Hermosa Formation Research Articles

Detrital Zircon Geochronology of Pennsylvanian-Permian Strata in Colorado: Evidence for Appalachian-Derived Sediment and Implications for the Timing of Ancestral Rocky Mountains Uplift

Pennsylvanian-Permian time in north-central Colorado corresponds with uplift of the Ancestral Front Range and deposition of the Fountain, Ingleside, and Lyons Formations along its flanks. In southwestern Colorado, deposition of the Molas and Hermosa Formations along the flanks of the Uncompahgre Highlands largely represents Pennsylvanian time. We present new detrital zircon U-Pb geochronology data for the Ingleside and Lyons Formations in north-central Colorado and the Molas and Hermosa Formations in southwestern Colorado to understand sediment provenance and dispersal patterns. We determined U-Pb ages using LA-ICPMS on 120-150 zircon grains from five sandstone samples collected from shallow marine and eolian facies within the Ingleside, Lyons, Molas, and Hermosa Formations. All sandstone samples display a mixed Laurentian derivation, with age populations that record local and distal sediment sources. All samples also contain between 5% and 10% concordant Paleozoic-age zircon grains ranging from 330–490 Ma, coinciding with high magmatic flux during the Taconic and Acadian orogenies in the Appalachian orogen. Ultimate derivation from the Appalachians are also interpreted for zircon age populations ranging from 500-750 Ma and 1000-1300 Ma that likely originated from Pan-African and Grenville terranes respectively. This study detects the earliest documented appearance of Paleozoic zircons along the northern Ancestral Front Range, corresponding to deposition of the lower Ingleside Formation. We compare our data along the Front Range to previous detrital zircon studies from the underlying Fountain Formation to conclude that the Fountain-Ingleside transition was accompanied by a decrease in locally sourced detrital zircons, most likely marking the cessation of Ancestral Front Range uplift. Conversely, deposition across the Molas-Hermosa contact in southwestern Colorado was accompanied by an increase in locally-sourced detrital zircon grains, most likely marking the initiation of the Uncompahgre uplift.

The late Neoproterozoic Sierra de las Ánimas Magmatic Complex and Playa Hermosa Formation, southern Uruguay, revisited: Paleogeographic implications of new paleomagnetic and precise geochronologic data

A new paleomagnetic study was carried out on 15 sites of the Sierra de las Ánimas complex, exposed in SE Uruguay. A paleomagnetic remanence was isolated at six sites, and this direction is likely to be pre-tectonic and primary. Mean directions from five of these sites are consistent with the SA2 paleomagnetic pole, which is based on a previous paleomagnetic study of the complex. A new paleomagnetic pole was computed for compiled results from the Sierra de las Ánimas complex (SAn, 12.2°S, 258.9°E, A95: 14.9°). A geochronological study based on U/Pb (SHRIMP) dating of magmatic zircons was also carried out on three samples of the Sierra de las Animas Complex, yielding a grand mean age of 578±4Ma. The new SAn paleomagnetic pole anchors the Early Ediacaran segment of the Apparent Polar Wander Path for the Río de la Plata craton. Consistency of the new pole with other Ediacaran poles from the Congo-São Francisco craton suggests a coherent Central Gondwana by this time. These data also support a wide Clymene Ocean between the conjoined Río de la Plata–Congo-São Francisco blocks and Western Gondwana, i.e., Amazonia–West Africa. Additional sampling (sixteen samples) was carried out on the basal levels of the glaciogenic Playa Hermosa Formation, exposed in the same area. These samples retain a paleomagnetic component consistent with the previously reported data, confirming the original pole position. U–Pb (SHRIMP) dating of detrital zircons reveal a bimodal distribution of Paleoproterozoic (around 2.1Ga) and late Neoproterozoic ages. The youngest zircons are interpreted as pre-dating to penecontemporaneous with deposition and points to a ≤594Ma age for deposition of the older levels of the Playa Hermosa Formation. A minimum age of deposition is set by the 578±4Ma age of the Sierra de las Ánimas magmatism. The low inclination of magnetization indicates that low-latitude (13.0°+9.5°/−5.5°), Gaskiers-aged glacial deposits are preserved atop the Rio de la Plata craton.

Chapter 52 The Playa Hermosa Formation, Playa Verde Basin, Uruguay

Abstract The Playa Hermosa Formation (Fm.) (Playa Verde Basin) is a volcano-sedimentary unit that crops out in the extreme south of the Dom Feliciano Belt in Uruguay. This formation has traditionally been interpreted as non-glacial in origin, but recently it has been suggested that the lower part at least may be glacially influenced. The stratigraphic position of the Playa Hermosa Fm. and its correlation with other Neoproterozoic units of Uruguay, Argentina and southern Brazil remains in dispute. An age of c. 580 Ma is indicated by a hornblende 39 Ar/ 40 Ar age on magmatism regarded as coeval with syn-sedimentary volcanism. Preliminary palaeomagnetic data suggest a primary remanence and a mean geomagnetic pole consistent with the proposed apparent polar wander path for the Río de La Plata craton. In combination, these data suggest mid-Ediacaran glacial sedimentation in low to intermediate latitudes on the Río de la Plata craton. However, this conclusion needs to be reinforced by more thorough studies of the age and origin of the Playa Hermosa Fm.

Chapter 53 Las Ventanas and San Carlos formations, Maldonado Group, Uruguay

Abstract Together with the Playa Hermosa Formation (Fm.), the Las Ventanas and San Carlos formations constitute the Maldonado Group, which is better developed in the southeastern part of Uruguay and covers an area of c. 200 km 2 . The total thickness of both units (i.e. Las Ventanas and San Carlos formations) reaches c. 1500 m, and comprises mafic and acidic volcanic rocks, pyroclastic rocks, diamictite, sandstone, conglomerate and pelite. Structurally, the Maldonado Group is extensively deformed, although variably, throughout the region. Strike–slip faults, westward-verging detachment faults, and folds with axis sub-parallel to the strike–slip planes are common features. The presence of pumpellyite, prehnite, chlorite and epidote in mafic rocks indicates very low- to low-grade metamorphic conditions. The Las Ventanas Fm. is characterized by basal conglomerate, diamictite, sandstone and siltstone that pass upwards into fine-grained rhythmites (pelite), and is the thickest unit of the Maldonado Group ( c. 1250 m). The San Carlos Formation ( c. 250 m) comprises fine-grained conglomerate, sandstone and mudstone towards the top. Both units lie on an angular unconformity above Palaeo- and Neoproterozoic basement and are overlain unconformably by late Ediacaran–lowermost Cambrian units. Reliable palaeomagnetic data indicate that the Maldonado Group accumulated at high palaeolatitudes; however, the palaeogeographical evolution of the Río de la Plata Craton during the Neoproterozoic remains conjectural. Radiometric data from intrusive bodies and cross-cutting strike–slip faults place the minimum age of the group at c. 565 Ma, whereas basement volcanic rocks dated at 590 ± 2 Ma interbedded with meta-sandstones hosting detrital zircons c. 600 million years old provide the best constraint on the maximum age of deposition. Given the absence of carbonate rocks, no chemostratigraphic studies (e.g. C, O, Sr) are available. The Las Ventanas and San Carlos formations are largely interpreted as units within a thick glacially influenced fan-delta sedimentary system formed during the early Ediacaran in a strike–slip basin. Based on stratigraphic and sedimentological characteristics it has been suggested that this succession, containing glacially influenced diamictite and dropstones, records a glacial period that occurred sometime between c. 570 and 590 million years ago. Ongoing research is focused on establishing the precise age of deposition of the Maldonado Group and on reconstructing the tectonic evolution of the basin. Further palaeomagnetic studies will be especially useful for determining the palaeogeography of the Río de la Plata Craton during the Ediacaran and establishing its relationships with neighbouring strata hosting similar successions.

Neoproterozoic subaqueous extrusive–intrusive rocks in the Playa Hermosa Formation in Uruguay: Regional and stratigraphic significance

Volcanic rocks in southern Uruguay are linked to an extensional tectonic regime related to the post-collisional Brasiliano–Pan African orogenic cycle. The ca 580 Ma magmatic event is recorded in the Playa Hermosa Formation. In the upper section of this formation several units were defined. The basal unit is a quartz–syenite brecciated deposit, interpreted as the result of explosive episodes of shallow plutonic quartz–syenite intrusion with signs of hydrothermal alteration. Sedimentary and basaltic lithoclasts are occasionally present in the breccia, implying the emplacement of basalts before of that magmatic event. The middle unit is composed of trachytic hyaloclastites. Local distribution and textural evidence indicate that trachytic volcanic pulses have been followed by an explosive event that produced brecciated deposits. The upper unit of the studied sequence is composed of basalts with thin sedimentary intercalations. Most of the outcropping basalts display the typical micro- and macro-features of hyaloclastites; distinctive of extrusions occurred in wet sediments previous to lithification. Basaltic feeder dikes are common in the lower unit. The study of flow structures in the quartz–syenite brecciated deposits led to the recognition of two basaltic inputs. Such differences are related to the intensity of chloritic alteration, which would be related to water contamination. Basaltic peperites are one of the most impressive features of the Playa Hermosa Formation. Hyaloclastites and peperites largely represented among the studied volcanic rocks, and devitrification textures are all evidences of magma/seawater interactions. In the basaltic fragments of the peperites well preserved orange palagonite appears, while the sandstone fractions have lithic clasts of felsic volcanic rocks, such as rhyolitic lavas with perlitic cracks and lithophysae-like structures. Palagonite chemical analyses suggest marine environment and, at the same time, it proves that the studied section lacks of regional metamorphism. The felsic volcanic lithoclasts contained in sandstones could be related to the felsic volcanic lobes intercalations. The studied magmatic event is important in order to formulate correlations with other Neoproterozoic units distributed around the Dom Feliciano belt, like Camaquã and Campo Alegre basins (Brazil), and in the Kaoko belt (NW Namibia) in Africa. Also, Playa Hermosa Formation presents glacigenic features that are well documented, and reinforce the volcanism around 580 Ma in the Rio de la Plata Craton.

Integrated loessite-paleokarst depositional system, early Pennsylvanian Molas Formation, Paradox Basin, southwestern Colorado, U.S.A.

Mississippian paleokarst served as a dust trap for the oldest known Paleozoic loessite in North America. The early Pennsylvanian Molas Formation consists of loessite facies (sorted, angular, coarse-grained quartz siltstone), infiltration facies (loess redeposited as cave sediments within paleokarst features of the underlying Mississippian Leadville Limestone), colluvium facies (loess infiltrated into colluvium surrounding paleokarst towers) and fluvial facies (siltstone-rich, fluvial channel and floodplain deposits with paleosols). The depositional system evolved from an initial phase of infiltration and colluvium facies that were spatially and temporally related to the paleokarst surface, to loessite facies that mantled the paleotopography, and to fluvial facies that were intercalated with marine-deltaic rocks of the overlying Pennsylvanian Hermosa Formation. This sequence is interpreted as a response to the modification of the dust-trapping ability of the paleokarst surface. Loess was initially eroded from the surface, transported and redeposited in the subsurface by the karst paleohydrologic system, maintaining the dust-trapping ability of the paleotopographic surface. Later, the paleotopographic surface was buried when loess accumulation rates exceeded the transport capacity of the karst paleohydrologic system. These changes could have occurred because of (1) increased dust input rates in western Pangaea, (2) rising base levels and/or (3) porosity loss due to deposition within paleokarst passageways.

Southernmost Exposures of the Arroyo del Soldado Group (Vendian to Cambrian, Uruguay): Palaeogeographic Implications for the Amalgamation of W-Gondwana

The occurrence of the Vendian to lowermost Cambrian Arroyo del Soldado Group (ASG) is reported from an area located to the SW of Minas, which was formerly mapped as part of the Lavalleja Group. The Yerbal, Polanco and Cerro Espuelitas Formations of the ASG occur there as a rather continuous, microfossil-rich sedimentary cover, intruded by granites of probable Cambrian age (Minas Granite). Organic-walled microfossils recovered from the Yerbal and Polanco Formations include Bavlinella faveolata, Leiosphaeridia tenuissima, Soldadophycus bossii, Soldadophycus major, Siphonophycus solidum, Glenobotrydion aenigmatis, and filament mats. The acritarch Dyctiotidium sp. is described for the first time from the Yerbal Formation. Essentially the same microfossil assemblage occurs in five samples of this unit collected immediately to the north of Minas. No identifiable microfossils were found in fifteen samples of organic-rich lithologies of the Lavalleja Group. Based on detailed geological mapping, litho- and biostratigraphic data, we show that the contact between the Arroyo del Soldado and Lavalleja Groups is of tectonic nature (Arroyo La Plata Lineament), the latter being pre-Vendian in age. The Villa Serrana Block is defined, and consists of the Carapé and Lavalleja Groups, Las Ventanas and Playa Hermosa Formations, an unnamed siliciclastic-ultrabasic succession showing low-grade metamorphism, and various granitic intrusions mostly of Neoproterozoic to Cambrian age. Its boundaries are the Sarandí del Yí-Piriápolis shear zone to the W, the Sierra Ballena shear zone to the E, the Arroyo La Plata Lineament to the N, and the Río de la Plata to the S. The Villa Serrana Block could be an allochthonous terrane, or alternatively, a part of the Nico Pérez Terrane that was thrusted onto the ASG. No continuity exists between the Brusque and Porongos groups of southern Brazil and the Lavalleja Group. According to available data these units are probably not coeval, and do not represent the suture between the cratonic areas to the W (Río de la Plata Craton) and the remnants of a Neoproterozoic magmatic arc to the E (Cuchilla Dionisio-Pelotas Terrane). Accretion of these blocks was controlled by sinistral, continental-scale megashears. The geology of the area is far too complex to be explained in terms of a single orogeny following one Wilson cycle.

TEMPESTITOS COM INFLUÊNCIA GLACIAL DA FORMAÇÃO PLAYA HERMOSA (NEOPROTEROZÓICO), PIRIÁPOLIS, URUGUAI

Storm-wave influenced glacimarine deposits of the Playa Hermosa Formation (Neoproterozoic) crop out along the beaches of Playa Grande and Playa Hermosa (town of Piriapolis, southern Uruguay). These deposits are characterized by fining-upward cycles composed, from the base to the top of successions of: (i) conglomerates and sandy-rhythmites in decimetric layers showing similar processes of rain-out, which pass to, (ii) finely laminated and rippled centimetric sandstones with deformed beds and outsized clast up to 71cm of size, with associated bending and rucking structures, (iii) well-sorted fine centimetric sandstones with swaley and hummocky cross-stratification and wave ripples interpreted as sandy tempestites which are recovered by (iv) finely laminated rhythmtes affected by intrusions of trachytes, syenites and rhyolites of the Sierra de Animas Suite (570-550 Ma). The main evidences for a glaciomarine origin includes: (i) the confirmed presence of a dropstone in fine-grained sediments indicating a hydrodynamic paradox, (ii) the presence of massive conglomeratic sandstone facies formed by rain-out processes and (iii) according to specialized literature the paleomagnetic data indicative of a low to intermediate position for the Rio de La Plata Craton in the Neoproterozoic. The loneclast was interpreted as a dropstone due to the anomalous size, the hydrodynamic paradox and the composition, farther that other forms of transport are rejected here ( e . g . organic transport).

The Record of the Varanger Glaciation at the Río De La Plata Craton, Vendian-Cambrian of Uruguay

A sedimentary succession included in the lower section of the Playa Hermosa Formation from the Playa Verde Basin, Uruguay, is described. Two facies associations, one mainly coarse- to medium-grained and other one fine-grained, have been defined (FA I-II). In the first one, breccias, conglomerates, sandstones and minor mudstones were deposited in a subaqueous depositional setting (proximal) suggesting slope instability and resedimentation processes. The second one contains diamictites, rhythmites, sandstones and mudstones and presents abundant evidence of soft-deformation, also interpreted to be deposited in a subaqueous environment (distal). Dropstones, clast layers, diamictites, rhythmites and varve-like deposits are interpreted as ice rafting processes generated during a glacial episode. This glacial-related succession constitutes the first record from the Varanger glaciation at the Río de la Plata Craton of the late Neoproterozoic age and also represents one of the oldest sedimentary records after the collision of the Río de la Plata and Kalahari Cratons. A combined interaction of extensional faulting and glaciation in a tectonically active basin with locally high subsidence rates, resulted in high rates of sedimentation and resedimentation processes. As a whole, the sedimentary succession sets a relevant datum to be used in future paleogeographic reconstructions of the Vendian glacial record in southern South America.

Paleomagnetism of the Sierra de Las Animas Complex, southern Uruguay: its implications in the assembly of western Gondwana

Global paleogeographies in the Neoproterozoic are poorly known. In particular, the assembly of Gondwana at the end of the Proterozoic is controversial. In order to constrain the kinematic evolution of the Rio de la Plata craton, a paleomagnetic study was carried out in the Vendian to Cambrian Sierra de Las Animas magmatic Complex, exposed in southeastern Uruguay (34.7°S, 55.3°W). One hundred and forty-seven samples from 23 sites were studied. These included two sites on the Vendian glaciogenic sediments of the Playa Hermosa Formation. After stepwise AF and thermal cleaning a characteristic remanence was isolated at several sites. According to the available isotopic data for the complex and the correlation between remanence directions and lithologic type and age, two paleomagnetic poles were computed for the Sierra de Las Animas complex. SA1 (338.1°E, 5.9°N, dp=19.6°, dm=26.7°, N=7 sites, mean declination: 44.5°, mean inclination: 58.0°, α 95: 18.1°), with a likely age around 520 Ma, shows a distribution of VGPs that suggests a remanence acquisition that spanned a long time during a period of fast apparent polar wander for Gondwana. SA2 (250.9°E, 16.9°S, dp=15.9°, dm=21.5°, N=6 sites, mean declination: 95.3°, mean inclination: −58.5°, α 95: 14.5°) corresponds to a likely age around 550 Ma and falls close to a pole of similar age from the Congo craton. This suggests that the Rio de la Plata craton was at that time part of Gondwana and confirms previous postulates that a single APWP can be defined for the supercontinent since 550 Ma. A preliminary mean geomagnetic pole obtained from the Playa Hermosa Formation (PH, 198.4°E, 43.0°S, dp=8.6°, dm=16.0°, n=6 samples, mean declination: 226.0°, mean inclination: 24.2°, α 95: 15.0°) is consistent with a previous pole of 595 Ma from the Campo Alegre (CA) lavas and may suggest, if confirmed, that the glaciogenic deposits were produced at low to intermediate latitudes. Despite high uncertainties, a new speculative model on the process of assembly of Gondwana is proposed.

Seismic reflection study of Upheaval Dome, Canyonlands National Park, Utah

The origin of Upheaval Dome, in Canyonlands National Park of southeastern Utah, has been a topic of controversy among geologists and planetary scientists. The structure has long been thought to have been created by salt diapirism from the underlying Paradox Formation. Recent studies have suggested that impact could have formed the dome. To test the various hypotheses, we acquired, processed, and interpreted seismic reflection data within and adjacent to the structure. Both conventionally stacked and prestack‐migrated images show <100 m relief in the Paradox Formation, contrary to salt diapirism hypotheses. Further, we have identified features within the images typical of impact structures, such as listric normal faults having displacements toward the center of the dome. Deformation occurs in two depth ranges, with the faulting that created the central uplift appearing only above the Hermosa Formation, in the upper 800 m of the structure. The images also suggest limited fracturing of the Hermosa and salt flow in the Paradox Formation, perhaps due to gravitational relaxation of the crater form. Our image of a nearly flat top of the Paradox salt strongly favors an impact origin for Upheaval Dome.

S, C, Sr, and Pb sources in the Pliocene Silver Creek porphyry, Mo system Rio, Colorado

The Pliocene Silver Creek porphyry Mo deposit at Rico, southwest Colorado, is the youngest known porphyry Mo deposit in the Colorado mineral belt. Associated sulfate replacement deposits, epithermal Pb-Zn-Ag vein and replacement deposits, and Recent hot springs also occur. We investigated the S, C, Sr, and Pb sources at the various levels of this well-preserved hydrothermal system by isotopic studies of the mineralization, and of sedimentary and igneous rocks in the area.Hydrothermal sulfide delta 34 S values are tightly grouped at ca. 0 per mil (x = 0.3 + or - 2.0ppm, 1Sigma ; n = 107) and are substantially lower than those of the hydrothermal sulfates (x = 15.2 + or - 1.6ppm, 1Sigma ; n = 14). Pliocene hydrothermal carbonate delta 13 C values (-7.9 to +1.1; x = -4.9 + or - 2.7ppm, 1Sigma ; n = 16) are similar to those for the Pennsylvanian Hermosa Formation carbonate (delta 13 C = -6.7 to +2.6ppm; x = -3.0 + or - 3.2, 1Sigma ; n = 5), which hosts most of the base and precious metal mineralization, whereas Recent hot spring tufas have somewhat higher values (delta 13 C = 1.8-5.5ppm; x = 3.7 + or - 1.3ppm, 1Sigma ; n = 5). Pliocene hydrothermal carbonates have delta 18 O values between -5.1 and 12.3 per mil (x = 5.0 + or - 6.3ppm, 1Sigma ; n = 16). The tufas have higher delta 18 O values (x = 15.4 + or - 2.6ppm, 1Sigma ; n = 5).Hydrothermal carbonates (including tufa) and sulfate have initial 87 Sr/ 86 Sr (sub (i)) values between 0.70700 and 0.71168. Pennsylvanian sedimentary rocks in the Rico district had 87 Sr/ 86 Sr ratios between 0.70877 and 0.71694 at the time of mineralization. Fresh and altered Pliocene monzonite intrusions at Rico have initial 87 Sr/ 86 Sr (sub (4)) values of 0.70581 and 0.70563, respectively, whereas variably altered rholite dikes have initial 87 Sr/ 86 Sr (sub (4)) values of 0.70987, 0.71574, and 0.71716. The 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 206 Pb/ 204 Pb ratios of molybdenite, pyrite, and galena taken together are 18.27 to 19.65, 15.55 to 15.70, and 37.75 to 38.62. Pliocene lamprophyre and rhyolite dikes at Rico have equivalent Pb isotope ratios of 18.26, 15.48, and 37.81 and 18.79, 15.65, and 38.02, respectively. These Pb isotope data are typical of Tertiary volcanic rocks and associated sulfides in southwest Colorado.Our delta 34 S data indicate a magmatic S source for the porphyry Mo and epithermal base metal sulfide mineralization. Most of the sulfide Pb is also of magmatic origin. In contrast, hydrothermal sulfate replacement deposits peripheral to the porphyry Mo deposit, carbonate gangue from the base metal deposits, and the Recent hot spring tufa derived their respective S, C, and Sr from local sedimentary rocks. Our delta 18 O data indicate that the hydrothermal carbonates were deposited from 18 O-shifted meteoric waters. Our comprehensive isotopic data set presents a similar genetic picture to that of other molybdenite deposits in the mineral belt, in that the porphyry Mo mineralization was dominated by magmatic components, whereas the peripheral base metal mineralization contains both magmatic and upper crustal components. These important genetic similarities are independent of fundamental differences in tectonic setting.

Borings in an Oomoldic Rockground, Pennsylvanian of Southeast Utah

The upper surface of an ooid grainstone within the Upper Pennsylvanian Hermosa Formation of southeastern Utah bears abundant borings and is encrusted by syringoporid coral colonies. Aragonite-dominated ooids were cemented on the sea floor by high magnesium calcite and were later leached by meteoric waters to form abundant oomolds. Two lines of evidence show that the bored, encrusted, marine-cemented surface is not a simple synsedimentary hardground: 1) The margins of the micrite-filled borings are mammillate: micrite fills truncated oomolds along the perimeter of the boring. Oomolds (and a leaching episode) therefore predate borings. 2) Rhizoliths within the ooid grainstone (previously interpreted as penetrating from a higher unconformity) are also bored along the encrusted surface, providing corroborative avidence of an ex- posure evnt that predated boring and postdated marine cementation

Clay-Mineral Transformation as an Indicator of Depositional and Diagenetic Conditions in Pennsylvanian Black Shales, Paradox Member of the Hermosa Formation, Utah and Colorado

The dominant clay minerals in the Paradox basin black shale facies include a detrital, aluminum-rich illite/smectite and a series of magnesium-rich, interstratified chlorite/smectite minerals. Black shales from the Gothic and Chimney Rock layers of the Paradox Member of the Hermosa Formation were sampled from 13 wells in southwestern Colorado and southeastern Utah. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Inductively-Coupled Plasma (ICP), and Rock-Eval pyrolysis were used to determine the nature of a reaction believed to consume aluminum-rich detrital clays to precipitate magnesium-rich authigenic clays. The potential for magnesium enrichment in the clay assemblage was found to be influenced by the following processes: particle size as a function of distance from detrital source area, paleosalinity as a function of transgressive-regressive stage, and the presence of large amounts of organic matter. The basic reaction that forms chlorite/smectite or corrensite (perfectly ordered chlorite/smectite with 50% smectite layers) from detrital clays is Illite/Smectite (detrital) + Mg(aq)[sup 2+] + (OH)[sup [minus]] [yields] Corrensite + K[sup +] + Al[sup 3+]. Farther from detrital source areas, this recreation proceeds easily. Within this constraint, the remaining variables either aid or inhibit the proposed reaction from taking place. Higher salinity during deposition results in more chlorite layersmore » within the chlorite/smectite structure. Large amounts of organic matter strongly inhibit the reaction from occurring, possibly due to pH effects from the production of organic acids during burial.« less

Evaporite Cycles and Cycle Boundaries in the Upper Part of the Paradox Member, Hermosa Formation of Pennsylvanian Age in the Paradox Basin, Utah and Colorado

The evaporites of the Paradox Member of the Hermosa Formation of Pennsylvanian age in southeastern Utah and southwestern Colorado are direct precipitates from marine brines and have been changed only slightly by subsequent events. Geophysical logs of deep wells indicate that the Paradox Member is composed of at least 30 evaporite cycles. Lithologies that make up the cycles, in order of increasing salinity, are organic carbon-rich carbonate shale (black shale), dolomite, anhydrite, and halite (with or without potash). Studies of core from two wells in the central part of the basin show that some of the cycles in the upper part of the Paradox Member are remarkably symmetrical, indicating regular changes in salinity. Detailed petrologic studies have revealed newly recognized lithologic textures and cycle boundaries in 11 evaporite cycles, indicating very regular cyclicity of subaqueous sedimentation in a basin in which salinity was probably controlled by Gondwana glaciation.

Authigenic Barite as an Indicator of Fluid Movement through Sandstones Within the Colorado Plateau

ABSTRACT Sandstones of the Salt Wash Member, Late Jurassic Morrison Formation, contain abundant authigenic barite on the east side of the northern Colorado Plateau (mean = 0.5 g barite/kg rock) and systematically less barite (mean = 0.02 g barite/kg rock) in the western part. The area containing abundant barite coincides approximately with the extent of bedded evaporites in the underlying Pennsylvanian Hermosa Formation. A genetic link between the barite and solutes from the Hermosa Formation is supported by sulfur isotope data. Most barites from the abundant barite zone have 34S values that range from +8 to +14 per mil, which is similar to the range of bedded gypsum and anhydrite in the Hermosa Formation. Sulfate derived from t e evaporites probably entered the Morrison Formation in solutions that moved along faults. Measured 87Sr/86Sr ratios of barite in the abundant barite zone are indicative of fault-controlled solution flow. Strontium contents and 87Sr/86Sr ratios (0.7103 to 0.7084) in barite decrease with increasing distance from faults. These trends reflect mixing of radiogenic strontium in the saline waters that ascended faults (87Sr/86Sr = 0.7100), with strontium originally contained in early diagenetic carbonate cements within the sandstones (87Sr/86Sr = 0.7080). Because measured 87Sr/86Sr values exceed the ratios in Hermosa anhydrite, it is concluded that radiogenic strontium was added to the rising brines by reaction between the evaporite-derived saline solutions and arkosic san stones along the flow path. Morrison sandstones within the abundant barite zone were altered by saline waters that added sulfate and strontium. Barite formation in this zone was limited by the availability of dispersed barium in the host rock. The zone of sparse barite on the western side of the Colorado Plateau was not affected by the same warm saline waters. Instead, intraformational redistribution of small amounts of barium, strontium, and sulfate formed the relatively sparse barite in this area.

Petrology of Eolian Carbonates, Upper Hermosa Formation (Pennsylvanian), Southeastern Utah: ABSTRACT

Wind-deposited calcarenite, or eolian carbonates, are present in the upper member of the Hermosa Formation (Missourian-Virgilian), southeastern Utah. This interval is a 250 m-thick carbonate-dominated unit composed of repetitively interbedded fluvial, eolian, and marine strata. Sharp contacts exist between subwave-base wackestones and overlying eolian carbonates. Skeletal and nonskeletal allochems are found in the eolian deposits. Cyclic marine/nonmarine units of the Hermosa are comparable in age and character to upper Paleozoic cyclothems of the Mid-Continent and were deposited during a period of rapid changes in sea level. Field and petrographic evidence suggests that the eolian carbonates are the product of regression, exposure, and deflation of shallow marine carbonates. The coarse red chert in the eolian units indicates that selective silicification in the marine carbonates occurred early.

Pennsylvanian Fossils Replaced by Red Chert: Early Oxidation of Pyritic Precursors

ABSTRACT Upper Pennsylvanian cyclothems of western Nebraska and southeastern Utah contain abundant brachiopods and crinoids replaced by red quartzine (length-slow chalcedony). Red fossils typically occur within silty wackestones that were deposited below fair-weather wave base during eustatic high stands. Within the D zone cyclothem of the Lansing-Kansas City Group (Nebraska), a well-documented paleosol recording an episode of subaerial exposure lies several meters above the chert-bearing rocks. The upper member of the Hermosa Formation (Utah) contains large-scale cross-stratified eolianites resting directly upon three red-chert-bearing limestones. Petrographic study of the silicified fossils from these two sequences shows that although fossils are presently hematitic, iron was initially con entrated within the skeletal material as scattered pyrite framboids and euhedra. Anaerobic decay of the organic material within shells during their replacement by quartzine may have led to the precipitation of pyrite. During a subsequent event, acid generated by the oxidation of pyrite inclusions allowed migration of dissolved iron through the quartzine. Buffering of these solutions by surrounding, unreplaced skeletal material caused preferential accumulation of iron oxide at the perimeters of the silicified zones. Silicification, pyrite precipitation, and pyrite oxidation all predated the crushing of shells during mechanical compaction of the matrix. We hypothesize that oxidation took place when the regional groundwater table dropped as the sea regressed and overlying beds were subaeria ly exposed. Cherts with similar histories should be present in other strata that bear internal sources of silica, iron, and organic material, and record large-scale, rapid changes of sea level.

Giant desiccation fissures filled with calcareous eolian sand, Hermosa Formation (Pennsylvanian), southeastern Utah

At two stratigraphic intervals within the upper member of the Upper Pennsylvanian Hermosa Formation, calcareous eolian sand fills downward-tapering fissures that are as much as 18 cm wide and 5.7 m deep. Fissure fillings define orthogonal polygons 10 m or more in diameter. One of the host beds is primarily composed of subtidally deposited limestone, the other is a thinly laminated, nonmarine red siltstone. Both systems of fissure fillings are directly overlain and underlain by large-scale cross-stratified, calcareous eolianites. The limestone host bed contains chert pseudomorphs after gypsum. Compaction of host rocks contorted fissure fillings and caused doming of eolian strata over each fissure. Platy mineral grains in fissure fillings are aligned subparallel to bedding in the host rocks, supporting the view that the fissures were passively filled rather than forcefully injected. These ancient fissure systems are similar in scale and pattern to those that define giant desiccation polygons in numerous Great Basin playas. The Pennsylvanian fissures, like their Holocene counterparts, probably formed when groundwater tables dropped from shallow levels within fine-grained, impermeable deposits into underlying aquifers, greatly decreasing the extent of the capillary fringe. Our study of the fissures and host rocks supports the hypothesis that carbonate grains within the eolianites were deflated from uncemented marine sediments that were broadly exposed during regressive intervals.

Mustang Flat--Significant New Paradox Basin Field: ABSTRACT

The Mustang Flat oil and gas field in T36S, R23E, Utah, was discovered by Texas Eastern Skyline Oil through the integration of regional geologic concepts and seismic stratigraphic techniques. Mustang Flat currently is capable of production from five wells in the Ismay (Des Moines) Member of the Hermosa Formation. The reservoir is developed from limestones and dolomites associated with the deposition of algal (Ivanovia) mounds. The porous section ranges from 30 to 104 ft (9 to 32 m) thick, using a 7% porosity cutoff (as indicated on a compensated neutron log). A hydrocarbon column of at least 180 ft (55 m) has been established; a water contact has not been encountered. Developmental drilling is continuing. Two wells are capable of production from the Desert Creek. The Patterson field in T37S, R24E, served as a model for exploration. Regional seismic and subsurface studies revealed the Patterson field to be associated with a break in the slope of a postulated paleoshelf. In addition, modeling indicated that porosity within the Ismay could be recognized from seismic data. From regional seismic control, the paleoshelf break was traced to the northwest into T36S, R23E. Seismic data revealed a broad northeast-southwest-trending nose that intersected the paleoshelf break, and porosity indicators were noted. Seismic and leasing programs were subsequently begun. Portable shothole seismic equipment was used because of terrain considerations. High-resolution acquisition and processing techniques were utilized. Careful analysis of isochron and time struct re maps, and use of porosity indicators resulted in the Mustang 1 discovery location. Subsequent drilling and seismic work have revealed pitfalls of which the seismic interpreter should be aware. End_of_Article - Last_Page 844------------