 Fitted-Fabric Grainstones—Commonly Overlooked Evidence for Vadose Diagenesis and Subaerial Exposure

Abstract

Some carbonate grainstones have distinctive fitted fabrics that form due to dissolution in the vadose zone that are only rarely recognized or correctly interpreted. These grainstones have flattened and concavo-convex grain contacts where the grains fit together like puzzle pieces and are commonly lined with early marine or meteoric cement. Examples of these fitted fabric grainstones have been identified in carbonates from the Archean to the Holocene and likely occur in shallow marine, eolian and lacustrine carbonate grainstones throughout the geologic record. Fitting occurs due to dissolution at grain contacts by meteoric or mixed marine-meteoric fluids that over time flattens the grain contacts. Although these grainstones may at first appear to be compacted, burial compaction can be ruled out because there is commonly no sign of pressure solution, the early cement that forms at the surface clearly postdates the fitting of the grains and is unaltered by the fitting process, and because these fabrics are found in Pleistocene and Holocene grainstones that have never been buried. Because this feature forms during periods of subaerial exposure, it can help to identify cryptic exposure surfaces and sequence boundaries.