Diagenetic Processes That Affect Provenance Determinations in Sandstone

Abstract

Diagenetic processes that alter the depositional composition of sands must be considered when making provenance interpretations. These modifying processes operate from the zone of weathering to the deep subsurface where diagenesis grades into metamorphism. Of greatest importance is the total dissolution of grains of feldspar, rock fragments, and heavy minerals by corrosive meteoric or subsurface waters to severely modify original sand composition. Some “diagenetic quartzarenites” have been produced by essentially wholesale dissolution of non-quartz grains under severe weathering. Even partial dissolution of some rock fragments can alter them beyond identification. Of great importance also is the replace­ment of detrital grains by authigenic carbonates, clays, zeolites, and many other minerals. Although the identity of pseudomorphous grains of distinctive size and shape can logically be deduced for some sandstones, the identity of most totally replaced grains cannot be determined with confidence. The albitization of plagioclase more calcic than An₁₀ and of K-feldspar should be expected for all deeply buried sandstones. In the Texas Gulf Coast basin, albitization is important for sandstones that reach burial temperatures in excess of 120° C.

There are several additional processes that can complicate provenance determinations. In environments where detrital ferromagnesian minerals, magnetite and rutile are undergoing oxidation to produce the pigment of secondary red beds, hematite, and its precursors can invade micropores in rock fragments sufficiently to prevent their identification. Of lesser importance, but locally troublesome, is the loss of primary textures of carbonate and other rock fragments that undergo recrystallization or severe compaction.