Abstract: Interpretation of Carbonate-Rock Sequences; Sedimentary or Metamorphic Models?

Brian W. Logan

The development of contemporary thought about carbonate rocks and their use as indices of paleoenvironment has been laid on a foundation of knowledge assembled during the past three decades of intensive research on modern carbonate sediments. This foundation has seemed sound and most of us have studied the ancient rocks with the confident assumption that most of the observed phenomena are sedimentary or the result of diagenesis either at normal temperature and pressure (NTP) or under overburden pressure alone. Is this assumption valid or are many carbonate phenomena the product of low-grade metamorphic processes?

The high susceptibility of carbonate sediments to diagenesis generally is recognized and thermodynamic considerations also suggest that carbonate sedimentary deposits should be altered profoundly in low-grade metamorphism where other sedimentary rocks such as sandstone and shale are stable. Strangely there has been little documentation of processes operating in ranges of pressure and temperature above normal or of carbonate-rock types produced. The reason is that many metamorphic features and rock types have been interpreted as sedimentary or early diagenetic. This applies particularly to carbonate rocks in structurally complex areas, faulted terrains, or those known to have been buried deeply.

Examples are drawn from Middle to Upper Devonian carbonate rocks cropping out along the northern Canning basin, Western Australia. These rocks have been subjected to dynamic metamorphism under conditions of relatively low temperature.

Processes of pressure solution and shear fracture are present in compressional-stress fields and are linked through the gradational relations evident in their products. The most obvious expressions of pressure solution and shear are ubiquitous subparallel stylolites that impart a strongly bedded appearance to rocks, but pressure-solution/shear surfaces are at all scales--separating tabular rock masses in apparently conformable sequence, circumscribing discordant bodies of different size (blocks to grains), and truncating carbonate grains and crystals.

Abundant pressure-solution surfaces imply volume reduction. Consequently, modifications are not only at the scale of grains, textures, and structures but also at larger scales of stratigraphic and structural relations. Rocks and rock components also have variable resistance as units in a pressure-solution field, and thus more susceptible units will tend to be removed, less soluble units will tend to remain. Fundamentally then, pressure solution concentrates residual materials at interfaces. It follows that: (1) residual materials form bodies of shape and dimension related to the disposition of pressure-solution surfaces in the host rocks, and (2) discrete insoluble bodies agglomerate into larger masses by pressure-solution loss of intervening host.

Metasomatism and recrystallization, penecontemporaneous with pressure solution and shear fracture, are additional factors in rock alteration under compressive stress. Dolomitization occurs in the calcite pressure-solution field. Dolomite also is less pressure soluble than calcite and dolomitization proceeds further via passive accumulation at pressure-solution surfaces.

Processes of tensional fracture, solution, and emplacement are present in tensional-stress fields and are linked through gradational relations in their products. Tensional fracture and solution are primary mechanisms that create cavities. Emplacement is a general term for processes of precipitation and infiltration (internal sedimentation) that result in filling of cavities.

AAPG Search and Discovery Article #90973©1976-1977 AAPG Distinguished Lectures