Field Test of Models of Sedimentation in Continental Half Grabens

一个有影响力的构造-地层模型表明，冲积扇相和湖相或轴流相在半地堑中的不对称分布主要受构造倾斜和这种倾斜所施加的集水区大小的控制。此外，在盆界正断层活动期间，由于上盘派生水道的切割、下盘派生水道的回填以及盆轴沉积体系的下盘向侵入，相带向下盘块体移动。后一种观点通常被认为是事实，但在很大程度上没有经过岩石记录的检验，而且关于相转移的规模、机制和速度，以及构造特征是否与其他外在变量的特征区分开来，还有许多问题有待解决。新墨西哥州里约热内卢大裂谷南部现存的帕洛玛斯半地堑是验证构造诱发相移假说的绝佳场所，因为(1)里约热内卢大裂谷及其支流早更新世后的沟槽暴露出了上新世-早更新世冲积扇和轴流地层，达100米；(2)暴露地层以高分辨率反转磁地层学的形式存在一个年代地层格架；(3)碳酸盐岩的稳定氧碳同位素组成提供了该地区在上新世和更新世时期的古气候记录，而在该地层的沉积过程中，海平面上升不是一个因素。对帕洛马斯盆地三维结构和跨盆地相对比的描绘，不仅将提供构造诱发相转移假说可行性的检验，而且将解决有关沉积体系跨盆地转移的过程和速率的许多问题。数据收集将包括(1)沿峡谷壁的25个测量剖面，5个切下的上盘流域和6个切下的下盘流域；(2)以1:12 000的比例在照片镶嵌上绘制出暴露良好的荒地区域的相图；(3)21个地层剖面的反转磁地层学。补充P.I.已经分析过的11个磁地层剖面，古地磁取样将在0.5-2米的垂直尺度上进行，将涉及沉积岩的标准钻孔和软沉积物取样技术。在古地磁取样期间，私家侦探将由加州大学洛杉矶分校的Stephen Salyards博士陪同，他将进行后续的实验室分析，其现场和实验室费用包括在本提案中。此外，在一些实地工作期间，私家侦探将由Michael Leeder博士（英国利兹大学）陪同，他将由NERC独立资助。

Mack 9627468 An influential tectono-stratigraphic model suggests that asymmetrical distribution of alluvial-fan and lacustrine or axial-fluvial facies in half grabens is primarily controlled by tectonic tilting and the size of catchment areas imposed by that tilting. Moreover, the model states that during periods of activity on the basin-bounding normal fault, the facies belts shift towards the footwall block, as a result of incision of hanging wall-derived drainages, backfilling of footwall-derived drainages, and footwall-directed incursion of the basin-axis depositional system. This latter idea is often accepted as fact, but is largely untested against the rock record, and many questions remain concerning the scale, mechanism and rate of facies shift, as well as whether the tectonic signature is distinguishable from that of other extrinsic variables. The extant Palomas half graben in the southern Rio Grande rift, New Mexico, is an excellent place to field test the hypothesis of tectonically induced facies shift, because (1) up to 100m of Pliocene-early Pleistocene alluvial-fan and axial-fluvial strata have been exposed by post-early Pleistocene entrenchment of the Rio Grande and its tributaries, (2) a chronostratigraphic framework exists for the exposed strata in the form of high-resolution reversal magnetostratigraphy, and (3) the stable oxygen and carbon isotopic composition of pedogenci carbonate provides a paleoclimatic record of the region in Pliocene and Pleistocene time, and eustacy was not a factor during deposition of the strata in question. Delineation of three-dimensional architecture and across-basin correlation of facies in the Palomas basin will not only provide a test of the biability of the hypothesis of tectonically induced facies shift, but will address many of the questions concerning the processes and rates at which the depositional systems shifted across the basin. Data collection will include (1) 25 measured sections along the canyon walls of 5 incised hanging wal l drainages and 6 incised footwall drainages, (2) facies mapping of regions of well exposed badlands at the scale of 1:12,000 and on photo mosaics, and (3) reversal magnetostratigraphy of 21 stratigraphic sections, supplementing the11 magnetostratigraphic sections already analyzed by the P.I. Paleomagnetic sampling will be on the vertical scale of 0.5-2 m and will involve standard drilling of sedimentary rock and a soft-sediment sampling technique. The P.I. will be accompanied during paleomagnetic sampling by Dr. Stephen Salyards (UCLA), who will perform the subsequent laboratory analyses and whose field and lab expenses are included in this proposal. In addition, the P.I. will be accompanied during some of the field work by Dr. Michael Leeder (University of Leeds, U.K.), who will be independently funded by NERC.