Abstract

The late early Permian coincided with a short-lived cold episode in Western Australia, which occurred long after the demise of late Paleozoic ice sheets, and is manifested by conspicuous dropstones in corresponding marine strata. In the northern Perth Basin, this time interval is represented by the Carynginia Formation, which has long been regarded as a record of low-energy marginal-marine sedimentation. The low-diversity body-fossil content and impoverished trace-fossil assemblages in the formation were recognised as evidence of low salinities, low dissolved-oxygen levels and high suspended-sediment concentrations presumably related to freshwater influxes into a restricted basin. However, sedimentary facies indicative of river processes have yet to be documented to explain the sedimentological and paleontological character of the formation. This study examines an overlooked sandstone-rich interval of the Carynginia Formation in the Irwin Sub-basin, a marginal half graben faulted against crystalline basement of the Yilgarn Craton and forming part of a series of late Paleozoic intracratonic rift basins in Western Australia. Facies analysis indicates that the lower part of the interval is dominated by storm-generated deposits containing extraformational pebbles and boulders. These deposits are interpreted as recording lower-shoreface sedimentation under the influence of seasonal sea ice, and constitute a shallower, sandstone-rich equivalent of mudstone-prone strata that otherwise characterise the Carynginia Formation. The upper part of the interval consists of river-borne deposits representing friction-dominated deposition of a river-mouth bar, the first direct record of deltaic sedimentation in the Carynginia Formation. The stratal organisation of mouth-bar bedsets is likely the result of successive sediment-laden pulses related to seasonal floods in a shallow marine basin, and the autogenic evolution of the mouth-bar complex in response to fluctuating river discharge and compensational stacking. Formative runoff events were arguably responsible for reductions in salinity and the introduction of turbid buoyant plumes into the shallow sea that were adverse to faunal development. A fluvial influence may explain the sedimentary character and fossil content of coeval mudstone-prone formations in Western Australian rift basins, especially in the vicinity of cratonic margins.

1. KEY POINTS

2. The lower Permian (late Artinskian–Kungurian) mudstone-prone Carynginia Formation in the northern Perth Basin has long been regarded as the record of low-energy, marginal-marine conditions in a cold-water seaway.

3. The low-diversity fossil content in the lower part of the formation indicates stressed physico-chemical conditions influenced by freshwater, sediment-laden influxes.

4. The described mouth-bar deposits are the first direct evidence of deltaic sedimentation in the Carynginia Formation, demonstrating that river outflows likely controlled water chemistry and impacted faunal development.

摘要

早二叠纪晚期恰逢西澳大利亚短暂的寒冷事件，该事件发生在晚古生代冰盖消亡很久之后，并在相应的海相地层中表现出明显的水滴石。在珀斯盆地北部，这一时间段以 Carynginia 组为代表，长期以来被认为是低能边缘海相沉积的记录。地层中的低多样性身体化石含量和贫乏的微量化石组合被认为是低盐度、低溶解氧水平和高悬浮沉积物浓度的证据，这可能与淡水流入受限盆地有关。然而，尚未记录表明河流过程的沉积相来解释地层的沉积学和古生物学特征。本研究检查了欧文次盆地中一个被忽视的富含砂岩的层段，这是一个边缘半地堑，与 Yilgarn 克拉通的结晶基底断层并形成了西澳大利亚一系列晚古生代克拉通内裂谷盆地的一部分。相分析表明，该层段的下部以风暴生成的沉积物为主，其中含有形成外的卵石和巨石。这些沉积物被解释为在季节性海冰的影响下记录下海岸沉积物，并构成了较浅的、富含砂岩的泥岩多发地层，这些地层在其他方面是 Carynginia 组的特征。区间的上部由河床沉积物组成，代表河口坝的摩擦为主的沉积物，Carynginia 组三角洲沉积的首次直接记录。河口坝床的地层组织可能是与浅海盆地季节性洪水相关的连续含沙脉冲的结果，以及河口坝复合体响应波动的河流流量和补偿堆积而自生演化的结果。形成性径流事件可以说是造成盐度降低和将混浊的浮力羽流引入浅海的原因，这对动物群的发育不利。河流的影响可以解释西澳大利亚裂谷盆地，特别是克拉通边缘附近的同时代泥岩易发地层的沉积特征和化石含量。河口坝床的地层组织可能是与浅海盆地季节性洪水相关的连续含沙脉冲的结果，以及河口坝复合体响应波动的河流流量和补偿堆积而自生演化的结果。形成性径流事件可以说是造成盐度降低和将混浊的浮力羽流引入浅海的原因，这对动物群的发育不利。河流的影响可以解释西澳大利亚裂谷盆地，特别是克拉通边缘附近的同时代泥岩易发地层的沉积特征和化石含量。河口坝床的地层组织可能是与浅海盆地季节性洪水相关的连续含沙脉冲的结果，以及河口坝复合体响应波动的河流流量和补偿堆积而自生演化的结果。形成性径流事件可以说是造成盐度降低和将混浊的浮力羽流引入浅海的原因，这对动物群的发育不利。河流的影响可以解释西澳大利亚裂谷盆地，特别是克拉通边缘附近的同时代泥岩易发地层的沉积特征和化石含量。以及口坝复合体响应波动的河流流量和补偿叠加的自生演化。形成性径流事件可以说是造成盐度降低和将混浊的浮力羽流引入浅海的原因，这对动物群的发育不利。河流的影响可以解释西澳大利亚裂谷盆地，特别是克拉通边缘附近的同时代泥岩易发地层的沉积特征和化石含量。以及口坝复合体响应波动的河流流量和补偿叠加的自生演化。形成性径流事件可以说是造成盐度降低和将混浊的浮力羽流引入浅海的原因，这对动物群的发育不利。河流的影响可以解释西澳大利亚裂谷盆地，特别是克拉通边缘附近的同时代泥岩易发地层的沉积特征和化石含量。

1. 关键点

2. 珀斯盆地北部的二叠纪下层（阿尔丁斯克纪晚期-昆古纪晚期）泥岩多发的 Carynginia 组长期以来一直被认为是冷水航道中低能、边缘海相条件的记录。

3. 地层下部的低多样性化石含量表明受淡水、沉积物流入量影响的压力物理化学条件。

4. 所描述的口坝沉积物是 Carynginia 组三角洲沉积的第一个直接证据，表明河流流出可能控制了水化学并影响了动物群的发育。