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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 56 (1972)

Issue: 3. (March)

First Page: 627

Last Page: 628

Title: Proterozoic Stromatolites of Cyclic Shelf, Mounded Shelfedge and Turbidite Offshelf Facies, Northwestern Canadian Shield: ABSTRACT

Author(s): P. Hoffman

Article Type: Meeting abstract


The shelf facies of the Rocknest Formation (lower Proterozoic) consists of almost 200 cycles, each 5-50 ft thick. Each cycle has, in ascending order, a thin transgressive beach deposit of intraclast and ooid packstone, a subtidal graded and rippled dolomitic shale with syneresis dikes, an intertidal dolomite with columnar and elongate-domal stromatolites and edgewise conglomerate, and a cherty black dolomite with microdigitate stromatolites deposited by direct precipitation in supratidal algal marshes. Individual stromatolite beds can be traced for hundreds of miles along strike and tens of miles across strike. On the landward extremities of the shelf, low compound stromatolite mounds are surrounded by silty shale and interbedded with crossbedded quartz sandstone of delta c origin.

The shelfedge facies of the Rocknest and the Pethei Group (lower Proterozoic) consists of compound columnar stromatolite mounds, each up to 60 ft thick. The mounds are elongate and separated laterally by relatively narrow anastomosing channels oriented normal to the trend of the shelfedge. The channels are filled with bimodally crossbedded, intraclast grainstone and their floors were less than 8 ft below the crests of adjacent mounds. The mound-and-channel belt is only 2 mi wide and marks the outer limit of surf zone deposits on the shelf.

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The offshelf facies of the Pethei contains shale with abundant stromatolitelike digitate growth structures of poorly laminated limestone. They are rhythmically interbedded with graywacke turbidites, thin evenly bedded calcareous mudstone with shale partings (rhythmite) and beds of cherty limestone submarine-slide breccia. The growth structures certainly formed below surf-base and probably below the photic zone of muddy water. They may be bacteriagenic.

The mere presence of stromatolites in Proterozoic carbonates cannot be used to infer a specific depositional environment, but by mapping different types of stromatolites and their associated sediments, paleogeographic facies can be established as in any Phanerozoic succession.

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