Synsedimentary diagenesis of Cambrian peritidal carbonates: evidence from hardgrounds and surface paleokarst in the Port au Port Group, western Newfoundland

Erosional surfaces in peritidal platform carbonates are products of synsedimentary lithification and subsequent erosion under submarine and subaerial conditions. Submarine hardgrounds formed in parted limestone (thinly interbedded, fine-grained carbonates and shale) and ooid grainstone sequences, and surface paleokarst developed on carbonate laminites and stromatolite and thrombolite mounds. Paleokarst features include scalloped surfaces (kaminitzas) and solution-enlarged fractures (grikes) in supratidal carbonate laminite, and planar to sub-planar surfaces on stromatolite and thrombolite mounds.

Parted limestone hardgrounds are characterized by mineralized, sub-planar to irregularly undulose surfaces on top of lime mudstone and skeletal packstone. These hardgrounds formed during intermittent periods of relatively slow sedimentation, reduced energy conditions and stable bottom sediments in the subtidal zone of muddy tidal flats. In contrast, oolitic hardgrounds developed in high-energy ooid shoals in which irregular topography and fluctuating energy conditions permitted lithification during short-lived periods of stable sediment conditions. Bored encrustations of interlaminated, fibrous low-Mg calcite cement and silty micrite occur on top of some oolitic hardgrounds. Reworking of parted limestone and oolitic hardgrounds by high-energy events, probably storms, resulted in the deposition of intraclast rudstones and floatstones.

Surface paleokarst developed on bare carbonate rock under subaerial conditions in the supratidal and/or intertidal environment. Paleokarst features include scalloped surfaces (kaminitzas) and solution-enlarged fractures (grikes) in supratidal carbonate laminite, and planar to sub-planar surfaces on stromatolite and thrombolite mounds. Other recognizable meteoric features, such as calcrete and subsurface leaching, are limited in the Port au Port Group, in spite of its peritidal origin. This is attributed to one or more the following: (1) insufficient periods of subaerial exposure; (2) pervasive synsedimentary lithification; and (3) predominance of calcitic components.