Search
Search Results
-
The central part of the Bayhorse Mining District is located in the Salmon River Mountains in north-central Idaho between the towns of Challis and Clayton. The area is underlain by metasedimentary rocks ...
Citation Citation
- Title:
- Igneous petrology, structural geology, and mineralization of the central part of the Bayhorse Mining District, Custer County, Idaho
- Author:
- Hodges, Wade Allan
The central part of the Bayhorse Mining District is located in the Salmon River Mountains in north-central Idaho between the towns of Challis and Clayton. The area is underlain by metasedimentary rocks of Early to Middle Paleozoic age that were profoundly affected by the emplacement of a plutonic complex in Middle Cretaceous time, both of which were later intruded and covered by volcanic rocks of Early Tertiary age. Both intrusive events were accompanied by significant mineralization. These basement rocks and the associated mineral deposits have been partly exposed by post-Miocene uplift and subsequent glacial and deep stream erosion. The stratigraphic succession within the Bayhorse area consists of a series of alternating pelitic, carbonate and quartzite units that range from Latest Cambrian to Middle Ordovician in age. Five sedimentary rock units have been described within the area of study and consist, from oldest to youngest, of the Garden Creek Phyllite, Bayhorse Dolomite, Ramshorn Slate, mixed lithology sequence and Clayton Mine Quartzite. The lithologic and textural varieties of the Lower Paleozoic rocks, combined with regional considerations, collectively indicate that the Bayhorse area was transitional between marine shelf areas to the west, north and east and deeper miogeosynclinal areas to the south for most of Paleozoic time. Central Idaho was affected by magmatic activity continually from Late Jurassic to Middle Cretaceous time. Synchronous with the onset of batholithic scale magmatism was folding and thrust faulting of the Paleozoic sedimentary rocks. The magmatic activity culminated in the formation of the Idaho Batholith and related outlying plutons, one of which is locally represented by the Juliette Creek intrusive complex. Geologic evidence indicates that the Juliette Creek intrusive complex represents the upper parts of a much larger and somewhat deeper plutonic mass that was forcefully emplaced into the surrounding sedimentary rocks at depths ranging from 4 to 5 miles along anticlinal axes that paralleled the north-south structural grain of the region. In approximate order of emplacement the exposed part of the intrusive complex consists of quartz diorite, granodiorite grading to granite, and quartz-feldspar porphyry. The effects of thermal metamorphism were variably imposed upon the adjacent sedimentary rocks and the resulting changes in the lithologic characteristics of the country rocks aided in the modification of the pre-existing local structure by the forceful emplacement of the intrusive complex. Hydrothermal alteration and sulfide metallization are predominantly structurally controlled and spatially, temporally and probably genetically related to the Juliette Creek intrusive complex. Fluorite mineralization is related to the later igneous activity of Early Tertiary age. The emplacement of the intrusive complex was of major importance in preparing the ground for the two later episodes of mineralization by significantly altering the pre-existing local structure and lithologic characteristics of the sedimentary rocks The predominant structural feature of the district consists of two parallel elongate folds that formed in the Paleozoic sedimentary rocks by eastwardly directed compressional movement. Subsequent ethplacement of the Juliette Creek intrusive complex has locally modified the pre-existing structure and caused the sedimentary rocks to break along predictable zones of weakness. The sulfide metallization is related to the upper parts of a large hydrothermal system that may be associated with stock-work molybdenum or porphyry-copper type mineralization at depth. After this major period of magmatic, tectonic and hydrothermal activity the rocks of the district were again affected by a later, but similar sequence of events that culminated in the eruption of the rhyodacitic, andesitic and basaltic flows and pyroclastic deposits of the Challis Volcanics and the deposition of significant fluorspar in Early Tertiary time.
-
72. [Article] The Deschutes Formation-- High Cascade transition in the Whitewater River area, Jefferson County, Oregon
The Whitewater River area is located directly east of Mt. Jefferson in the Cascades of central Oregon. Approximately 90 mi2 (230 km2) were mapped (scale 1/24,000) and four new K-Ar ages and 151 major element ...Citation Citation
- Title:
- The Deschutes Formation-- High Cascade transition in the Whitewater River area, Jefferson County, Oregon
- Author:
- Yogodzinski, Gene M.
The Whitewater River area is located directly east of Mt. Jefferson in the Cascades of central Oregon. Approximately 90 mi2 (230 km2) were mapped (scale 1/24,000) and four new K-Ar ages and 151 major element analyses were obtained in a study of the stratigraphic and magmatic transition from the Miocene - Pliocene Deschutes Formation on the east to the Pliocene - Pleistocene High Cascades on the west. Deschutes strata in the Whitewater River area overlie late Miocene (8-11+ m.y.) andesites, dacites, and rhyodacites along an erosional unconformity. The oldest Deschutes rocks exposed in the Whitewater River area are approximately 6 m.y. old, and the youngest are probably between 4.5 and 5 m.y. old. The oldest High Cascade rocks exposed in the Whitewater River area are approximately 4.3 m.y. old. There is no evidence for a hiatus in volcanic activity between Deschutes and High Cascade time in the Whitewater River area. Late Pleistocene explosive volcanism, probably free Mt. Jefferson, is evidenced in a hornblende rhyodacite pyroclastic-flow deposit which occurs within the glacial stratigraphy and is tentatively thought to be between approximately 60,000 and 20,000 years old. Deschutes strata are dominated by pyroclastic lithologies (mostly ash-flow tuffs) with some lava flows and minor epiclastic sediment. Compositions range mostly between basaltic andesite and dacite. Many Deschutes-age rocks are aphyric, high in Fed, TiO2, and alkalies, and low in MgO, CaO, and A12O3. They define a tholeiitic trend extending at least from basaltic andesite to dacite that can largely be derived through fractional crystallization of plagioclase, olivine, magnetite, and clinopyroxene from a parent magma, probably of basaltic composition. These rocks are compositionally similar to "tholeiitic anorogenic andesites" that are most commonly associated with areas of crustal extension. Rocks of High Cascade age in the Whitewater River area are mostly lava flaws that range in composition from basalt (high-alumina, olivine tholeiite) to rhyodacite. The High Cascade suite forms a calc-alkalic association that is typical of subduction-related magmatic arcs. Fractional crystallization of the basalts leads to iron-enrichment. Fractional crystallization of the basaltic andesites might lead to calc-alkalic compositions, but the mineral phases necessary to deplete the magmas in FeO, TiO2, and CaO (magnetite and clinopyroxene) are not common phenocryst phases in the basaltic andesites or andesites. Two northwest-trending, down-to-the-west normal faults with sane possible strike-slip motion have been mapped in the upper Whitewater River area, directly west of Lion's Head. Motion on these faults occurred after approximately 4 m.y. ago, but probably began prior to that time. There is between 200 and 400 ft (60-120 m) of apparent vertical separation on the western side of these faults. There may be a large, northwest-trending fault running from the south end of Green Ridge, through Bald Peter and the Whitewater River area, but this structure is largely buried by younger volcanic rocks. There is no evidence for a northern extension of the north-trending Green Ridge faults, and there is no evidence for large structural displacement in the lower Whitewater River along north- or northwest-trending structures. The Deschutes Formation - High Cascade transition in the Whitewater River area is marked by a switch in the eruptive style and in the dominant magmatic compositions during Deschutes and High Cascade times. Volcanism in the Whitewater River area does not appear to have been episodic with respect to volume and/or intensity; rather, the character of magmatism has varied with time and with the tectonic style through the period immediately prior to and following the formation of the High cascade graben.
-
Sedimentary records from the North Atlantic, instrumental in the development of modern paleo-geomagnetic concepts, show a highly variable field even during times of constant polarity. Yet, our understanding ...
Citation Citation
- Title:
- Deep-sea sediment paleomagnetism : a case study from the North Atlantic
- Author:
- Strano, Sarah Elianna
Sedimentary records from the North Atlantic, instrumental in the development of modern paleo-geomagnetic concepts, show a highly variable field even during times of constant polarity. Yet, our understanding of how the magnetization is acquired in the sediments is poorly understood. Primary magnetizations preserved in deep-sea sediments are known to be acquired through a depositional or possibly, a post-depositional remanent magnetization (DRM or pDRM). A pDRM process implies that the magnetization is locked-in at depth creating an offset between the age of the magnetization and the age of sediment. The process is not currently accounted for in paleomagnetic records despite the wide use of magnetic records to elucidate the timing and rate of change of many paleomagnetic and environmental processes. This dissertation uses seven Northern North Atlantic (NNA) deep-sea sediment cores that were studied by alternating field demagnetization of natural and laboratory imposed remanence on uchannel samples, providing for detailed paleomagnetic and environmental magnetic records. These high-quality Holocene and deglacial magnetic data are combined with independent radiocarbon chronologies to better understand the: (1) magnetic acquisition process, (2) the NNA paleo-geomagnetic signal and (3) the influence of rock magnetic parameters on the sedimentary paleomagnetic record. Under the traditional paradigm of magnetostratigraphy, sediment deposition and magnetization are assumed to occur synchronously and with little to no signal attenuation. In Chapter 2, we compare independently dated Holocene paleomagnetic records from the seven deep-sea sediments cores across the North Atlantic with regional paleo-geomagnetic reconstructions derived from ultra-high resolution sediment records. We find variable delays between the timing of these records, consistent with a magnetization “locked-in” at depth and over an interval that results in smoothing of the geomagnetic signal. Optimization modeling of the post-depositional remanent magnetization (pDRM) accounts for both offset and some of this smoothing. It also demonstrates that the preserved magnetization is acquired ~20 cm below the sediment-water interface. Consistent with previous observations, this potentially ubiquitous process results in age offsets of 350-2000 years even in deep-sea sediment accumulation rates in excess of 10 cm/kyr that is rarely if ever accounted for in magnetostratigraphy or paleomagnetic records. In Chapter 3, we assume that the new pDRM-corrected chronologies developed for Chapter 2 more accurately represent each paleomagnetic record and create a NNA stack of both direction and intensity from ~15,000 years ago to present (NAPstack15). Uncertainty analyses and comparison to data derived from global field models at the same locations suggest that both, our directional and intensity stacks robustly capture the evolution of the mean geomagnetic field variations of the NNA. Broader regional comparisons with data from North America and Europe begin to define the evolution of the geomagnetic field during this time interval. Geomagnetic morphology and spatial/temporal variability can be roughly broken into three time intervals consistent with the evolution of global intensity and implicate the dynamics of the high-latitude Holocene flux patches as a source of this variability. We evaluate the effect of rock magnetic properties on the fidelity of the NNA paleomagnetic record from the Holocene through the last deglaciation. NNA records have been argued to consistently record high-quality paleomagnetic records over millennial to orbital time scales with little concern for lithologic variability resulting from glacial-interglacial environmental changes. We find that rock magnetic variability has little effect on the fidelity of the NNA's directional record, but has a variable and sometimes large influence on normalized remanence records, which are commonly used as a relative paleomagnetic intensity proxy. We find that the eastern NNA records are most affected by the use of different normalizers during the Holocene. The western NNA cores are more affected by the use of different normalizers during the deglacial period but to a lesser extent. The Iceland Basin cores are an exception, providing consistent normalized remanence records regardless of the normalizer during both the Holocene and deglacial interval. This likely reflects their proximity to Icelandic basaltic sources and the consistent magnetite grain-size regardless of physical grain-size that these sources provide.