MgO, MnO, CaO, SrO, BaO, TiO₂, Na₂O, and K₂O and the concentration in parts per million of Ni, V, Cu, Be, Y, Zr, La, Sc, and Cr.

Concepts and hypotheses based on previous investigations were used to construct models of the distribution of variability in specific variates and these were tested against the patterns derived from the observations collected in the present investigation.

If areas containing coarsest grain size may be taken to indicate direction of source area, data of the present investigation suggests the Rensselaer Graywacke received material from four different directions; north, south, southwest, and west.

Trend-surface maps indicate that unusual grain elongation (b/a ratios < 0.61) is confined to eastern and northern margins of the Rensselaer and is of regional magnitude; possibly reflecting a shear zone of regional extent.

Sixth-order trend surfaces for Na₂O, K₂O, Cu, MgO, and MnO are very similar in that they each indicate north-northeast axial trends. It is believed that the distribution pattern for the above elements reflects internal structure of the Rensselaer, and therefore, structure of the Graywacke appears to conform with the general structure of the Taconics.