Chronology of deposition and unconformity development across the Cretaceous Paleogene boundary, Magallanes-Austral Basin, Patagonian Andes
Chronology of deposition and unconformity development across the Cretaceous Paleogene boundary, Magallanes-Austral Basin, Patagonian Andes
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DOI：
10.1016/j.jsames.2019.102237
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发表时间：
2020-01-01
影响因子：
1.8
通讯作者：
Clarke, Julia A.
中科院分区：
文献类型：
--
作者：
George, Sarah W. M.;Davis, Sarah N.;Clarke, Julia A.
马加兰-南方前陆盆地保存了智利和阿根廷巴塔哥尼亚安第斯山脉造山作用和地貌演变的重要记录。在整个弧后前陆盆地，一个几乎没有角度不整合的区域不整合将上白垩统-下古新世地层与历时始新世至中新世的上覆沉积分隔开来。本文报道了巴塔哥尼亚中南部中国河谷(S 50度-51度)含恐龙化石陆相和海相混合碎屑序列中11个砂岩样品的碎屑锆石U-Pb年代学结果和6个有机质样品的镜质组反射率数据，以：(1)利用U-Pb最大沉积年龄约束确定不整合的时间和持续时间；(2)重建沉积物的物源和扩散模式；(3)评估弧岩浆作用可能的时间变化；(4)评估不整合发展过程中沉积覆盖的量。(5)确认了南半球白垩纪-古近纪(K/PG)界线化石的存在。多罗泰组的样品获得了从马斯特里赫特时代到达尼安时代的最大沉积年龄(年龄类似于65-63 Ma)，证实了在最近发现的恐龙、其他陆生脊椎动物和植物化石的剖面中保存了K/PG边界。满艾克组直接位于不整合之上的样品，获得了始新世中期的最大沉积年龄(具有突出的45-40 Ma年龄组)，表明一个类似于20Myr断裂带的长寿命，代表了不沉积或侵蚀。对保存在最上部Dorotea组多个煤层中的有机质的分析表明，镜质组反射率值始终很低，需要有限的沉积埋藏，这与未沉积或沉积物绕过沉积一致，而不是沉积和后来侵蚀去除先前提议的古新世至始新世中期的厚厚的碎屑物质包。根据不整合时代和几何学的区域趋势、弧岩浆活动的时间和沉积物来源的时间变化，我们认为不整合成因的一系列潜在机制，包括(1)与前缘褶皱冲断带有关的缩短抬升，(2)挠性前隆的克拉通推进，(3)区域或海平面变化驱动的可容纳变化，(4)海脊碰撞和板窗成因，(5)构造平静(或小幅伸展)期间的均衡反弹，或(6)平板俯冲期间的区域前陆隆起。
The Magallanes-Austral foreland basin preserves an important record of orogenesis and landscape evolution in the Patagonian Andes of Chile and Argentina. Throughout the retroarc foreland basin, a regional disconformity with little to no angular discordance separates Upper Cretaceous-lower Paleocene strata from overlying deposits of diachronous Eocene to Miocene age. Here we report detrital zircon U-Pb geochronological results for 11 sandstone samples, and vitrinite reflectance data for 6 samples of organic matter from a fossiliferous dinosaur-bearing mixed nonmarine and marine clastic succession in the Rio de las Chinas valley (50-51 degrees S) of central-southern Patagonia to: (1) determine the timing and duration of the unconformity using U-Pb maximum depositional age constraints, (2) reconstruct sediment provenance and dispersal patterns, (3) assess possible temporal variations in arc magmatism, (4) evaluate the amount of sedimentary overburden removed during unconformity development, and (5) confirm the presence of a fossiliferous southern hemisphere Cretaceous-Paleogene (K/Pg) boundary site. Samples from the Dorotea Formation yield maximum depositional ages spanning Maastrichtian to Danian time (with ages as young as similar to 65-63 Ma), confirming preservation of the K/Pg boundary in a section with recently discovered fossils of dinosaurs, other terrestrial vertebrates, and plants. Samples from directly above the unconformity in the Man Aike Formation, yield middle Eocene maximum depositional ages (with a prominent 45-40 Ma age cluster), indicating a long-lived similar to 20 Myr hiatus representative of nondeposition or erosion. Analyses of organic matter preserved in multiple coal horizons of the uppermost Dorotea Formation show consistently low vitrinite reflectance values, requiring limited sedimentary burial, consistent with nondeposition or sediment bypass rather than deposition and later erosional removal of a previously proposed thick package of Paleocene to middle Eocene clastic material. On the basis of regional trends in the age and geometry of the unconformity, timing of arc magmatism, and temporal variations in sediment provenance, we consider a range of potential mechanisms for unconformity genesis, including (1) shortening-related uplift of the frontal fold-thrust belt, (2) cratonward advance of a flexural forebulge, (3) accommodation changes driven by regional or eustatic variations in sea level, (4) ridge collision and slab-window genesis, (5) isostatic rebound during tectonic quiescence (or minor extension), or (6) regional foreland uplift during flat-slab subduction.