Abstract: Marine Calcite Cement Stratigraphy of Neptunian Fractures, Devonian Reef Complexes, Canning Basin, Western Australia

W. Bruce Ward, Milton H. Kwong, Jay L. Banner

A marine cement stratigraphy recorded in the exceptionally large amount of coarse marine calcite cement in the Devonian reef complexes of the Canning basin, Western Australia, has been correlated over 10s of meters of stratigraphic section and 100s of meters along depositional strike. Sequences of sub-millimeter growth zones in these marine cements are continuous within individual neptunian fractures and have been correlated between different fractures that occur in fore-reef slope strata in the Dingo Gap and McWhae Ridge areas. Zones of different thicknesses, fluid-inclusion densities, and crystal morphologies define these sequences. The continuity of fine-scaled growth zones within a fracture indicates synchronous cementation. Correlation of such zonations between fractures suggests that fractures were open and connected at the same time on a scale at least as large as that of the correlations. This implies factors controlling zoning in these open systems were external to that scale.

Neptunian fractures at Dingo Gap contain marine oxide encrustations encased between calcite cements. These fractures occur in slope strata for at least three kilometers along strike. The cement zones and oxides are correlative between fractures throughout an eighty-meter stratigraphic interval and up to 600 meters along strike. In addition to constraining the extent of synchronous seawater flow through these strata, these relationships indicate that (1) fractures formed within a relatively narrow time interval, which suggests a single event (perhaps tectonic) caused the multiple fractures; and (2) a significant period of time (greater than approx. 0.5 m.y.) occurred between deposition and marine cementation in at least the fractures within the lower part of the stratigraphic interval. The latter implies that fractures prolonged the time during which porosity was opened to seawater flow and marine cementation. This, in addition to the observation that correlative fracture-filling calcites account for most of the coarse marine cements in the Dingo Gap slope strata, suggest that fracturing (which ultimately may have had a tectonic control) was important in the accumulation of the large amount of marine cements in the Devonian reef complexes.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994