Carl F. Vondra

Stratigraphy and Petrology of the Lower Eocene Willwood Formation, Bighorn Basin, Wyoming

The Willwood Formation (late Paleocene–early Eocene) consists of a complex association of marginal alluvial fan conglomerate grading basinward into stream channel and floodbasin deposits in the Bighorn Basin in northwestern Wyoming. Conglomerate beds are exposed primarily along the western flank of the basin. Conglomerate beds bordering the Beartooth Mountain front contain an assemblage of igneous, metamorphic, and sedimentary rock fragments reflecting a proximal source area. Willwood conglomerate beds along the Absaroka Mountains are composed of quartzite pebbles and cobbles derived from more distant westerly sources and recycled Fort Union Formation sediments. Willwood sandstone units are generally of a subarkose variety, with detrital grains consisting of quartz (68 percent), chert (13 percent), feldspar (10 percent), metaquartz (7 percent), and “heavy” minerals (2 percent). Sandstone textures average fine grained and moderately sorted. Willwood siltstone and clay-stone (mudstone) are mainly quartz, with illite and montmorillinite dominant and kaolinite subordinate among the clay minerals. Vertical changes in Willwood lithology include an upward increase in Precambrian metamorphic detritus, improved textural sorting, and the proportional increase in red beds. Willwood sandstone beds contain large- and small-scale cross-stratification, horizontal stratification, laterally extensive sand bodies indicative of flow regimes, and sediment accumulation processes characteristic of shallow gradient meandering streams. Localized channel fills record chute and neck cut-off during abandonment of a stream channel. Thin sheet sandstone (natural levees) and variegated mud-stone beds of the floodbasin record overbank deposition. Markov chain analysis of Willwood strata demonstrates the presence of repetitive depositional sequences that change at higher stratigraphic levels. Illite and montmorillinite clay minerals in the Willwood Formation, in association with subordinate amounts of kaolinite, indicate the absence of extensive lateritic weathering of source material in adjacent upland areas. Modification of bedrock appears analogous to a red-yellow Mediterranean type of soil formation. Modification of Willwood sediment at the site of deposition was greatly influenced by the development of appreciable alluvial relief through meander belt confinement. Red mud-stone shows low organic carbon amounts coupled with high free iron and manganese, an association expected of a relatively high-standing, well-drained (oxidizing) depositional site. Conversely, greenish-gray (drab) mud-stone shows high organic carbon amounts and low free iron and manganese levels, an association typical of low standing, poorly drained (reducing) sites of deposition. Detailed chemical analysis of a selected mudstone unit shows profile development of free iron, aluminum, and manganese, and unusually high levels of organic carbon for red mudstone. Such a unit is interpreted to be an immature soil developed on the Willwood landscape through leaching of mobile constituents and organic matter concentration.

Use of oxygen isotope ratios in correlation of tuffs, East Rudolf Basin, northern Kenya

Abstract Oxygen isotope determinations were made using CoF 3 to extract oxygen from 27 volcanic glass samples from the East Rudolf Basin, northern Kenya. Results show that the older tuffs are progressively enriched in 18 O and that this index can be used in the correlation of volcanic ash units. This method could not distinguish individual samples from the youngest units studied because their ranges of δ 18 O overlap. The δ 18 O values for the shards in the Tulu Bor Tuff, the KBS Tuff, the Koobi Fora Tuff and the Chari Tuff range from 14.5 to 16.4, from 8.9 to 9.5, from 6.6 to 7.0 and from 7.0 to 7.2, respectively, in decreasing age. Determinations from pumice cobbles are consistently higher than the above values.

Provenance of the plio-pleistocene sediments in the East Turkana Basin, northern Kenya

Abstract The fossiliferous Plio-Pleistocene fluvio-lacustrine sediments in the East Turkana Basin of northern Kenya were derived from two major source areas: (1) Ethiopian plutonic igneous and metamorphic rocks of the Precambrian basement and (2) Kenyan Cenozoic volcanic rocks. Interpretations of provenance are based on heavy mineral analyses of 123 sandstone samples and data collected during stratigraphic studies. Thirty heavy minerals have been identified in the East Turkana Basin and commonly compose 5–20% of the fine sand fraction which is compositionally representative of the entire heavy fraction. Morphological properties of the grains, geometric form, roundness and surface textures indicate that the mineral distribution primarily reflects source rocks lithology. Seven distinct heavy mineral associations have been distinguished which are grouped into two suites: a plutonic suite dominated by blue-green hornblende, apatite, sphene, garnet and clinozoisite-epidote and a volcanic suite dominated by augite and opaque minerals. Both suites form mappable heavy mineral provinces. Plutonic provinces occur throughout the basin interior, and volcanic provinces are distributed along the basin margin in association with alluvial fan deposits. Sedimentation began in the Pliocene following formation of the Turkana half-graben and the Chew Bahir (Stefanie) graben. The sediments are predominantly first cycle with limited reworking in fluvial and lacustrine environments. Two perennial stream systems transported plutonic detritus to the basin interior from basement rocks of high relief exposed to the northeast in the Chew Bahir basin and eastern Omo valley. Short ephemeral streams transported volcanic detritus to alluvial fans along the basin margins from volcanic rocks of low relief which border the basin to the east and south.

Mesotidal Barrier Complex, Sundance Formation, North-Central Wyoming: ABSTRACT

The sandstones and coquinas of the upper 20 m of the Sundance Formation are a tidal inlet, back-barrier shoal, and sandy tidal-flat sequence deposited at the close of marine Jurassic sedimentation in north-central Wyoming. The lateral migration of these interbarrier tidal inlets along the regressive shoreline of the late Sundance sea caused the coquinas and sandstones of the uppermost Sundance Formation to be deposited as tabular, laterally extensive units. Earlier models, which attach an offshore environment of deposition to this sequence, fail to explain the tabular gross geometry of the unit and its conformable stratigraphic relationship with the overlying nonmarine sediments of the Morrison Formation. Within the sandstone of the uppermost Sundance Formation, tidal bundles, sigmoidal reactivation surfaces, herringbone cross-lamination, and abundant mud drapes present considerable evidence for tidal influence during the deposition of the unit. The neap-spring cyclicity of the tidal bundles implies they were developed in a diurnal tidal setting. A meso-paleotidal range along the Late Jurassic shoreline is estimated, based on calculations of sediment transport rates during the tidal bundle development.

Stratigraphy of Upper Jurassic Morrison and Lower Cretaceous Cloverly Formations of Big Horn Basin, Northern Wyoming: ABSTRACT

The Morrison and Cloverly Formations in the Big Horn basin of northern Wyoming and southern Montana are part of a distal edge of a westward-thickening clastic wedge of sediments deposited in an elongate intracontinental basin in the western North American craton. These formations reflect orogenic and volcanic activity in the western Cordillera during Late Jurassic and the subsequent eastward migration of volcanic centers during Early Cretaceous. The Morrison Formation (Upper Jurassic) conformably overlies the Jurassic marine Sundance Formation and consists of light olive-green, lenticular, calcareous siltstones and mudstones interbedded with white to buff or yellowish green, massive and cross laminated, calcareous quartzarenites. The olive-green mudstones in the upper portion of the formation alternate with red-brown calcareous mudstones or shales producing red banding. A lenticular bed of siliceous accretionary lapilli is present in the upper portion of the formation along the west flank of Sheep Mountain anticline, north of Greybull, suggesting a closer proximity to volcanic vents than previously hypothesized. The Cloverly Formation (Lower Cretaceous) consists of three members: the Pryor Conglomerate, the Little Sheep Mudstone, and the Himes; it overlies the Morrison Formation both conformably and unconformably. Its basal contact is sometimes marked by a lenticular, cross-bedded, conglomeratic quartzarenite or a pebble conglomerate consisting of clasts of black chert (Pryor Conglomerate) derived from uplifted Paleozoic deposits to the west and deposited as channel lag. The Little Sheep Mudstone Member is composed of variegated mudstones which are generally noncalcareous bentonitic, and laced with abundant chalcedony and barite concretions and veinlets. The Himes Member also contains a stacked sequence of devitrified tuffs interbedded with bentonitic mudstones, which locally may be several m ters thick. The individual tuffs, however, are generally less than 1 m (3 ft) thick and are very fine grained, buff to white, and massive. Also, relatively thin lenticular lithic wackes occur in the Himes Member. These are anomalous to the quartzarenites of the rest of the Morrison and Cloverly. A thick lenticular, trough-cross-bedded, quartzarenite caps the formation. Both the Morrison and Cloverly Formations are characterized by high ratios of overbank fines relative to coarse channel sands. It has been assumed, but not documented by detailed sedimentologic study, that the deposits were part of an aggrading alluvial flood plain complex dotted by seasonal lakes and swamps and crossed by braided rivers. This model deviates from most modern braided systems which are characterized by rapid lateral mobility and the lack of fine-grained overbank material. The large ratio of fine-grained siltstones and mudstones to coarser grained sandstones can be explained by a number of processes, the most probable being rapid overbank aggradation as a result of a large influx of wind-blown volcanic material from vents to the west. However, this ratio could also be ob ained in a rapidly subsiding basin whose river systems are characterized by long periods between avulsion, which is common in an arid to semiarid environment. End_of_Article - Last_Page 1346------------

Translocation of Chemical Constituents and Pedogenesis in Early Eocene Willwood Formation in Northwestern Wyoming: ABSTRACT

Chemical data on a 4-ft red mudstone, in association with intense color mottling, high fossil content, and great lateral extent, indicate the development of a soil (paleosol) within the red-banded, fluviatile Eocene Willwood Formation in northwestern Wyoming. Quantitative analyses of free iron, aluminum, and manganese from a vertical section through the mudstone indicates zones of concentration at the 16-30 in. and 35-42 in. intervals from the top. Mudstone textural analysis indicates increased amounts of fine clay (less than 10 ^Phgr) in the 16-30 in. level. Organic carbon content throughout the profile is unusually high compared with similar Willwood mudstones. Carbonate minerals are essentially absent. The distribution of free aluminum through the profile displays a st ong statistical correlation (r = + .748) to fine clay distribution, whereas free iron (r = + .094), free manganese (r = + .160), and organic carbon (r = +.004) show essentially no correlation to fine clay content. Pedogenesis was characterized by organic matter concentration and mobile chemical constituent translocation to lower levels within a parent material of alluvium. Solution and movement resulted primarily from a shallow, fluctuating groundwater table, which produced alternating oxidizing and reducing conditions. Low concentration values and deeper movement of manganese compared with iron may reflect its higher solubility in the reduced state. Mobile ion concentration at the 35-42 in. level probably resulted from minor water table fluctuations directly above an underlying, more permeable sandstone. A significant reduction in the rate of sediment accumulation appears to be the major factor allowing for in-situ development of a soil upon the Willwood alluvial plain. End_of_Article - Last_Page 355------------