Stratigraphy and sedimentary petrology of the Mascall Formation, eastern Oregon Public Deposited

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  • The type section of the Mascall Formation, which is located in the John Day Valley, is interpreted to represent a sequence of paleosols. These fossil soils were formed on a floodplain during the middle Miocene. The measured thickness of this section is 1340 feet, and although the top of the section is truncated by an erosion surface, the original thickness was probably not much more than 2000 feet. Sediment accumulation rates were high in the vicinity of the type section with deposits being predominantly of the overbank type. Minimum sediment accumulation time at the type section is thought to have been several hundred thousand years. A concretionary horizon, which occurs within the type section is determined to represent a significant temporal hiatus. Because of the absence of caliche in this layer and elsewhere in the type section, and because of the occurrence of moisture-loving plants, a wet, temperate climate is envisioned for the type section during the middle Miocene (Barstovian). The floodplain sediments of the type section are predominantly composed of ash which was produced by nearby silicic volcanism. This ash was mostly washed in from the surrounding highlands, but on occasion the floodplain was blanketed by air fall debris. Scanning electron microscopy demonstrates that this ash is of the type erupted by plinian and pelean type volcanoes. The ash has been mostly altered to clay minerals, and SEM, TEM, and XRD analyses, show these clays to consist principally of smectite (Ca, Mg), with lesser amounts of kaolin and tubular halloysite. Deposits west of Picture Gorge are predominantly of floodplain origin; however, a limited lacustrine sequence also occurs. East of the type section the floodplain deposits tend to become coarser and reflect main channel deposition. Farther to the east, near the mouth of Fields Creek a 300 foot thick lacustrine sequence occurs, representing a shallow eutrophic lake which was at least 3.5 miles in east-west dimension. Mascall deposits of the Paulina Basin also were formed in a floodplain environment. The area was characterized by slow sediment accumulation rates, and river meandering resulted in deposition of large tabular sandstone bodies. Meander loop cut-off probably occurred often, and as a result the floodplain was probably dotted with oxbow lakes. Volcanoes were active nearby, and on occasion covered the floodplain with pyroclastic debris. The Mascall Formation is believed to have been deposited only in the structural and topographic lows of the time. Present occurrences of Mascall rocks in the John Day Valley, Paulina Basin, Fox Basin, and Miocene rocks which may or may not be Mascall in the Bear Valley and Unity Basin, are not the remnants of a huge alluvial fan. Rather each of these structurally lows was filled in by sediment and pyroclastic material from their respective adjacent highlands. Pumices ranging from white to black in color, representing a zoned eruption, were collected from Mascall deposits in the John Day Valley. Chemical analysis of these pumices precludes magma mixing as a means for producing the zoned eruption. It is not known whether crystal fractionation or assimilation of wall rock material represented the mechanism involved. High K₂0 values in the Mascall pumice show that the magma had a continental source.
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