Paleozoic Stratigraphy and Structure at Iron Point, Humboldt County, North-Central Nevada
AuthorKey, Erica Leanne
AdvisorCashman, Patricia H
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Detailed mapping and reconsideration of biostratigraphic data provide new insights into how the Comus Formation at its type locality at Iron Point, Humboldt County, Nevada fits into the regional stratigraphic framework. The age designation of the Comus Formation was reevaluated by this study using the most current understanding of Ordovician graptolite biostratigraphy. Previous studies at Iron Point had determined that the graptolites found in the siltstone units in the Comus Formation were middle Ordovician. This study determined that the species of graptolites found at Iron Point had been reclassified as late Ordovician since the original biostratigraphic study had been performed. A portion of the Vinini Formation at Iron Point was remapped in this study as the uppermost unit of the Comus Formation. Originally, this part of the Vinini Formation was mapped as a thrust “klippe” over the Comus Formation. This area originally mapped as “Vinini” is a very different rock type than the rest of the Vinini mapped at Iron Point. The lithology was much more similar to the underlying Comus Formation. Additionally, the geometry of the contact did not make sense; the thrust contact between the Vinini and the Comus was mapped as subhorizontal, however all of the strata dipped moderately to the west. The boundary between the "Vinini" and Comus Formation in this area was remapped and determined to be depositional, not structural. The Comus Formation in its type locality at Iron Point is not correlative with the "Comus Formation" that hosts Carlin-style gold deposits to the north in the Osgood Mountains. The Comus Formation at Iron Point is a sequence of interbedded carbonate and siliciclastic rocks deposited on the continental slope during the late Ordovician. The “Comus Formation” mapped in the Osgood Mountains is a sequence of carbonate, siliciclastic, and mafic volcanic rocks deposited on or near a carbonate seamount from the late Cambrian to late Ordovician (Hotz and Willden, 1964; Breit et al., 2005). The Comus at Iron Point and the “Comus” in the Osgood Mountains are composed of some similar types of Ordovician rocks, but their internal stratigraphy is too different to be classified as the same continuous unit. The Comus Formation at Iron Point is here interpreted to be correlative with the late Ordovician Hanson Creek Formation. These units have similar internal stratigraphy and timing of deposition. The Hanson Creek Formation was deposited on the continental shelf during the late Ordovician, and the Comus Formation is interpreted here to be the continuation of the Hanson Creek Formation onto the continental slope. A new unit composed of conglomerate, breccia, and a mature quartzite was identified at Iron Point underlying the Comus Formation. The quartzite portion of the unit was previously associated with the lower part of the Comus Formation, but the conglomerate and breccia were never recognized. The quartzite is composed entirely of quartz, and the conglomerate and breccia have a quartz sand matrix. The quartzite may be correlative with the middle Ordovician Eureka Quartzite. The Eureka Quartzite is the only widespread siliciclastic deposit on the continental shelf or slope during Paleozoic time. Additionally, the Eureka Quartzite underlies the Hanson Creek Formation and its correlative units in other areas in the Great Basin. Structural analyses using the new detailed mapping yielded evidence of six different deformational events at Iron Point. Their relative ages were determined through cross-cutting relationships and comparison to deformation recorded at Edna Mountain less than a kilometer east of Iron Point. The first fold set (F1) is west-vergent, and likely correlative to mid-Pennsylvanian folds observed at Edna Mountain (Villa, 2007; Cashman et al., 2011). F1 folds are asymmetric, steeply inclined, and locally overturned to the west. The second fold set (F2) records north-south contraction and is likely correlative to early Permian folds observed at Edna Mountain (Villa, 2007; Cashman et al., 2011). F2 folds are upright, symmetrical, and trend west-southwest. The King fault is a normal fault that strikes north-south and dips east. It post-dates the first two fold sets, and has not been active since the early Permian. The Silver Coin thrust strikes east-west, places the Vinini Formation over the Comus Formation, truncates the King fault, and is not affected by the first two fold sets. The West fault strikes southeast and dips southwest. The West fault truncates the Silver Coin thrust and juxtaposes the Comus and Vinini Formations in the footwall with the Cambrian Preble Formation in the hanging wall. Finally, Iron Point is bounded on the east side by the Pumpernickel fault, a normal fault that strikes north-south and dips east. The Pumpernickel fault Eureka Quartzite and Comus Formation in the footwall and the rock unit in the hanging wall is covered by Quaternary alluvium, so is not exposed. The movement on this structure is likely related to Basin and Range faulting starting in the Miocene.