Gary D. Stricker

High chromium contents in Tertiary coal deposits of northwestern Washington — A key to their depositional history

Abstract Chromium contents obtained from 20 coal and 5 associated rock samples collected from the basal part of the Eocene Chuckanut Formation, in Skagit and Whatcom counties, northwest Washington, range between 30 and 300 ppm (mean 120 ppm whole-coal basis). The lenticular coals, ranging in rank from subbituminous to anthracite, and with an ash content of 12–46%, crop out along the western flank of the Cascade Range. Results of X-ray diffraction analysis of low-temperature ash show that the mineral matter in the coal samples consists predominantly of quartz and clay (kaolinite, illite and chlorite group). However, accessory minerals, isolated from the coal samples and analyzed by X-ray diffraction, scanning electron microscope and optical methods, contain angular fragments and euhedral crystals of the spinel group (chromite, magnetite and trevorite ), kaolinite-serpentine group (antigorite and chrysotile), chlorite group, amphibole group and pyroxene group minerals (augite, diopside and enstatite), all of which are commonly enriched in chromium. Although associated primarily with the inorganic fraction of the coal, concentrations of chromium in the samples show no statistically significant correlation with ash content. Localized concentrations of chromium in the coal are the result of natural contamination from the alteration of detrital chromium-bearing mineral grains introduced into the peat-forming mires from nearby Jurassic ophiolite bodies. The coals formed in the early Eocene, in rapidly subsiding small basins that developed during the uplift and erosion of the pre-Tertiary ophiolite terrain. Scattered bodies of source rock, random distribution of chromium-bearing minerals within the coal and sample heterogeneity account for the variation in Cr contents of the samples.

Coal in National Petroleum Reserve in Alaska (NPRA): Framework Geology and Resources: ABSTRACT

The North Slope of Alaska contains huge resources of coal, much of which lies within NPRA. The main coal-bearing units, the Corwin and Chandler Formations of the Nanushuk Group (Lower and Upper Cretaceous), underlie about 20,000 mi2 (51,800 km2) of NPRA. They contain low-sulfur, low-ash, and probable coking-quality coal in gently dipping beds as thick as 20 ft (6.1 m) within stratigraphic intervals as thick as 4,500 ft (1,370 m). Lesser coal potential occurs in other Upper Cretaceous units and in Lower Mississippian and Tertiary strata. The river-dominated Corwin and Umiat deltas controlled the distribution of Nanushuk Group coal-forming environments. Most organic deposits formed on delta plains; fewer formed in alluvial plain or delta-front environments. Most NPRA coal beds are expected to be lenticular and irregular, as they probably accumulated in interdistributary basins, infilled bays, or inland flood basins, whereas some blanket beds may have formed on broad, slowly sinking, delta lobes. The major controls of coal rank and degree of deformation were depth of burial and subsequent tectonism. Nanushuk Group coal resources in NPRA are estimated to be as much as 2.75 trillion short tons. This value is the sum of 1.42 trillion short tons of near-surface (< 500 ft or 150 m of overburden) bituminous coal, 1.25 trillion short tons of near-surface subbituminous coal, and 0.08 trillion short tons of more deeply buried subbituminous coal. These estimates indicate that the North Slope may contain as much as one-third of the United States coal potential. End_of_Article - Last_Page 677------------

Geochemistry of Coal from Cretaceous Corwin and Chandler Formations, National Petroleum Reserve in Alaska (NPRA): ABSTRACT

The Cretaceous, coal-bearing Corwin and Chandler Formations accumulated in two river-dominated deltas on the North Slope. The larger Corwin delta (Corwin Formation), in the western portion of NPRA, prograded northeastward and eastward, and the smaller Umiat delta (Chandler Formation), in the southeastern part of NPRA, prograded northward. Ninety coal samples from these formations within NPRA were collected and analyzed in order to evaluate coal quality and elemental distribution. Their apparent rank ranges from lignite A in the northern part of NPRA to high-volatile A bituminous coal in the southern part. Mean vitrinite reflectance values range from 0.65 to 0.74%. Some Corwin Formation coal samples west of NPRA have coking potential with free-swelling indexes between 3.0 and 5.0. Compared to other western United States Cretaceous coal, NPRA coal is significantly lower in ash, volatile matter, 0, Si, Al, Ca, Fe, Ti, Cu, F, Li, Mn, Mo, Pb, Sb, Se, Th, and Zn. Statistical comparisons of element concentrations indicate that the mean content of Si, Al, K, Li, Sc, Y, and Yb increases as the mean ash content increases (correl tion coefficient at least 0.7). Sulfur values are extremely low (0.1%), and elements that normally show positive correlation with sulfur, such as Fe, As, Cd, Co, Cu, Mo, Pb, and Zn, are also low. Therefore, coal from NPRA can be characterized by low ash and sulfur contents and low contents of elements of environmental concern, such as As, Be, Hg, Mo, Sb, and Se. The elements found to have positive correlations with ash content are probably present as aluminosilicate or stable oxide minerals. Variations in element content and quality of NPRA coal were probably influenced by the geochemical conditions that existed in the Corwin and Umiat delta systems. End_of_Article - Last_Page 655------------

Pre-Laramide Tectonics--Possible Control on Locus of Turonian-Coniacian Paralic Coal Basins, West-Central New Mexico: ABSTRACT

Published evidence indicates that Late Cretaceous shorelines trended northwest through west-central New Mexico and adjacent Arizona. Our investigations delineate these shorelines through time and relate them to the prominent northwest-trending monoclinal flexures in the Zuni and southwestern San Juan basins. We related the transgressive (T)-regressive (R) marine cycles (T2-R2, T3-R3, T4-R4) of C. M. Molenaar to deep-rooted monoclinal or asymmetric anticlinal structures. The T2-R2 turn-around is coincident with the Pinon Springs anticline in the northern part of the Zuni basin and appears to be controlled by the Atarque and Gallestina monoclines in the southern part of this basin. Shoreline configurations during the T3 and T4 transgressive maximums coincide with the axis o the Nutria monocline and relate to some subtle pre-Laramide movements along this structure. The R2 regression is unique to New Mexico, suggesting local tectonic control on the configuration of the seaway. The subsequent T3 transgression, which was a major widespread event elsewhere in the Western Interior, was abbreviated in west-central New Mexico near the location of the Nutria monocline. The T2-R2 through T4-R4 shoreline turnarounds produced numerous paralic basins favorable for the accumulation of organic detritus. A turn-around probably represents a period of slow rates of shoreline migration which allowed a thicker, more extensive accumulation of plant material and hence thicker coals. The present and most of the past coal production in the Zuni and southwestern San Juan basins is from coals formed in paralic basins just landward of the turnarounds caused by pre-Laramide tectonics. End_of_Article - Last_Page 868------------