Burro Canyon Formation Research Articles

Hydrogeochemical evolution of formation waters responsible for sandstone bleaching and ore mineralization in the Paradox Basin, Colorado Plateau, USA

AbstractThe Paradox Basin in the Colorado Plateau (USA) has some of the most iconic records of paleofluid flow, including sandstone bleaching and ore mineralization, and hydrocarbon, CO2, and He reservoirs, yet the sources of fluids responsible for these extensive fluid-rock reactions are highly debated. This study, for the first time, characterizes fluids within the basin to constrain the sources and emergent behavior of paleofluid flow resulting in the iconic rock records. Major ion and isotopic (δ18Owater; δDwater; δ18OSO4; δ34SSO4; δ34SH2S; 87Sr/86Sr) signatures of formation waters were used to evaluate the distribution and sources of fluids and water-rock interactions by comparison with the rock record. There are two sources of salinity in basinal fluids: (1) diagenetically altered highly evaporated paleo-seawater-derived brines associated with the Pennsylvanian Paradox Formation evaporites; and (2) dissolution of evaporites by topographically driven meteoric circulation. Fresh to brackish groundwater in the shallow Cretaceous Burro Canyon Formation contains low Cu and high SO4 concentrations and shows oxidation of sulfides by meteoric water, while U concentrations are higher than within other formation waters. Deeper brines in the Pennsylvanian Honaker Trail Formation were derived from evaporated paleo-seawater mixed with meteoric water that oxidized sulfides and dissolved gypsum and have high 87Sr/86Sr indicating interaction with radiogenic siliciclastic minerals. Upward migration of reduced (hydrocarbon- and H2S-bearing) saline fluids from the Pennsylvanian Paradox Formation along faults likely bleached sandstones in shallower sediments and provided a reduced trap for later Cu and U deposition. The distribution of existing fluids in the Paradox Basin provides important constraints to understand the rock record over geological time.

Coarse-grained meandering distributive fluvial system of the basal Cedar Mountain Formation, U.S.A.

ABSTRACT The analysis of downstream changes in ancient fluvial systems can better inform depositional models for foreland-basin systems. Herein we analyze the basal deposits of the Early Cretaceous Cedar Mountain Formation of Utah to better understand the variety of fluvial deposits present and to develop a depositional model for the Sevier foreland basin. We also evaluate the long-held interpretation of a braided origin for these deposits and document numerous examples of point-bar deposition in highly sinuous meandering rivers by analysis of large (20 to 60 km2) plan-view exposures. These plan-view exposures allow comparisons between planform and cross-sectional geometries. The study utilizes outcrop data, virtual outcrop models, and satellite imagery to develop a facies model and analyze the architecture of channel bodies in the Buckhorn Conglomerate and Poison Strip Sandstone of the Cedar Mountain Formation. We document downstream (west to east) decreases in lateral channel migration, sinuosity, channel amalgamation, grain size, and percent of fluvial channel facies (conglomerate and sandstone). Fluvial channel deposits occur arranged into larger stratal bodies: multistory–multilateral channel bodies that are dominantly composed of clast-supported conglomerate in the west to a mix of multistory, multilateral, and isolated channel bodies composed of matrix-supported conglomerate in the east. The median width of highly sinuous point bars is similar across the field area (344 m to 477 m), but the inclusion of narrower (median = 174 m), low-sinuosity bar elements in the east indicates an overall reduction in lateral channel migration and sinuosity downstream. Net-to-gross values range from 100% in much of the western outcrops to as low as 38% in the east. Paleocurrent analysis reveals a transverse (west to east) paleoflow for the study interval that merges with axial (south–north) paleoflow near the Utah–Colorado state line. We estimate 104 m3/s-scale discharge and 106 kilometer-scale drainage area for axial rivers based on paleohydraulic analysis which represents a significant part of the Early Cretaceous continental-scale drainage. The observed downstream trends in lateral channel migration, sinuosity, channel amalgamation, grain size, and net-to-gross for the basal Cedar Mountain Formation are consistent with expected trends for sinuous single-thread distributive fluvial systems and are similar to observed trends in the Jurassic Morrison Formation. Medial (Buckhorn Conglomerate) to distal (Poison Strip Sandstone) zones are preserved and span the forebulge to backbulge depozones of a foreland-basin system. Postulated deposits of the proximal distributive fluvial system have been removed during erosion of the foredeep depozone. The easternmost Poison Strip Sandstone and coeval Burro Canyon Formation represent deposits of an axial system at which western-sourced distributive fluvial systems end. Distributive fluvial systems dominate modern foreland basins, and this study suggests that they may constitute a significant proportion of ancient successions.

Fluvial architecture and sequence stratigraphy of the Burro Canyon Formation, southwestern Piceance Basin, Colorado

The Lower Cretaceous Burro Canyon Formation in the southwestern Piceance Basin, Colorado, is composed of deposits that represent a braided fluvial system with high net to gross that transitions stratigraphically upward into a low net-to-gross, low-sinuosity, meandering fluvial system. The fluvial deposits are composed of multiple upward-fining, conglomeratic-to-sandstone successions forming bars and bar sets that exhibit inclined heterolithic strata that we have interpreted to have formed by oblique and downstream accretion. We used well-exposed outcrops, detailed measured sections, and unmanned aerial system-based imagery to describe the fluvial architecture of the Late Cretaceous formation using a hierarchical approach. We described the Burro Canyon Formation as comprising sandstone-rich amalgamated channel complexes (ACC) overlain by non- to semiamalgamated channel complexes. The lower interval of the formation is composed of ACC that contain channel-fill elements with cross-stratification and numerous truncated contacts. These stacked channel-fill elements exhibit an apparent width range of 137–1300 ft (40–420 m) and a thickness range of 5–60 ft (1.5–18 m). The upper interval of the Burro Canyon Formation comprises mudstone-prone intervals of the nonamalgamated channel complex with isolated channel-fill elements interbedded with floodplain mudstones that represent a period of relatively high base level. Associate channel fill elements range in apparent width from 200 to 1000 ft (60 to 300 m) and thickness from 20 to 30 ft (6 to 18 m). The characteristics and spatial distribution of architectural elements of the Burro Canyon Formation correspond to one depositional sequence. The erosional basal surface of the formation, as well as lateral changes in thickness and net to gross, suggest that the Burro Canyon Formation within this study area was deposited as an incised valley fill. Fluvial deposits of the Burro Canyon Formation serve as outcrop analogs for subsurface interpretations in similar reservoirs.

The Morrison Formation and its bounding strata on the western side of the Blanding basin, San Juan County, Utah

In 2016 and 2017, the Utah Geological Survey partnered with the U.S. Bureau of Land Management to conduct a paleontological inventory of the Morrison Formation south and west of Blanding, Utah, along the eastern margin of the Bears Ears National Monument. The Morrison in this region is critical to understanding Upper Jurassic stratigraphy across the Colorado Plateau because it is the type area for the Bluff Sandstone, Recapture, Westwater Canyon, and Brushy Basin Members of the Morrison Formation, which are the basis for nomenclature in New Mexico and Arizona as well. Researchers have disagreed about nomenclature and correlation of these units, which transition northward in the study area into the Tidwell, Salt Wash, and Brushy Basin Members. Numerous vertebrate localities make inclusion of the Bluff Sandstone and Recapture Members in the Middle Jurassic San Rafael Group, as suggested by some previous workers, unlikely. The Salt Wash Member does not separate the Bluff Sandstone and Recapture Members at Recapture Wash, but sandstone lenses of Salt Wash facies occur higher in northern Recapture exposures. Northward, along the outcrop belt east of Comb Ridge, the Bluff-Recapture interval thins, interlenses, and pinches out into the Tidwell and lower Salt Wash, with the main lower sandstone interval of the Westwater Canyon merging northward into the upper Salt Wash Member.
 The partly covered, 1938 type section of the Brushy Basin Member is identified along Elk Mountain Road at the southern end of Brushy Basin. We describe a detailed, accessible Morrison Formation reference section about 11.2 km (7 mi) to the south along Butler Wash. There, 81.68 m (268 ft) of Brushy Basin Member is well exposed along a road between the top of the Westwater Canyon Member and the base of the Lower Cretaceous Burro Canyon Formation. We informally call the upper sandstone bed(s) of the Westwater Canyon Member that cap mesas and benches in the region “No-Mans Island beds.” Smectitic mudstones between the No-Mans Island beds and the main sandstone body of the Westwater Canyon suggest that the Salt Wash-Brushy Basin contact to the north may be somewhat older than the base of the Brushy Basin Member as originally defined in its type area. Determining whether the No-Mans Island beds pinch out to the north or are removed by erosion below the regional basal Brushy Basin paleosol requires further research. Several significant fossil vertebrate and plant sites have been documented in the Brushy Basin type area. Newly identified volcanic ashes provided zircons for U-Pb ages of 150.67 ± 0.32 Ma from near the top of the Brushy Basin Member and of 153.7 ± 2.1 Ma and 153.8 ± 2.2 Ma for two zircons in lower part of Recapture Member. At the top of the Brushy Basin Member, ferruginous paleosols commonly overlying conglomeratic sandstone are speculated to be of Early Cretaceous age (detrital zircon age pending) and are assigned herein to the Yellow Cat Member of the Burro Canyon Formation. These iron-rich paleosols suggest wetter climatic conditions during the Jurassic-Cretaceous transition in the Blanding basin.

Fluvial architecture of the Burro Canyon Formation using unmanned aerial vehicle-based photogrammetry and outcrop-based modeling: Implications for reservoir performance, Escalante Canyon, southwestern Piceance Basin, Colorado

The Cretaceous Burro Canyon Formation in the southern Piceance Basin, Colorado, represents low sinuosity to sinuous braided fluvial deposits that consist of amalgamated channel complexes, amalgamated and isolated fluvial-bar channel fills, and floodplain deposits. Lithofacies primarily include granule-cobble conglomerates, conglomeratic sandstones, cross-stratified sandstones, upward-fining sandstones, and gray-green mudstones. To assess the effects of variable sandstone-body geometry and internal lithofacies and petrophysical heterogeneity on reservoir performance, conventional field methods are combined with unmanned aerial vehicle-based photogrammetry to create representative outcrop-based reservoir models. Outcrop reservoir models and fluid-flow simulations compare three reservoir scenarios of the Burro Canyon Formation based on stratigraphic variability, sandstone-body geometry, and lithofacies heterogeneity. Simulation results indicate that lithofacies variability can account for an almost 50% variation in breakthrough time (BTT). Internal channel-bounding surfaces reduce the BTT by 2%, volumetric sweep efficiency by 8%, and recovery efficiency by 10%. Three lateral grid resolutions and two permeability-upscaling methods for each reservoir scenario are explored in fluid-flow simulations to investigate how upscaling impacts reservoir performance. Our results indicate that coarsely resolved grids experience delayed breakthrough by as much as 40% and greater volumetric sweep efficiency by an average of 10%. Permeability models that are upscaled using a geometric mean preserve slightly higher values than those using a harmonic mean. For upscaling based on a geometric mean, BTTs are delayed by an average of 17% and the volumetric sweep efficiency is reduced by as much as 10%. Results of the study highlight the importance of properly incorporating stratigraphic details into 3D reservoir models and preserving those details through proper upscaling methods.

Fluvial architecture of the Burro Canyon Formation using UAV-based photogrammetry and outcrop-based modeling: implications for reservoir performance, Rattlesnake Canyon, southwestern Piceance Basin, Colorado

By Kelsey D. Lewis, Matthew J. Pranter & 2 more. Analysis of the fluvial architecture and facies heterogeneity of the Lower Cretaceous Burro Canyon Formation provides insight regarding their impact on reservoir performance.

First report on dinosaur tracks from the Burro Canyon Formation, San Juan County, Utah, USA – evidence of a diverse, hitherto unknown Lower Cretaceous dinosaur fauna

The newly discovered White Mesa tracksite in the Burro Canyon Formation represents a snapshot of a diverse, Lower Cretaceous dinosaur fauna from south-eastern Utah. The tracks were found at a construction site where the sandstone had been bulldozed and broken up. All tracks were found as deep, well-preserved natural casts on the underside of the sandstone slabs. Individual theropod tracks are 19–57 cm in length; one peculiar track shows evidence of a possible pathological swelling in the middle of digit III and an apparently didactyl track is tentatively assigned to a dromaeosaurid. Individual sauropod tracks are found with pes lengths of 36–72 cm, and interestingly, three distinct shapes of manus tracks, ranging from wide banana shaped to rounded and hoof-like. Ornithopods are represented with individual tracks 18–37 cm in length; a single track can possibly be attributed to the thyreophoran ichnogenus Deltapodus . Zircon U-Pb dating places the track-bearing layer in the Barremian, contemporary to the lower Yellow Cat Member of the Cedar Mountain Formation, which has a similar faunal composition based on both tracks and body fossils. This new track-fauna demonstrates the existence of a diverse dinosaurian assemblage in the lower part of the Burro Canyon Formation, which hitherto is not known to yield skeletalre mains.

Mid-Cretaceous alluvial-plain incision related to eustasy, southeastern Colorado Plateau

Eustatic effects on the deposition of ancient coastal and marine rocks are well known, but eustasy also can affect depositional patterns and processes well inland from the sea and play an important role in the development of nonmarine unconformities. In the southeastern part of the Colorado Plateau, fluvial rocks of the lowermost Cenomanian (lowermost Upper Cretaceous) Encinal Canyon Member at the base of the Dakota Sandstone fill paleovalleys incised into underlying formations. In most of region, the Encinal Canyon disconformably overlies fluvial rocks of the Aptian to middle Albian (upper Lower Cretaceous) Burro Canyon Formation. Along the southern margin of the plateau, the Encinal Canyon overlies progressively older Mesozoic and Paleozoic formations toward the south. In the latter part of the Early Cretaceous, an epicontinental sea lay about 240 km east of the southeastern Colorado Plateau and was base level for streams in the plateau region. Near the end of the Early Cretaceous, sea level fell, base level was lowered, and streams incised valleys into alluvial deposits of the Burro Canyon Formation and into older formations. The resulting incised paleodrainage surface was preserved as the sub-Dakota unconformity when the succeeding sea-level rise, in earliest Late Cretaceous time, caused Dakota streams to aggrade and backfill the paleovalleys with alluvial sediments of the Encinal Canyon Member. Unconformities in continental rocks have rarely been attributed to sea-level changes. The example of sub-Dakota unconformity, however, suggests that eustatic lowering of sea level can result in erosion far inland from the sea and play an important role in the formation of unconformities in continental strata.

Dakota Sandstone and Associated Rocks Adjacent to San Juan Sag Near Gunnison, Colorado: ABSTRACT

The stratigraphy and depositional systems of the Dakota Sandstone and associated rocks were studied in outcrop at the northeastern margin of the San Juan volcanic field in southern Gunnison and northern Saguache Counties, Colorado. This study fills in a major gap in regional Mesozoic stratigraphy and provides a last view of these rocks before they are concealed to the south by the volcanic cover of the San Juan sag, a frontier hydrocarbon basin. Locally, the Burro Canyon Formation is interpreted as a dominantly meandering fluvial system formed under oxidizing conditions similar to the underlying Morrison Formation. The Burro Canyon Formation pinches out along a roughly east-west line just south of US Highway 50 and is missing for about 15 mi south to the edge of the continuous volcanics. The Dakota Sandstone consists of a lower, low-sinuosity fluvial system abruptly overlain by an upper, mostly nearshore marine sequence which grades upward into the offshore Mancos Shale. While the total Dakota thickness is relatively constant, the fluvial system thickens markedly to the south where marine rocks are quite thin. To the north, marine rocks thicken irregularly and fluvial rocks are thin. In this area, the Mancos Shale can be readily subdivided intomore » Graneros, Greenhorn, Carlile, Juana Lopez, and upper Niobrara formations, although the Fort Hays limestone is locally missing. The upper Mancos Shale and Mesaverde Group are also missing, presumably due to late Eocene prevolcanic erosion.« less

Contrasting Styles of Fault Control on Position of Pre-Dakota Unconformity and on Depositional Environments of Dakota Sandstone, Northern San Juan Basin, Colorado: ABSTRACT

Detailed subsurface analyses of depositional environments and sandstone geometry of rocks from the upper part of the Upper Jurassic Morrison Formation through the Upper Cretaceous Dakota Sandstone indicate that faulting was a major control on distribution and preservation of lithofacies in the northern San Juan basin. The faults have a predominant northwest trend and were episodically active from Late Jurassic through early Late Cretaceous time. The main effect of faulting on the Morrison Formation and Lower Cretaceous Burro Canyon Formation was in the preservation and/or erosion of rock. These formations were uniformly deposited across the study area and then locally removed across horsts. Although depositional thinning of the Morrison occurs from west to east across the northern part of the basin, thickness differences (as much as 200 ft) between the Bridge Creek Limestone Member of the Mancos Shale and the Todilto Limestone of the Wanakah Formation cannot be solely attributed to regional thinning. Among closely spaced drill holes (1-3 mi apart) erosion is evidenced by thinning or local absence of the Burro Canyon and/or loss of section at the top of the underlying Morrison. The unconformity below the Dakota is not a simple regionally inclined surface from north to southmore » across the San Juan basin. Rather, its development in the northern part of the basin is more intense on uplifted areas among a series of parallel northwest-trending vertical faults.« less

Depositional Environments of the Dakota Sandstone and Adjacent Units in the San Juan Basin Utilizing Discriminant Analysis of Trace Elements in Shales

Thirteen chemical and ten mineralogical variables were evaluated as possible indicators of depositional environments. Samples from 300 shale and coal outcrops of the Morrison Formation (Jurassic), and the Burro Canyon Formation, Dakota Sandstone, and Mancos Shale (all Cretaceous) were collected at 29 localities in and north of the San Juan Basin. Rock samples of marine and nonmarine origin were selected as control groups on the basis of their stratigraphic positions and physical and paleontological indicators of environment. Discriminant analyses were performed on the control group samples to establish criteria for estimating marine or nonmarine character in samples from uncertain depositional environments. Discriminant analysis using both trace element and mineral variables effectively identified all of the samples in the control groups as marine or nonmarine. Discriminant analysis using chemical variables alone correctly identified 91 percent of the control group samples. The most significant chemical variables for distinguishing between marine and nonmarine depositional environments are phosphorus, vanadium, iron, chromium, nickel, and zinc. The minerals illite, dolomite, mixed-layer clay, plagioclase, and percent HCl soluble minerals were also significant in characterizing marine and nonmarine environments. The discriminant classification of the shales in the Dakota and lower Mancos interval in addition provided a tool for stratigraphic correlation of the various sandstone and shale members of this interval. Discriminant analysis helped correlate the Paguate Sandstone Member of the Dakota Sandstone northward along the east side of the basin, westward to near Cortez, Colorado, and around the west side of the basin to near the San Juan River in New Mexico. The Twowells Sandstone Member of the Dakota Sandstone was correlated as a series of discontinuous sandstone lenses that extend into the Four Corners region. The numerous pinch-outs of the Paguate and Twowells Sandstone Members offer stratigraphic trap opportunities for future oil and gas exploration in the San Juan Basin.

Paleocurrents of Jackpile Sandstone, Burro Canyon Formation, and Dakota Sandstone of San Juan Basin Region, New Mexico, Colorado, and Utah: ABSTRACT

Paleocurrents of Jackpile Sandstone, Burro Canyon Formation, and Dakota Sandstone of San Juan Basin Region, New Mexico, Colorado, and Utah: ABSTRACT

Age of Karla Kay Conglomerate Member of Burro Canyon Formation, Colorado and Utah: GEOLOGICAL NOTES

Age of Karla Kay Conglomerate Member of Burro Canyon Formation, Colorado and Utah: GEOLOGICAL NOTES

Relations of Lower Cretaceous and Upper Jurassic Rocks, Four Corners Area, Colorado

In many parts of the Colorado Plateau continuous conglomerate and sandstone lenses within the Burro Canyon formation (Early Cretaceous age) present a contrast to the dominantly mudstone upper part of the Morrison formation (Late Jurassic age). In the Four Corners area of southwestern Colorado, however, the lithologic character of the Burro Canyon is extremely variable, and in places the formation consists entirely of mudstone. The mudstone of the Burro Canyon formation and mudstone of the Brushy Basin member of the Morrison formation characteristically weather differently, the mudstone of the Burro Canyon to hackly fragments and mudstone of the Brushy Basin member to a hard frothy crust. This difference is believed due to original differences in composition rather than diagenetic processes, and it was found to persist throughout southwestern Colorado. The compensatory thickening of the Burro Canyon formation in relation to the Morrison formation, lateral change within short distances from mudstone of the Burro Canyon to mudstone of the Brushy Basin member, lack of evidence of a persistent unconformity between the Morrison and Burro Canyon, and the partial interbedding and intertonguing of the lowest conglomerate and sandstone of the Burro Canyon with mudstone of the Brushy Basin indicate that the Burro Canyon formation and Brushy Basin member of the Morrison formation are conformable and intertongue. The lack of fossil evidence prohibits precise placement of the Jurassic-Cretaceous time boundary. The lowest conglomerate in the Burro Canyon formation is part of a system of channel-filling conglomerate and conglomeratic sandstone lenses. The name Karla Kay conglomerate member is proposed for this system of lenses. A new sedimentary cycle accompanied by reduction in volcanic ash deposition is suggested by the Karla Kay conglomerate and adjacent mudstones of the Burro Canyon formation. The basal conglomerate of the Burro Canyon and Stokes' Buckhorn conglomerate member of the Cedar Mountain formation of central Utah may be lithogenic equivalents. The general decrease in the number of conglomerate and sandstone lenses southward in the Four Corners area of Colorado suggests that this area may be near the southern edge of deposition of the coarse clastic material of the Burro Canyon.

Contact of Burro Canyon Formation with Dakota Sandstone, Slick Rock District, Colorado, and Correlation of Burro Canyon Formation

Weathering of shales in the upper part of the Burro Canyon formation and the lower part of the overlying Dakota sandstone has made recognition of the disconformity between the formations difficult in the Disappointment basin area of the Slick Rock district, San Miguel and Dolores counties, Colorado. In lieu of the basal conglomerate of the Dakota sandstone, which marks the contact between the formations in the surrounding region, the presence of abundant carbonaceous material in shales of the Dakota sandstone differentiates them from the green shales in the Burro Canyon formation. The contact of the two formations, where clearly exposed in Disappointment basin, is conformable, sharp, and not gradational. Correlation of the Burro Canyon formation of western Colorado with the Cedar Mountain formation of central and eastern Utah is substantiated by the discovery in the Burro Canyon formation of two pelecypods, Protelliptio douglassi and Unio farri, with the conifer, Frenelopsis varians. The pelecypods also occur in the Kootenai-Cloverly fauna of Montana and Wyoming. The Kootenai-Cloverly fauna contains the pelecypod Eupera onestae which is found in the Cedar Mountain formation. The conifer also occurs in the Trinity group of Texas. The Trinity group contains the charophyte Clavator harrisi which is found in the Cedar Mountain formation.

DISCONFORMITY BETWEEN LOWER AND UPPER CRETACEOUS IN WESTERN COLORADO AND EASTERN UTAH

In a part of western Colorado and eastern Utah the contact between the Burro Canyon formation (Lower Cretaceous) and the Dakota sandstone (Upper Cretaceous) is a broadly undulating erosional surface. Angular and subangular pebbles, cobbles, and boulders of sandstone, chert, and quartzite derived from the Burro Canyon have been found in the basal part of the Dakota sandstone. These fragments and a thin quartzite bed at the top of the Burro Canyon indicate that the formation was lithified, probably under subaerial conditions, before its erosion and before deposition of the Dakota sandstone. On the basis of fossil plants R. W. Brown (1950) placed the boundary between the Lower and Upper Cretaceous at the top of the Burro Canyon. The field evidence described here supports this division and further suggests a hiatus between the deposition of Lower and Upper Cretaceous sediments in part of the Colorado Plateau.

Glauconitic Mica in the Morrison Formation in Colorado

AbstractGlauconitic mica occurs as interstitial clay in a bed of fine-grained sandstone, 15 feet thick, in the upper part of the Brushy Basin shale member of the Morrison formation of Late Jurassic age on Blue and Lone Tree Mesas near Uravan, Montrose County, Colorado. This glauconitic mica, similar to other material ordinarily called glauconite, is interesting because the Morrison formation is generally regarded as nonmarine in origin, whereas glauconite commonly forms in a marine environment.The identifying properties of the glauconitic mica are: strong 001 d spacing, 9.97A; very weak 002, 5.01A; strong 003, 3.34A; 060, 1.51A; green color, pleochroic; X and Z indices, approximately 1.578 and 1.600; SiO2 49 percent, Al2O3 18 percent, Fe2O3 13 percent, FeO 1.3 percent, MgO 2.8 percent, K2O 7.8 percent, TiO2 1.1 percent, ignition loss 6 percent, others approximately 1 percent.The glauconitic sandstone is underlain by variegated mudstones about 480 feet thick, which contain montmorillonite (with very sparse relicts of shards), illite, quartz, and also, in several layers, scanty analcime. The Burro Canyon formation of Early Cretaceous age overlies the glauconitic sandstone.This paper discusses alternative origins of the glauconitic mica possible within the framework of an appropriate chemical environment other than, and apart from, one typically marine.