Mixed Siliciclastic-Skeletal Carbonate Facies on Wanganui Shelf, New Zealand: A Contribution to the Temperate Carbonate Model
Published:January 01, 1997
Jeanette L. Gillespie, Campbell S. Nelson, 1997. "Mixed Siliciclastic-Skeletal Carbonate Facies on Wanganui Shelf, New Zealand: A Contribution to the Temperate Carbonate Model", Cool-Water Carbonates, Noel P. James, Jonathan A. D. Clarke
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Mixed siliciclastic-carbonate sediments are presently accumulating on the inner to mid Wanganui shelf (20 to 110-m water depth) off central western New Zealand (40°S). Cluster analysis identifies five surficial sediment facies across this sector of the shelf. A modern, high- carbonate (avg. >70%) skeletal sandy gravel (Facies 2) occupies an irregular lobe, up to 2000 km2 and 1 m thick, over the inner to mid shelf (30- 90 m). At inner shelf depths (<50 m), Facies 2 passes relatively abruptly shorewards into a modern, very low carbonate (avg. <10%) siliciclastic sand (Facies la) or a relict, low carbonate (avg. 20%) bivalve-bearing gravelly sand (Facies lb). Offshore, the skeletal carbonates (Facies 2) pass more gradually via a moderate carbonate (avg. 30%) bivalve-bearing muddy sand (Facies 3; 75- 110 m) into a low carbonate (avg. <15%) siliciclastic mud (Facies 4; 85 110 m) and micaceous sand (Facies 5; 95-105 m), all predominantly modern. The high carbonate facies (Facies 2) occurs in a region of relatively reduced siliciclastic sediment input and is maintained by recurring colonization by epibiota upon the coarse shelly substrate. The nearshore siliciclastic facies are sourced mainly from southwestern North Island rivers and southeastwards longshore drift. The offshore siliciclastic facies include both North and South Island-derived sediments, the latter introduced by the prevailing oceanic currents.
Analogues of the five facies defined for the surficial sediments are identified in short cores (<2.5 m) taken from Wanganui shelf. The ideal facies motif in these cores changes upwards from siliciclastic gravelly sands (Facies lb) to skeletal-rich carbonates (Facies 2 and 3) to siliciclastic sandy muds and muddy sands (Facies 4 and 5). This vertical succession formed in response to Holocene transgression associated with the last postglacial rise of sea level and the accompanying changes in hydraulic regime on Wanganui shelf. In a sequence stratigraphic context, Facies lb gravelly sands represent transgressive systems tract deposits. The overlying skeletal carbonates (Facies 2 and 3) form a condensed shellbed up to about 1 m thick. The locally capping fine siliciclastics (Facies la, 4 and 5) equate to highstand systems tract sediments, fed from opposite directions by dual North and South Island sources, which are slowly encroaching upon and covering the surficial carbonate deposits. The Holocene Wanganui shelf record affords a modern analogue of comparable glacio-eustatic cyclothemic facies widely developed in uplifted Plio-Pleistocene deposits on the North Island.
In common with many other temperate-region carbonates, the Wanganui shelf carbonates are dominated by bivalve molluscan and bryozoan remains, comprise mixtures of fresh and moderately to highly degraded skeletal material, have low positive (c. +1 to 4-2%) 8lsO and 8I3C values and are closely associated with siliciclastic sediments. The Wanganui carbonates also exhibit characteristics that contrast with the larger Australasian temperate carbonate platform deposits. These characteristics include: (1) moderately high total mud (up to 50%) and carbonate mud (avg. 27%) contents, (2) abundant living and diverse fauna in areas with otherwise moderately high terrigenous mud inputs, (3) a significant proportion (up to c. 50%) of aragonitic components, mainly infaunal bivalves, (4) relatively high accumulation rates (c. 5-10 cm/ky), and (5) bivalve molluscan shells, rather than rocky surfaces, as the main substrate for attachment and encrustation. These kinds of differences exemplify the substantial variations that can occur amongst temperate carbonate facies, and require consideration as models of temperate carbonate sedimentation continue to develop.
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This book is a collected series of papers on the sedimentary geology of carbonate sediments deposited on shelves and offshore banks in cool to cold oceans. Contributions come mainly from a workshop organized by Jonathan Clarke held in Geelong, Victoria from January 14 to 19, 1995. Most earth scientists have traditionally viewed carbonate sediments as warm-water deposits and interpreted them as such in most of the geological record. Yet large areas of the modern seafloor are covered with neritic carbonate sediments formed in seawater that is colder than 20ºC. Such environments are not easily studied. Thus, our knowledge of cool-water carbonates has lagged far behind our understanding of their warm-water counterparts. This situation has changed somewhat as more and more investigators have braved the chill waters and rough seas. This book brings together a group of studies that illustrate the present status of our understanding and current research in a field that is in mid-life.