Carboniferous Continental Sedimentation, Atlantic Provinces, Canada

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Abstract
Carboniferous continental strata in eastern Canada are classed into (1) fanglomerate, (2) fluvial, (3) lacustrine, and (4) mixed fluvial and lacustrine facies. Each facies contains formations that are widely distributed geographically and vertically within the Carboniferous strata; each is related genetically to a rift valley framework, basement mobility, and climate. The fanglomerate facies grade laterally into the fluvial facies. They are poorly sorted and crudely stratified. Angular to subrounded clasts, ranging in size from 2 mm to over 1,220 mm, correspond in composition to underlying or nearby upfaulted basement. The fluvial facies contain thick sandstone units alternating with lutite and thin sandstone units. Many thick sandstone units contain one or more cycles of primary structure sequences suggestive of fluvial channel deposits. A typical cycle consists of (1) mud-pebbles at base, (2) cross-stratified sandstone, (3) horizontally stratified sandstone, and (4) ripple-laminated sandstone at top. This sequence suggests fluctuating velocities during the formation of bedforms such as subaqueous dunes, plane bed, channel bars, point bars, or ripples. Horizontal stratification is considered upper flow regime; cross-stratification and ripples, lower flow regime. Channel sandstone units contain as many as four cycles. Rate of valley floor subsidence, rate of lateral channel migration (or rate of channel jumping), rate of base-level changes, and climatic fluctuations are possible controls on the number of cycles within a channel sandstone unit. The interchannel units consist of lutite with persistently uniform sandstone interbeds, and contain mud cracks, current ripples, flutes, grooves, raindrop impressions, amphibian trackways, and horizontal stratification. The sandstone interbeds are interpreted as the basal member of the overbank cycle. The lutite is interpreted as the upper member of the overbank cycle. The lacustrine facies consist of dark- to medium-gray calcareous lutite with interbeds of medium-gray fine-grained sandstone and of dusky-red lutite and sandstone. In addition to the typical lithology described, 6 of the 10 formations assigned to the lacustrine facies are known to contain less typical units of dark-gray calcareous lutite with interbeds of tan-weathering calcilutite. These less typical units contain hopper-shaped casts, mud cracks, red lutite beds, algal biolithites, and a sparse fauna of conchostracans and pelecypods, and are interpreted as a fluctuation from deeper to a shallower water (and perhaps partially subaerial), more saline environment. The more typical rock units of gray calcareous lutite and sandstone show graded and current-worked laminae, plant detritus, pyrite, and contain a fauna of fresh-water fish, pelecypods, and arthropods, which are interpreted as a deeper water, less fluctuating environment of deposition. The lakes thus fluctuated in depth and salinity and covered up to 11,000 square miles during lower Cansoan (middle Carboniferous) time. Hortonian (lower Carboniferous) and lower Cansoan lake deposits exceed 1,000 feet in thickness and pass laterally over short distances into fine and eventually coarse fluvial facies near the margin of the depositional basins. Fine-grained facies of mixed origin consist of intertonguing fine fluvial and lacustrine deposits. These were laid down during the expansion and contraction of the lakes within what was probably a flat central-basin region. A proposed model of Carboniferous continental deposition consists of fanglomerates at the basin margin (or above pre-Carboniferous basement) grading laterally basinward into coarse and then to fine fluvial facies that grade ultimately into basin-center lacustrine facies.

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