ABSTRACT

Sedimentary facies analysis of conglomerate deposits in the Greymouth Rift Basin has identified the latest Cretaceous to Paleocene alluvial fan and fan delta environments on the northwestern side. The Gravelly Braided River Facies Association is interpreted as high energy, braided river streamflow in a streamflow-dominated alluvial fan environment. The Gravelly Delta Front Facies Association was deposited by high bedload mouth bars and channel avulsion. The Gravelly Prodelta Facies Association is interpreted as high-density turbidity currents, and subaqueous debris flows in a fan delta slope environment. Bedding geometries suggest the fan deltas were Hjulström-types formed on lower angle slopes. The gradual decrease in conglomerate thickness from northwest to southeast indicates that the primary basin bounding fault was located offshore to the northwest, most likely the Cape Foulwind-Canoe Fault Zone. Overall facies distribution and paleoflow directions indicate the Greymouth Basin formed as a half-graben in a purely extensional setting with no strike-slip movement. The presence of contemporaneous sub-basins in the West Coast region suggests they likely experienced similar depositional history to the Greymouth Basin. Our findings agree with previous models from the Taranaki Basin that indicate that rifting was purely extensional, suggesting that the West Coast-Taranaki Rift System recorded primarily orthogonal extension.

摘要

格雷茅斯裂谷盆地砾岩矿床的沉积相分析已经确定了西北侧最新的白垩纪至古新世冲积扇和扇三角洲环境。砾石辫状河相组合被解释为在以水流为主的冲积扇环境中的高能量辫状河水流。砾石三角洲前缘相组合由高床负荷口坝和河道撕脱沉积而成。砾石前三角洲相组合被解释为扇三角洲斜坡环境中的高密度浊流和水下碎屑流。层理几何形状表明扇三角洲是在较低角度的斜坡上形成的 Hjulström 型。砾岩厚度从西北向东南逐渐减小表明主要盆地边界断层位于西北近海，很可能是逆风角-独木舟断层带。总体相分布和古流向表明格雷茅斯盆地在纯伸展背景下形成半地堑，没有走滑运动。西海岸地区同时期次盆地的存在表明它们可能经历了与格雷茅斯盆地相似的沉积历史。我们的研究结果与塔拉纳基盆地之前的模型一致，这些模型表明裂谷是纯粹的伸展，表明西海岸-塔拉纳基裂谷系统记录的主要是正交伸展。总体相分布和古流向表明格雷茅斯盆地在纯伸展背景下形成半地堑，没有走滑运动。西海岸地区同时期次盆地的存在表明它们可能经历了与格雷茅斯盆地相似的沉积历史。我们的研究结果与塔拉纳基盆地之前的模型一致，这些模型表明裂谷是纯粹的伸展，表明西海岸-塔拉纳基裂谷系统记录的主要是正交伸展。总体相分布和古流向表明格雷茅斯盆地在纯伸展背景下形成半地堑，没有走滑运动。西海岸地区同时期次盆地的存在表明它们可能经历了与格雷茅斯盆地相似的沉积历史。我们的研究结果与塔拉纳基盆地之前的模型一致，这些模型表明裂谷是纯粹的伸展，表明西海岸-塔拉纳基裂谷系统记录的主要是正交伸展。