Introduction
Big Hole River Area of Block Mountain is situated in one of Montanas states – Madison Country. Currently Big Hole River area is surrounded by the Anaconda Range, the Beaverhead Range and the Pioneer Mountains. The mountain ranges have experienced Quaternary glaciations, which embedded the high location and hidden valley bottoms. The southwestern Montana Big Hole basin is north-south extensional basin, which is filled with approximately 5000m of under formed Cenozoic sediment. Beaverhead Range rocks are mostly Mesoproterozoic Lemhi Group argillites. The Anaconda Range contains muscovite and biotite units, which intruded Mesoproterozoic met sediments. The Pioneers Range rich for biotite Late Cretaceous granodiorites, which are related to Mesoproterozoic met sediments. Overall, the cenozoic tectonic, the regions sedimentology history, and the relation to the adjacent basins, has not been well understood because of the lack of exposure and the plain valley floor. In neighboring basins, stratigraphy, sedimentology, structural geology, fossil dating, zircon geochronology and ash-tuff geochronology have been used in order to recreate paleogeography, date sediments, and lift the extended episodes of deformation in eastern Idaho and southwestern Montana.
Sedimentary Units
Sedimentary Units can be divided to the following three types: Quadrant Dunwoody, which are marine; Morrison, which are non-marine; and Kootenai Black leaf, which are non-marine. Though 4850m deep, the sediments of the Big Hole River area is rarely outcrops and set only the top 200m of the basin, mainly produce Early Miocene and Oligocene fossils. Sedimentary units in these basins consisted of conglomerates, silts tuffaceous, calcareous muds, sands and megabreccias, which has been deposited in fluvial, alluvial, and lacustrine settings. Gravel, sand and clastic origin zircon provenance became an indicator of intra-basin sources derivation, in addition, there is no evidence of the river systems, which could connect the basins along extensional axis and transport the sediments among the basins.
Deformation and Post-Deformation History
It is known that the first stage of deformation has appeared in Late Jurassic Period and led to early Paleogene. The events in this period caused the deformation of Proterozoic and Phanerozoic layers. Early Cenozoic enlargement in Rocky Mountains in the north has immediately post-dated the middle Eocene Challis-Absaroka volcanism, furthermore in the late Eocene to early Miocene the syn-extensional basin has been filled in the southwestern Montana and eastern Idaho. Extremely hot hinterland Sevier gravitational collapse is mostly known to be a mechanism, which has initiated the Eocene Rockies expansion in the north. Some researches assume that the Renova Formation has been deposited in huge, sheet-like east flowing fan of alluvial and fluvial detritus, which was later dissected by faulting and uplift. Many scientists came to the conclusion that Revona Formations begun the first deformational process. This can be judged from the existence of the gneiss clasts in Middle Miocene gravels. Actually, Sixmile Creek Formation can be divided into Sweetwater Creek formation, Anderson Creek formation, and Big Hole River formation, which consisted of clast-supported cobbles and gravels, of distal and exotic origins from central Idaho. The late stage of deformation or post-deformation has provided the territory with these unique layers.
Igneous Geology
It has constrained by the strata increase, radiometric dating of igneous layers, and transverse field relationships. There is a huge possibility of existence of the gold and silver resorts in the evaluated area, it is common to the places with high metamorphic provinces or elevation igneous. The igneous deposits have been created out of mechanical erosion, turbulent stream transportation, and sedimentation in water weak, and increased roughness areas. Thus, the igneous surveys are very relevant in this area.