Types and Facies Models of Platform Margins and Slopes

Deep-water carbonate slope sediments are significantly different from shallow marine carbonate sediments. Platform margin or slope sequences are divided into five major sedimentary units: (1) Shallow-water complex of banks, reefs and ooid/pellet shoals which may occur on platform margins ; (2) undisturbed in situ pelagic and hemipelagic sediments; (3) gravity-induced mass transport deposits;(4) periplatform sediments; (5) deep-water organic baildups. The proportions of these units vary in different settings, but because slopes are especially susceptible to mass failure, a high percentage of the section may contain allochthonous material. One of the most distinct differences between basin and slope sediments, however, is reflected in the instability of slope and base-of-slope environments. The disruption and reorientation of sediments into slides and chaotically deformed masses may be very common on slopes. In contrast, the frequency and scale of these features in carbonate basins are usually relatively minor. Stratigraphic models are employed in an attempt to explain the vertical facies changes (i. e. the stratigraphic sequences) that occur at the platform margin-slope transition. They may be divided into models of: upbuilding, aggrading, retreating and drowned or a combination of these. Ancient carbonate facies patterns in platform margin, slope and basin sequences can be quite varied. Various types of carbonate platform margins are characterized by distinctive profiles, facies and evolutionary sequences. Ramps may be homoclinal or distally steepened. Homoclinal ramps pass seaward into deeper waters without a major break in slope, and lack shallow-water breccias as well. Distally steepened ramps have broad slopes over a deeper ramp. Clasts of shallow-water facies are rare. Thus slope facies may contain abundant breccias of slope-derived clasts. Rimmed platform margins have relatively flat tops and show marked break in slopes in the highenergy zone. Rimmed platform margins and slopes may be divided into: (1) depositional or accretionary ; (2) by-pass, and (3) erosional margins. Depositional or accretionary margins show both upbuilding and outbuilding; they generally lack marginal escarpments; and platform margin and foreslope/slope facies may intertongue. By-pass margins occur in areas of rapid upbuilding where shallowwater sedimentation keeps pace with sea level rise. Bypassing may be associated with a marginal escarpment and/or a gullied by-pass slope, whereas erosional margins are characterized by high, steep escarpments. Debris flows and periplatform talus are obvious features.