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About 130 m (427 ft) of Lower Cambrian Weymouth Formation is exposed at Nahant, Massachusetts, 11 km (7 mi) northeast of Boston. Several beds of white to light-gray, fossiliferous limestone, up to 3 m (9.8 ft) thick, occur in a sequence of dark, very thinly bedded argillite. Portions of the argillite contain altered and chertified carbonate nodules. Limestone beds contain irregular, very thinly laminated chert layers with structures characteristic of silicified laminate stromatolites.
The limestone is comprised of 4 microfacies: (1) thinly bedded unfossiliferous micrite, (2) irregular intraclasts surrounded by sparry cement or biomicrite, (3) biomicrite (wackestone to packstone) containing small shelly fossils, primarily hyolithids, and (4) biosparite (hyolithid grainstone).
Conoidal hyoliths do not show a strong preferred current orientation on slabs and peels, but are often irregularly disposed with long axes at high angles to bedding. Biomicrites (packstones) and biosparites occasionally overlay irregular scour surfaces. Irregular bioclastic-rich pockets are surrounded by and grade into micrite or biomicrite.
A shallow subtidal, partially protected shelf of platformal environment best explains the textural and bedding characteristics of limestones. Storm events are recorded as bioclastic-rich and intraclastic sediments over scour surfaces. Irregular cyclic repetition of microfacies and lack of progressive shoaling or deepening during carbonate deposition suggest that limestones represent periods of stillstand at relatively low sea level positions. Carbonate deposition ceased when extrabasinal mud input increased, possibly during episodes of rapid sea level rise.
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