STRUCTURAL CONTROL ON CARBONATE FACIES DISTRIBUTION ON ACTIVE AND PASSIVE CARBONATE PLATFORMS

Tectonism commonly is a controlling factor on the regional distribution of carbonate facies on ancient and modern carbonate platforms and the current understanding of these facies is a direct outgrowth of the studies of Recent carbonates initiated by Bob Ginsburg's in the early 1950's. In foreland basin settings the forebulge can be a site of preferential carbonate sedimentation in an otherwise carbonate-poor environment. In the Late Ordovician Taconic foreland basin peritidal carbonates are restricted to a narrow geographic band in the TST and late HST along the forebulge. Correlation of these tidal flats is complex indicating autocylic processes predominated but increasing amplitudes of high-frequency sea level fluctuations may also have played a part in their distribution. Peritidal carbonates of the Mississippian Antler foreland basin in Montana and Idaho also are restricted to a narrow band that never progrades far into the basin. A structurally uplifted block in this basin was the site of a shallow subtidal, muddy coral bank that is surrounded by deeper water facies.

Tectonism also can influence sedimentation along passive margins though sometimes this influence is subtle. In the Lower Cambrian Sekwi Formation of the Northwest Territories, Canada, debris flows, large allochthonous blocks and an irregular wedge of deeper water sediment indicates syndepositional down-to-the-basin faulting during the Nevadella biozone. This faulting partitioned the ramp into a localized area of active erosion that was flanked by coeval deep subtidal to shallow subtidal sedimentation on both the basinward and landward sides. The carbonate ramp became tectonically quiescent in the Bonnia-Olenellus biozone and there is little evidence of the previous tectonism during this period. The Late Ordovician passive margin of southern Laurentia preserves and extensive carbonate ramp that was influenced by prolonged upwelling as shown by abundant phosphate and spiculite. However, subtle tectonism on this low-relief ramp produced an areally restricted topographic depression that was the site of deep-water deposition and is surrounded by shallow-water carbonates.