Shallow-water Permian spiculitic ramp sedimentation: Sverdrup Basin, Arctic Canada.

摘要

Thick, widespread spiculitic chert and subordinate carbonate of the Late Guadalupian to Lopingian (Late Permian) Degerböls and Lindstrom formations record deposition in a third-order relative sea-level cycle in the Sverdrup Basin. This succession was deposited in a storm-dominated, distally-steepened ramp environment, one of the few occurrences of shallow-water biogenic cherts in the geological record.; Sediments are distributed within eight lithofacies and can be placed in an inner ramp to basinal setting. Inner-ramp facies include fossiliferous grainstone, fossiliferous spiculitic chert, and light spiculitic chert. Although siliceous sponge spicules greatly outnumber all other fossils, Heterozoan Association calcareous biota commonly occur in nearshore environments. Burrow-mottled and dark spiculitic chert typify the middle ramp. Finely-laminated spiculitic siltstone and fissile shale occur in the outer ramp, slope and proximal basinal settings. Ankeritic siltstone spans middle ramp to basinal environments, characterizes sediments deposited subsequent to the biosiliceous cherts, and may record continuous deposition across the Permian-Triassic boundary. Episodic storms affected nearshore environments, and hummocky cross-stratification characterizes spiculites on the middle ramp. During periods of slow accumulation, bioturbation is pervasive, with Zoophycos and Planolites common across the ramp profile. Ramp morphology evolved through the third-order sequence, with widespread fossiliferous grainstone, typical of Guadalupian time, reduced to patchy, isolated occurrences by Lopingian time.; This extensive chert occurrence is one of many accumulations of Late Permian age along the northwest margin of Pangea. The source of silica is interpreted to have been upwelled cold and nutrient-rich waters, with an abundant typical upwelling assemblage of authigenic minerals including glauconite, phosphate, and pyrite associated with the cherts.