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Sedimentology and diagenesis of the Lower Jurassic McCoy Brook Formation, Fundy Basin, Nova Scotia
Abstract
The 230+ m-thick McCoy Brook Formation was deposited during Early Jurassic time in the Minas Basin, the eastern extension of the Fundy basin. Outcrops form a narrow band along the northern side of the basin. During early McCoy Brook deposition, synsedimentary faulting created fault scarps along which talus accumulated, interfingering with lacustrine, playa, fluvial, and eolian sediments. During later deposition, broad sandflats extended southward from the toes of alluvial fans to playas on the valley floor, which were crossed by streams. Fluvial sequences comprise multi-storied channel sandstones interbedded with overbank flood deposits of sheet sandstones, graded-bed sequences, and desiccated claystones. Paleocurrent structures indicate that the channels were part of a southwest-flowing river. Sections comprised of alternating muddy sandstone and mudstone represent interfingering deposits on distal sandflats and playas. In the arid climate, bedding was disrupted by desiccation, formation of efflorescent salt crusts, and interstitial precipitation of evaporites. Progradation of an alluvial fan over sandflat and playa deposits is recorded where mudstone and muddy sandstone coarsen upward to beds of massive conglomerate. A 24-m thick section of basalt breccia that lacks bedding planes is interpreted as a talus deposit. Matrix- to clast-supported basalt conglomerates interbedded with redbeds are interpreted as 51 debris-flow beds. These beds are separated by cm-scale sandstone beds deposited by stream flow. Paleozoic aged volcanic, intrusive, and low-rank metamorphic rocks in the Cobequid Highlands were the source of much of the McCoy Brook sediments. The mean composition of sandstones was determined by modal analyses to be Q$\sb{64.2}$F$\sb{11.1}$R$\sb{24.8}$, feldspathic litharenite. The clay-mineral assemblage consists of illite, chlorite, and smectite. Diagenesis proceeded in three stages. Stage I occurred at shallow burial depths characterized by dehydration of limonite grain coatings to hematite. Stage II occurred at depths of 1 to 2 km, where K-spar overgrowing grew on K-feldspar grains, followed by calcite precipitation. Stage III followed uplift and exposure of the rocks to weathering. Earlier calcite cements were partially dissolved in some beds. Authigenic smectite and calcium zeolites formed in some sandstones by alteration of basalt.
Subject Area
Geology
Recommended Citation
Tanner, Lawrence Henry, "Sedimentology and diagenesis of the Lower Jurassic McCoy Brook Formation, Fundy Basin, Nova Scotia" (1990). Doctoral Dissertations Available from Proquest. AAI9110222.
https://scholarworks.umass.edu/dissertations/AAI9110222