--> Abstract: A Proposed Facies Model for Cool Water Carbonate Shelves, by T. D. Boreen, N. P. James, and Y. Bone; #91012 (1992).

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ABSTRACT: A Proposed Facies Model for Cool Water Carbonate Shelves

BOREEN, THOMAS D., and NOEL P. JAMES, Queen's University, Kingston, Ontario, Canada, and YVONNE BONE, University of Adelaide, Adelaide, South Australia

Detailed examination of Modern and Tertiary sediments from the southern margin of Australia and synthesis of data from globally dispersed temperate carbonate successions provides the framework to establish a preliminary model for carbonate deposition in temperate latitudes.

Cool water carbonate platforms are hydrodynamically and biologically partitioned, high-energy open shelves. There are two primary zones of carbonate production: (1) a shallow water, inner shelf, photic biostromal epifaunal bryozoan/echinoid-mollusc-algal assemblage, and (2) a deep-water, shelf edge aphotic biostromal/biohermal bryozoan-echinoid-coral community. Platforms are hydrodynamically graded into: (1) an inner shelf zone of erosion and particle abrasion, (2) a midshelf zone of storm/tide-assisted swell and shoaling, and (3) an outer shelf zone of alternating suspension/storm deposition. The inner shelf facies is characterized by extensive carbonate production (mollusc-encrusting bryozoa-red algal-sponge-barnacle), moderate to high terrigenous input, daily particle abrasion, reg lar traction bedform movement, erosion, winnowing, lag formation, sediment reworking, sea floor cementation, and intense destructive diagenesis. Deposits are thin discontinuous lenses of coarse-grained, poorly sorted, massive- to cross-bedded, quartz-rich rhodoidal grainstones overlying and encrusting hardground surfaces. The midshelf facies records high energy, bryozoan/echinoid-dominated sediment production and continuous traction deposition. Meadows of erect branching bryozoans inhabit gravelly substrates between mobile sheets of megaripples, sandwaves, and sand ribbons. Deposits occur as sharp-based, lensoid to sheet-like bodies of amalgamated tabular to trough cross-bedded grainstones punctuated by hardgrounds and clay-draped/burrowed omission surfaces. The outer shelf facies is dom nated by suspension deposition of terrigenous/carbonate fines, episodic storm emplacement of inner shelf-derived coarse bioclastic sands, intense bioturbation, bioerosion, and authigenic mineral precipitation. Deposits comprise interbedded successions of laminated/burrowed argillaceous wackestones and distally fining, sharp-based graded/burrowed bioclastic grainstone/packstone beds. The shelf edge facies is characterized by aphotic carbonate sediment production and accumulation, deep-water wave conditions, upwelling, winnowing, mass transport, bioturbation, and bioerosion. Two broad subfacies are present: (1) a laterally extensive biostromal/biohermal core of branching and fenestrate bryozoan/sponge/coral bafflestone/packstone, and (2) flanking units of bryozoan/echinoid-rich bioturbated fine sandy grainstone/packstone.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)