Abstract

The fluvial Chaswood Formation records a complex history of tectonic reactivation of basement horsts and shedding of clastic sediment. Previous mineralogical studies indicate sources from both local and more distant basement rocks of the Appalachians. Because such mineralogical studies identify only particular source rocks, the geochemistry of 60 samples from a complete section through the Chaswood Formation in borehole RR-97-23 was investigated to determine the character of bulk sediment supply. The detrital signature is partially obscured by diagenetic processes that caused concentration of K, P, Sr, and U at three regional unconformities. The elements Ti (in ilmenite and its alteration products), Zr (in zircon), Th, and Y are largely controlled by the abundance of heavy minerals in the rocks. Ilmenite is the dominant first-cycle heavy mineral, whereas much of the zircon is of polycyclic origin; therefore, the Ti/Zr ratio is a guide to the proportion of first-cycle sediment supply from crystalline basement. High Cr, Sr, and Ni/Co ratio in mudstones appear related to supply from weathered mafic crystalline basement. Three cycles of sediment supply are recognized, each overlying a regional unconformity. These reflect uplift of horsts that resulted first in shedding of readily eroded Carboniferous sandstones, then rapid erosion of crystalline basement and, finally, greater supply of deeply weathered regolith. This study provides a template for interpreting more distal sediments in the Scotian Basin, where the direct effects of source-area tectonics are not recorded in the stratigraphic succession.

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