Evidence of a Continuous Continental Permian-Triassic Boundary Section in western Equatorial Pangea, Palo Duro Basin, Northwest Texas, U.S.A.
Evidence of a Continuous Continental Permian-Triassic Boundary Section in western Equatorial Pangea, Palo Duro Basin, Northwest Texas, U.S.A.
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DOI：
10.3389/feart.2021.747777
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发表时间：
2022-05
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影响因子：
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通讯作者：
N. Tabor;J. Geissman;P. Renne;R. Mundil;W. S. Mitchell;T. Myers;Jacob Jackson;C. Looy;Renske Kirchholtes
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作者：
N. Tabor;J. Geissman;P. Renne;R. Mundil;W. S. Mitchell;T. Myers;Jacob Jackson;C. Looy;Renske Kirchholtes
Whitehorse Group和Quartermaster Formation是代表美国德克萨斯州中北部Palo Duro盆地沉积最后一幕的广泛的红层陆相层序。从区域上看，这些地层记录了始于二叠纪中晚期的长期海退层序的顶点。怀特霍斯群包括大量层状至块状红色石英粉砂岩至细砂岩和稀有白云石、层状至块状石膏和粘土岩以及成岩石膏。Quartermaster地层显示出从几乎等量的薄平面和透镜状细砂岩以及其下半部分的层状块状泥岩到上覆地层的变化，上覆地层中含有粗粒交错层砂岩，表明弯曲河道深达7 m，以及罕见的漫滩泥岩。上怀特霍斯群没有古土壤，仅在夸特马斯特组发育不良。火山灰落沉积（凝灰岩）存在于最上部的怀特霍斯群和较低的夸特马斯特组地层之间的对比，分布在150公里以上的五个地层剖面，并提供这些岩石的地质年代学信息。传统上，怀特霍斯群和夸特马斯特组都被归为晚二叠世Ochoan（长兴期）阶段，工作者认为，二叠-三叠系界线的特征是具有区域意义的不整合。然而，迄今为止，这一年龄分配的化学地层学或生物地层学证据一直缺乏。从Palo Duro盆地的五个不同地点识别和收集的Quartermaster组下部至少两个不同火山灰坠落层的单颗粒锆石U-Pb CA-TIMS分析，产生的沉积年龄范围为252.19 ± 0.30至251.74 ± 0.28 Ma。对位于夸特马斯特组顶部下方几米处的砂岩中碎屑锆石进行单颗粒锆石U-Pb CA-TIMS分析，获得了从中元古代（1418 Ma）到中三叠世的一系列年龄（244.5Ma; Anisian），后者被解释为最大沉积年龄，不早于Anisian，从而表明二叠纪-三叠纪界线位于下夸特马斯特组/上怀特霍斯组序列内的某个地方。来自180个早期埋藏的微晶灰岩胶结物样品的稳定碳同位素数据保存了一个化学地层学信号，该信号在各剖面之间是相似的，在怀特霍斯群-夸特马斯特组边界之下约20米处有一个大的<$−8‰负同位素偏移。这种大的负碳同位素偏移被解释为与二叠纪末灭绝相关的相同偏移，这与新的高精度放射性同位素年龄数据以及偏移位于正常极性地层磁性带内的事实是一致的。白云岩胶结物δ 13 C值在夸特马斯特地层的下部保持较小的负值（约在-5至-8 permil之间），然后在剖面的顶部变得更正值。这一长时间的负δ 13 C值在夸特马斯特组被解释为代表最早的三叠纪（印度）开始的生物和生态系统的“恢复”。氧同位素值的微晶灰岩水泥显示出一个渐进的趋势，通过边界间隔更积极的价值观，这表明大幅度升温的条件下结束二叠纪灭绝事件和冷却条件的趋势后，最早的三叠纪。我们对这些地层的观察表明，在西部，亚赤道盘古大陆的二叠纪-三叠纪边界的古环境和古气候的特点是沉积体系，不利于植物的保存。
The Whitehorse Group and Quartermaster Formation are extensive red-bed terrestrial sequences representing the final episode of sedimentation in the Palo Duro Basin in north-central Texas, U.S.A. Regionally, these strata record the culmination of a long-term regression sequence beginning in the middle to late Permian. The Whitehorse Group includes beds of abundant laminated to massive red quartz siltstone to fine sandstone and rare dolomite, laminated to massive gypsum, and claystones, as well as diagenetic gypsum. The Quartermaster Formation exhibits a change from nearly equal amounts of thin planar and lenticular fine sandstone and laminated to massive mudstone in its lower half to overlying strata with coarser-grained, cross-bedded sandstones indicative of meandering channels up to 7 m deep and rare overbank mudstones. Paleosols are absent in the Upper Whitehorse Group and only poorly developed in the Quartermaster Formation. Volcanic ash-fall deposits (tuffs) present in uppermost Whitehorse Group and lower Quartermaster Formation strata permit correlation among five stratigraphic sections distributed over ∼150 km and provide geochronologic age information for these rocks. Both the Whitehorse Group and Quartermaster Formation have traditionally been assigned to the late Permian Ochoan (Changhsingian) stage, and workers assumed that the Permian-Triassic boundary is characterized by a regionally significant unconformity. Chemostratigraphic or biostratigraphic evidence for this age assignment, however, have been lacking to date. Single zircon U-Pb CA-TIMS analyses from at least two distinct volcanic ash fall layers in the lower Quartermaster Formation, which were identified and collected from five different localities across the Palo Duro Basin, yield interpreted depositional ages ranging from 252.19 ± 0.30 to 251.74 ± 0.28 Ma. Single zircon U-Pb CA-TIMS analyses of detrital zircons from sandstones located only a few meters beneath the top of the Quartermaster Formation yield a range of dates from Mesoproterozoic (1418 Ma) to Middle Triassic (244.5 Ma; Anisian), the latter of which is interpreted as a maximum depositional age, which is no older than Anisian, thus indicating the Permian-Triassic boundary to lie somewhere within the lower Quartermaster Formation/upper Whitehorse Group succession. Stable carbon isotope data from 180 samples of early-burial dolomicrite cements preserve a chemostratigraphic signal that is similar among sections, with a large ∼−8‰ negative isotope excursion ∼20 m beneath the Whitehorse Group-Quartermaster Formation boundary. This large negative carbon isotope excursion is interpreted to be the same excursion associated with the end-Permian extinction and this is in concert with the new high precision radioisotopic age data presented and the fact that the excursion lies within a normal polarity stratigraphic magnetozone. Dolomite cement δ13C values remain less negative (between about −5 and −8 permil) into the lower part of the Quartermaster Formation before becoming more positive toward the top of the section. This long interval of negative δ13C values in the Quartermaster Formation is interpreted to represent the earliest Triassic (Induan) inception of biotic and ecosystem “recovery.” Oxygen isotope values of dolomicrite cements show a progressive trend toward more positive values through the boundary interval, suggesting substantially warmer conditions around the end-Permian extinction event and a trend toward cooler conditions after the earliest Triassic. Our observations on these strata show that the paleoenvironment and paleoclimate across the Permian-Triassic boundary in western, sub-equatorial Pangea was characterized by depositional systems that were not conducive to plant preservation.