Carbonates and relative changes in sea level

Abstract

In the geologic record some of the most accurate gauges of changes in sea level are the sediment type, geometry and diagenesis of carbonate shelves and platforms. This is because carbonates frequently occur at or very near sea level and are usually less compacted than siliciclastics. World-wide changes in relative sea level (the sum of eustatic sea-level changes, sedimentation and crustal movements) have occurred repeatedly and cyclicly through geologic time, producing characteristic responses in carbonates. 1. (1) Relative rises in sea level (usually caused by the cumulative effect of tectonic subsidence and eustatic rise) may result in the following: 1.1. (A) Drowned carbonate reefs or platforms. Here carbonate growth potential is exceeded by relative sea-level rise, and is characterized by shallow-water sediments, overlain by hardgrounds and/or deep-water sediments, some of which may be condensed sequences. 1.2. (B) Platforms where only the fast growing rim and patches of the interior are able to match sea-level rise while the remainder of the platform is drowned (temporarily). 1.3. (C) Platforms which keep up and maintain a flat top at sea level and contain shallow-water sediments whose thickness at the least matches the height of the sea-level rise. If terrigenous supply is limited, prograding sheets of shelf carbonate occur (frequently capped by supratidal evaporites), with prograding shelf-margin carbonate clinoforms and turbidites. If terrigenous supply is high, the shelf carbonates encroach on deltaics. 2. (2) Relative drops in sea level (caused by crustal uplift or by subsidence being outpaced by a eustatic drop in sea level) cause karst and soil development over shelves and platforms, deposition of “deep-water” evaporites in adjacent semi-enclosed basins and in open marine basins the deposition of deltaic and aeolian clastics that bypassed the shelf. Falls are accompanied by platform-wide fresh-water diagenesis. During relative sea-level rises marine diagenesis is common in the subtidal portions of the shoaling-upward carbonates, and fresh-water diagenesis and dolomitization and sulphate deposition is common in their intertidal and supratidal portions. The stratigraphic significance of these reponses to relative sea-level change is that many are tied to eustatic events and so are predictable within a basin of deposition.