Mixed Siliciclastic Skeletal Carbonate Facies on Wanganui Shelf, New Zealand: A Contribution to the Temperate Carbonate Model
Mixed Siliciclastic Skeletal Carbonate Facies on Wanganui Shelf, New Zealand: A Contribution to the Temperate Carbonate Model
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新西兰旺加努伊大陆架混合硅质碎屑骨架碳酸盐岩相：对温带碳酸盐岩模型的贡献
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发表时间：
1997
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通讯作者：
C. Nelson
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作者：
J. Gillespie;C. Nelson
目前在新西兰中西部外的旺格努伊陆架内侧至中部水深20至110米处堆积着混合硅屑碳酸盐沉积。400s聚类分析确定了该陆架上的五种表层沉积相现代高碳酸盐AVG 270骨架砂砾相2占据了一个可达2000 km2的不规则叶，在内陆架深度30 90 m内陆架上I厚90 m相2相经相对陡峭的海岸进入现代极低碳酸盐Avg 10硅屑砂相La或残留的低碳酸盐Avg 20双壳类含砾质砂相Ib近海骨架碳酸盐相2通过中等碳酸盐Avg 30双阀更缓慢地传递。含泥质砂相3 75 110 m，发育低碳酸盐平均15个硅质碎屑泥相4 85 110 m和云母砂相5 95 105 m，均以现代为主。高碳酸盐相2产于硅质碎屑沉积输入相对较少的区域，并由粗壳基底上的附生生物反复定居维持；近岸硅质碎屑相主要来源于西南部的北岛河流和东南沿海漂移；近岸硅质相包括南北两岛的沉积，后者是由盛行洋流引入的，为表层沉积定义的5个相的类似物是在旺内陆架的2 5 m短岩心中识别出来的。岩心中的砂岩母题从硅屑砾质砂相Ib向骨架富碳酸盐相2和3向硅屑砂泥和泥质砂相4和5向上变化。这种垂向序列形成于与末次冰期后海平面上升有关的全新世海侵以及伴随着层序地层背景中旺加努伊陆架水力状况的变化而形成的垂直序列。南岛碳酸盐岩与其他温带地区碳酸盐岩一样，都是由双壳类软体动物和苔藓虫残体组成，由新的和高度降解的骨骼物质组成，具有低的正Ci~21180和1IJC值，与硅屑沉积密切相关。Wanganui碳酸盐岩也表现出与澳大利亚温带碳酸盐台地沉积形成对比的特征，这些特征包括：I中高总泥量达50和27。2丰富的生物和多样的动物群地区有中等高的陆源泥浆输入3相当大的比例高达c50的文石组分主要存在于动物群4相对较高的堆积率c5 10 cmlky和5双壳软体动物贝壳而不是岩石表面作为附着和结壳的主要基质这些种类的差异说明了温带碳酸盐相之间可能发生的实质性变化，需要考虑作为温带碳酸盐沉积模式的继续发展。
Mixed siliciclastic carbonate sediments are presently accumulating on the inner to mid Wanganui shelf 20 to llO m water depth off central western New Zealand 400S Cluster analysis identifies five surficial sediment facies across this sector of the shelf A modern high carbonate avg 270 skeletal sandy gravel Facies 2 occupies an irregular lobe up to 2000 km2 and I m thick over theinner to mid shelf 30 90 m At inner shelf depths 50 m Facies 2 passes relativelyabruptly shorewards into a modern very lowcarbonate avg 10 siliciclastic sand Facies la or a relict lowcarbonate avg 20 bivalve bearing gravelly sand Facies Ib 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 lowcarbonate avg 15 siliciclastic mud Facies4 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 siliciclas tic 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 fa cies motif in these cores changes upwards from siliciclastic gravelly sands Facies Ib to skeletal rich carbonates Facies 2 and 3 to siliciclastic sandy muds and muddy sands Facies4 and 5 This vertical succession formed in response to Holocene transgression associated with the last post glacial rise of sea level and the accompanying changes in hydraulic regime on Wanganui shelf In a sequence stratigraphic context Facies I b grav elly sands represent transgressive systems tract deposits The overlying skeletal carbonates Facies 2 and 3 form a condensed shellbed up to about I m thick The locally capping fine siliciclastics Facies I a 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 shelfrecord 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 shelfcarbonates are dominated by bivalve molluscanand bryozoan re mains comprise mixtures offresh and moderately to highly degraded skeletal material have low positive c I to 2 1 180 and 1 IJC 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 I moderately high total mud up to 50 and carbonate mud avg 27 con tents 2 abundant living and diverse faunain areas with otherwise moderately high terrigenous mud inputs 3 a significant proportion up to c 50 of aragonitic components mainly infaunalbivalves 4 relativelyhigh accumulation rates c 5 10 cmlky and 5 bivalve molluscan shells rather than rocky surfaces as the main substrate for attachment and encrustation These kinds ofdifferences exemplify the substantialvariations that can occur amongst temperate carbonate facies and require consideration as models of temperate carbonate sedimentation continue to develop