Geologic Map of the Hogback Mountain Quadrangle, Lewis and Clark and Meagher Counties, Montana

The geologic map of the Hogback Mountain quadrangle, scale 1:24,000, was made as part of the Montana Investigations Project to provide new information on the stratigraphy, structure, and geologic history of an area in the geologically complex southern part of the Montana disturbed belt. In the Hogback Mountain area, rocks ranging in age from Middle Proterozoic through Cretaceous are strongly folded within and under thrust plates of equivalent rocks. Continental rocks of successive thrust plates have been telescoped eastward over a buttress of the stable continent. Erosional remnants of Oligocene andesitic basalt lie on highest surfaces eroded across the strongly deformed older rocks; younger erosion has dissected the terrain deeply, producing Late Tertiary and Quaternary deposits of alluvium, colluvium, and local landslide debris in the valleys and canyons. Different stratigraphic successions are exposed at different structural levels across the quadrangle. In the northeastern part of the quadrangle at the lowest structural level, rocks of the Upper Mississippian Big Snowy Group, including the Kibbey Formation and the undivided Otter and Heath Formations, the overlying Pennsylvanian Amsden and undivided Quadrant and Phosphoria Formations, the Ellis Group, and the Kootenai Formation, are folded and broken by thrust faults. The next higher structural level, the Avalanche Butte thrust plate, exposes strongly folded and, in places, attenuated strata of Cambrian (Flathead Sandstone, Wolsey Shale, Meagher Limestone, and undivided Pilgrim Formation and Park Shale), Devonian (Maywood Formation, Jefferson Formation, and most of the Three Forks Formation), and Mississippian (uppermost part of the Three Forks Formation and Lodgepole and Mission Canyon Limestones) ages. The overlying Hogback Mountain thrust plate contains strongly folded rocks ranging in age from the Middle Proterozoic Greyson Formation to the Upper and Lower Mississippian Mission Canyon Limestone and Cretaceous diorite sills. The highest structural level, the Moors Mountain thrust plate, contains the Middle Proterozoic Greyson and Newland Formations and discontinuous Upper Proterozoic diabase sills. Rocks are complexly folded and faulted across the quadrangle. At the lowest level in the northeastern part of the quadrangle, Upper Mississippian and younger strata are folded along northwest-trending axes and broken by thrust faults that at outcrop level displace the same rocks. The central core of the quadrangle is formed by the Avalanche Butte thrust plate, which contains recumbently folded and thrust faulted Paleozoic rocks. A succession of four tight recumbent folds within the plate have axial traces that trend northwest and north-northwest, and that are both arched and downfolded along east- and northeast-trending axes. Carbonate rocks of the Mission Canyon and Lodgepole Limestones in the upper part of the Avalanche Butte thrust plate exposed in the canyon of Trout Creek are folded and attenuated in stacked east-directed recumbent folds that developed as a succession of folded duplex thrust slices. The exposed remnant of the next higher structural level, the Hogback Mountain thrust plate, contains northeast- and east-trending folds that are inverted on the upper overturned limb of a younger northwest-trending recumbent fold. The Hogback Mountain thrust fault is itself folded and, in its northernmost exposures, is overturned to dip west beneath the overlying Moors Mountain thrust plate. During post-middle Tertiary deformation, the Hogback Mountain thrust fault moved as a normal fault, down on the east. The structurally highest Moors Mountain thrust plate rests on the Avalanche Butte thrust plate in the southwestern part of the quadrangle and across both the Avalanche Butte and Hogback Mountain thrust plates along the northwest edge of the quadrangle. In the central eastern part of the map area, the edge of a large klippen of the Moors Mountain thrust plate rests on a major inverted syncline of the underlying Hogback Mountain thrust plate. At the base of both the contiguous thrust plate and its klippen, the bounding Moors Mountain thrust fault ramps steeply upward from south to north across a structural culmination in underlying thrust plates. In the southwest corner of the quadrangle, strata of the Greyson Formation are faulted down on the west along a normal fault that, in the Nelson quadrangle adjacent on the west, is listric down to the west with the Moors Mountain thrust fault. Movement on the normal fault is younger than middle Oligocene.比例尺1:24000的霍巴克山图幅（Hogback Mountain quadrangle）地质图，是蒙大拿州调查项目（Montana Investigations Project）的成果之一，旨在为蒙大拿扰动带南部地质复杂区域的地层学（stratigraphy）、构造特征及地质演化历史提供全新资料。霍巴克山区域内，从中元古宙至白垩纪的岩石在同期岩石构成的逆冲岩片（thrust plate）内部及其下发生强烈褶皱。连续逆冲岩片的陆相岩石被向东推挤叠覆，覆盖于稳定大陆（stable continent）的推挡体之上。渐新世安山岩质玄武岩（andesitic basalt）的侵蚀残留体分布于强烈变形的古老岩石之上的最高侵蚀面；后期侵蚀作用对该区域进行深切剥蚀，在河谷与峡谷中形成了第三纪晚期及第四纪的冲积物（alluvium）、坡积物（colluvium）与局部分布的滑坡碎屑（landslide debris）堆积。 不同构造层次出露不同的地层序列。在本图幅东北部的最低构造层次中，上密西西比统比格斯诺伊群（Upper Mississippian Big Snowy Group）岩石——包括基比组（Kibbey Formation）、未分异的奥特组与希思组（Otter and Heath Formations）、上覆的宾夕法尼亚统阿姆登组（Pennsylvanian Amsden）及未分异的夸特兰组与磷灰岩组（Quadrant and Phosphoria Formations）、埃利斯群（Ellis Group）以及库特奈组（Kootenai Formation）——均发生褶皱并被逆冲断层（thrust fault）切割破坏。更高一级的构造层次——雪崩山逆冲岩片（Avalanche Butte thrust plate）——出露强烈褶皱且局部减薄的寒武系（Cambrian）地层（弗拉特黑德砂岩组、沃尔西页岩组、迈格灰岩组及未分异的皮尔格林组与帕克页岩）、泥盆系（Devonian）地层（梅伍德组、杰斐逊组及大部分三福克组）以及密西西比统地层（三福克组最上部、洛奇波尔灰岩与使命峡谷灰岩）。上覆的霍巴克山逆冲岩片包含从中元古宙格雷森组（Greyson Formation）至上、下密西西比统使命峡谷灰岩以及白垩纪闪长岩岩床（diorite sills）的强烈褶皱岩石。最高构造层次——穆尔斯山逆冲岩片（Moors Mountain thrust plate）——则包含中元古宙格雷森组与纽兰组（Newland Formation）以及不连续分布的上元古宙辉绿岩岩床（diabase sills）。 本图幅内岩石的褶皱与断裂作用整体十分复杂。在图幅东北部的最低构造层次中，上密西西比统及更年轻的地层沿北西向轴部发生褶皱，并被逆冲断层切割，这些断层在露头尺度上可使相同岩层发生位移。图幅的中央核部由雪崩山逆冲岩片构成，该岩片内发育平卧褶皱（recumbent fold）并被逆冲断层切割的古生界岩石。岩片内部发育4个紧密平卧褶皱，其轴迹走向为北西向与北西西向，且沿东向及北东向轴部兼具背斜拱升与向斜下弯的特征。在鳟鱼溪（Trout Creek）峡谷出露的雪崩山逆冲岩片上部的使命峡谷灰岩与洛奇波尔灰岩碳酸盐岩，发育一系列向东堆叠的平卧褶皱群，由一系列褶皱式双重逆冲岩片叠覆演化而成。下一级更高构造层次的出露残体——霍巴克山逆冲岩片——包含北东向与东向褶皱，这些褶皱在一期更年轻的北西向平卧褶皱的上部倒转翼部发生倒转。霍巴克山逆冲断层本身也发生褶皱，在其最北部的露头区域，断层发生倒转并向西倾斜，覆于上覆的穆尔斯山逆冲岩片之下。在中第三纪之后的构造变形阶段，霍巴克山逆冲断层作为正断层活动，东侧盘相对下降。构造位置最高的穆尔斯山逆冲岩片在图幅西南部覆于雪崩山逆冲岩片之上，并沿图幅西北边缘横跨雪崩山与霍巴克山两条逆冲岩片展布。在图幅中东部区域，穆尔斯山逆冲岩片的大型飞来峰（klippen）边缘覆于下伏霍巴克山逆冲岩片的一个大型倒转向斜之上。在连续逆冲岩片及其飞来峰的底部，限定性的穆尔斯山逆冲断层从南向北急剧抬升，切过下伏逆冲岩片中的构造高点。 在图幅西南角，格雷森组地层沿一条正断层西侧发生断陷；在西侧相邻的纳尔逊图幅（Nelson quadrangle）中，该正断层为向西变缓的犁式正断层，与穆尔斯山逆冲断层相接。该正断层的活动时代晚于渐新世中期。