Abstract

Limestone breccia, estimated to be as much as 100 feet thick, which disconformably overlies the basal limestone of the Mississippian Windsor Group in central Nova Scotia, has heretofore been considered sedimentary breccia. It is absent in deep borings; at depth, the basal limestone is apparently conformably overlain by interbedded limestone and anhydrite.

The breccia typically is a jumbled mass of unsorted angular limestone fragments in a matrix of muddy massive limestone. Most of the fragments are similar to the basal laminated limestone of the Windsor Group although some are of massive limestone and red marl apparently derived from beds higher in the section. In a few exposures, the breccia contains relict beds; in others, thin, graded calcarenite beds appear to fill pockets in the breccia. Numerous pipes, channels, and large masses of cavity fill containing sand and scattered quartzose pebbles occur within the breccia.

The breccia postdates the Late Paleozoic regional deformation that extensively folded the Windsor Group. Stratification in floored cavities is virtually horizontal and independent of the attitude of the adjacent Windsor strata. The deformation produced veins normal to bedding in limestone of the Windsor Group; many rotated fragments within the breccia contain similar veins normal to their bedding. Metamorphic fabric, absent in the breccia matrix, varies in nature and orientation from fragment to fragment. Sand grains, pebbles, and heavy minerals within the cavity fill appear to have been derived from Triassic rocks, suggesting that the breccia originated in either Triassic or post-Triassic time. The compositional dissimilarity between these pebbles and those of Pleistocene gravel indicates a pre-Pleistocene origin.

It is here proposed that the breccia formed only near an erosion surface by collapse and disintegration following solution of anhydrite interbeds and part of the limestone. The cavity fill appears to be composed of surficial debris which filled solution channels within the breccia.

The breccia formed long after the deposition of the Windsor Group and should not be regarded as part of the depositional sequence of the group. The name “Pembroke Formation” proposed for the breccia and cavity fill by Weeks (1948) is preempted and should be regarded as abandoned.

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