Abstract: Internal Structure of Lower Cambrian Archaeocyathid Reefs of Northern Maritime Appalachians

N. P. James, D. R. Kobluk

The complex organism-sediment-diagenetic mosaic so characteristic of both modern and ancient reefs has been discovered in some of the oldest metazoan structures, the archaeocyathid reefs of southern Labrador.

Reefs are made up of many overlapping small mounds with each mound a jumble of sticklike and vaselike archaeocyathids in a matrix of calcareous red mudstone. Mounds grade laterally into skeletal, reef-derived calcarenites; individual patch-reef complexes of mounds and lime sand commonly attain stratigraphic thicknesses of 20 m.

The pattern of biologic accretion, internal sedimentation, early cementation, and biologic destruction easily is recognized: archaeocyathid skeletons form a loose frame with mudstone deposited as internal sediment; cavities as large as 1.0 × 0.2 m and roofed by pendant Renalsis are partly filled by multigeneration, geopetal, marine sediment interlayered with cement; cavity walls are lined with laminated, sometimes botryoidal calcite; mound surfaces commonly are erosional with both cemented sediment and toppled archaeocyathids planed off; borings (Trypanites sp.) penetrate skeletons, lithified sediment, and synsedimentary pore-filling cement and in turn are filled with trilobite debris.

These attributes indicate that the basic internal structure of reefs was present at the very beginning of the Paleozoic and did not develop gradually during the Cambrian-Ordovician.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC