Frio Formation Research Articles

Tertiary volcanic rocks and the potassium content of Gulf Coast shales—The smoking gun

Research Article| June 01, 1998 Tertiary volcanic rocks and the potassium content of Gulf Coast shales—The smoking gun John Bloch; John Bloch 1Scealu Modus, 2617 Cutler Ave. NE, Albuquerque, New Mexico 87106 Search for other works by this author on: GSW Google Scholar Ian E. Hutcheon; Ian E. Hutcheon 2Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada Search for other works by this author on: GSW Google Scholar Patrice de Caritat Patrice de Caritat 3Cooperative Research Centre for Landscape Evolution and Mineral Exploration, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia Search for other works by this author on: GSW Google Scholar Geology (1998) 26 (6): 527–530. https://doi.org/10.1130/0091-7613(1998)026<0527:TVRATP>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation John Bloch, Ian E. Hutcheon, Patrice de Caritat; Tertiary volcanic rocks and the potassium content of Gulf Coast shales—The smoking gun. Geology 1998;; 26 (6): 527–530. doi: https://doi.org/10.1130/0091-7613(1998)026<0527:TVRATP>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The majority of Tertiary volcanic rocks of the western United States and Mexico are alkaline in composition and may contain as much as 50 wt% equivalent K-feldspar. Emplacement of these volcanic strata is coeval with Tertiary shale deposition in the Texas Gulf Coast, and they previously have been identified as likely sources of sediment for Gulf Coast shales. Evaluation of chemical trends in Gulf Coast shales, particularly K2O, indicates changes in sediment composition in the lower Eocene, Oligocene, and near the Oligocene-Miocene boundary. In particular, there is a 250% increase in K2O content from ∼2 wt% to ∼5 wt% from the late Eocene to the early Oligocene. Gulf Coast shale bulk-rock compositions are consistent with a Tertiary volcanic source. Estimates of erosion and mass balance calculations suggest that in the south Texas Gulf Coast, the Oligocene Frio Formation may contain between 60% and 85% volcanic detritus, and coeval Frio shales to the north contain ∼25%. Vertical and lateral compositional variations highlight variable abundances of source detritus and the effects of weathering and depositional processes on Gulf Coast shale composition. Trends of increasing K2O content with depth in Gulf Coast shales previously have been interpreted to result from open-system diagenesis and K-metasomatism at depth. The data presented herein suggest instead that these trends result from variable provenance and the influx of large volumes of Tertiary alkaline volcanic material. Therefore, diagenetic models that invoke a homogeneous initial shale composition and open-system behavior may be invalid. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Frio Shale Mineralogy and the Stoichiometry of the Smectite-to-Illite Reaction: The Most Important Reaction in Clastic Sedimentary Diagenesis

AbstractBurial diagenesis of shales of the Frio Formation resulted in an increase in the abundance of mixed-layer illite-smectite (I-S), albite and chlorite, and a decrease in the abundance of K-feldspar, illite and kaolinite. Some of the mineralogic trends determined in this study contrast with the results of Hower et al. (1976) and other studies of Frio shales. The differences are due to improvements in laboratory and clay quantification techniques since the time of the earlier research. I-S composition changed from ~20% to ≥80% illite, and mineralogic and chemical reaction of I-S continued throughout burial. Shale diagenesis was an open-system process that required addition of K2O and A12O3, and resulted in loss of SiO2. The amount of SiO2 made available by shale diagenesis is sufficient to be the source of the quartz-overgrowth cements in the associated Frio sandstones. The relationships between I-S diagenesis and fluid flow from shales into sandstones, generation of abnormal formation-water fluid pressure, onset of sandstone diagenesis and distribution of authigenic phases in sandstones indicate that reaction of the I-S in shales is one of the most important components of the sandstone/shale/formation water diagenetic system.

Burial diagenesis of illite/smectite in shales and the origins of authigenic quartz and secondary porosity in sandstones

Several gram-size samples of <0.1 μm, nearly monomineralic mixed-layer illite/smectite (I/S)were isolated from nineteen whole-cores of shale of the Frio Formation from the Texas Gulf Coast. Between ∼7,000 ft (2133 m) and ∼15,000 ft (4572 m) burial depth I/S changes from random-interstratification ( R = 0) with low %I to ordered-interstratification ( R = 3) with high %I. The accompanying chemical changes are loss of Si, Mg, and Fe +3, gain of K, Al, and Fe +2, and a shift in the REE content of the mineral. There is no chemical or mineralogical evidence for a cessation of I/S reaction at ∼80% I as is commonly accepted; both chemical and mineralogical changes occur throughout burial. Continuous change in the oxygen isotopic composition of I/S implies that the conversion of smectite-layers to illite-layers is a dissolution and reprecipitation process. Mass balance calculations indicate that burial diagenesis of Frio Formation shales is an open-system process that requires addition of K 2O and Al 2O 3 and results in loss of SiO 2. The amount of SiO 2 made available by shale diagenesis is sufficient to be the source of the quartz-overgrowth cements in the associated Frio sandstones. Iron reduction in I/S is a significant source of the acid required for the diagenesis of both Frio shales and sandstones.

Mineral/Water Interaction, Fluid Flow, and Frio Sandstone Diagenesis: Evidence from the Rocks: Discussion

The spatial distribution of burial diagenetic cements in sandstones of the Frio Formation is related to permeability and fluid flow. The relationship between sandstone diagenesis and fluid flow implies that the modification of the rocks is largely the outcome of reactions between the detrital and authigenic minerals and the formation water in the pores. During burial, formation water preferentially flowed through the most porous and permeable sandstones. Diagenetic alteration is more extensive in permeable sandstones because the minerals in those rocks were exposed to significantly more pore volumes of reactive water, each of which accomplished a small amount of diagenetic work. Consequently, porous and permeable units were also the preferred sites of diagenetic mineral p ecipitation. The heterogeneous distribution of authigenic minerals in sandstones is a result of changing preferential flow paths, due to diagenetic modification, during burial. Sandstones that are not the most porous or permeable at the time of deposition commonly preserve the best reservoir quality at depth because they are less modified by diagenesis. Petrographic analyses indicate that the SiO2 needed for the quartz overgrowths in the sandstones had an allochthonous source. ^dgr18O values for diagenetic quartz imply that the mineral precipitated from rapidly ascending, hot formation water. Quartz overgrowths are most abundant in distal-shelf facies sandstones, where the isolated nature of the sandstones in a predominately shaly section maximized preferential fluid flow. Mass balance calculations show that thousands of pore volumes of formation water are required to supply the SiO2 needed for the quartz overgrowths in the sandstones. The amount of water required to produce the diagenetic modification of the sandstones is much larger than the amount of water present in the entire Frio Formation, impl ing that reuse of formation water is an important and necessary process during diagenesis.

The Possible Role of Nannobacteria (Dwarf Bacteria) in Clay-Mineral Diagenesis and the Importance of Careful Sample Preparation in High-Magnification SEM Study

ABSTRACT Bacterial textures are present on clay minerals in Oligocene Frio Formation sandstones from the subsurface of the Corpus Christi area, Texas. In shallower samples, beads 0.05-0.1 µm in diameter rim the clay flakes; at greater depths these beads become more abundant and eventually are perched on the ends of clay filaments of the same diameter. We believe that the beads are nannobacteria (dwarf forms) that have precipitated or transformed the clay minerals during burial of the sediments. Rosettes of chlorite also contain, after HCl etching, rows of 0.1 µm bodies. In contrast, kaolinite shows no evidence of bacterial precipitation. We review other examples of bacterially precipitated clay minerals. A danger present in interpretation of our earlier work (and much work of other ) is the development of nannobacteria-looking artifacts caused by gold coating times in excess of one minute; we strongly recommend a 30-second coating time. Bacterial growth of clay minerals may be a very important process both in the surface and subsurface.

The origin of aqueous sulfate in Frio pore fluids and its implication for the origin of oil field brines

The O and S isotopic composition of aqueous S0 4 in saline formation waters from the Frio Formation of S Texas can be used to deduce the origin of the S0 4. The average O and S isotopic composition of aqueous S0 4 in Frio brines is + 13.4‰ SMOW and + 8.2‰ CDT, respectively. The O isotopic compositions fall within the range of Mesozoic and Cenozoic seawater values and are interpreted to be derived from dissolved Jurassic evaporites. The S values appear to have been modified by the addition of light S derived from the oxidation of pyrite. We conclude, therefore, that some of the chemical components in Frio brines are derived from water-rock interactions that occurred in much deeper units and that these components have migrated into the Frio.

Petrophysical Properties and Geology of Selected Intervals in the Frio Formation, Stratton Field, South Texas for Modeling Interwell Seismic Logging Response: ABSTRACT

ABSTRACT Seismic or continuity logging consists of locating a seismic source in one borehole near or in a low-velocity layer and deploying a detector array in a second borehole. Detection of guided waves transmitted between the two wells indicates bed connectivity. The guided wave signatures are either leaky modes or normal modes (or both). The technique has numerous applications in various types of heterogeneous geological environments, including many Gulf Coast gas reservoirs. It can be used to determine the continuity of beds between wells, estimate and locate variations in the thickness of beds, and estimate the average rock physical properties of the beds. Stratton field was selected as the Gulf Coast gas-play type field for a project to model interwell seismic logging responses. Stratton is a mature gas field located in the south Texas Gulf Coast, about 30 miles southwest of Corpus Christi. It encompasses over 120,000 acres in portions of Kleberg, Nueces, and Jim Wells counties. Stratton is one of 29 fields in the Frio Formation fluvial-deltaic play associated with the Vicksburg fault zone along the Texas Gulf Coast Basin. This poster presentation explains the technique of interwell seismic logging, documents the petrophysical properties and geology of intervals in the upper and middle Frio, and presents the results of the forward modeling tests. End_of_Record - Last_Page 472-------

Determining Between-Well Reservoir Architecture in Deltaic Sandstones Using Only Well Data: Oligocene Frio Formation, Tijerina-Canales-Blucher Field, South Texas: ABSTRACT

ABSTRACT Accurate prediction of compartment architecture and intracompartment heterogeneity is necessary to locate and recover the estimated 15 billion barrels of mobile oil remaining in U.S. fluvial-dominated deltaic reservoirs. Complex architecture and rapid 1ateral variability in such reservoirs complicate subsurface prediction, particularly in mature fields where well logs are the only available subsurface data. A genetic-stratigraphy-based methodology has been developed that improves between-well prediction of deltaic reservoir architecture and, thus, reduces risks associated with infill-drilling. In the area of Tijerina-Canales-Blucher (T-C-B) field, which lies on the northeast margin of the Oligocene-age Norias delta, the productive 3rd-order Lower Frio unit was subdivided into eight 4th-order genetic units. Delta-front positions were identified on the basis of regional and subregional cross sections. The 4th-order units (30 to 80 ft thick) were subdivided into two to five 5th-order units (10 to 30 ft thick). Log patterns and net sandstone maps were used to identify facies, which include (1) distributary channels (up to 25 ft thick, 8,000 ft wide, and commonly narrower than 40-acre well spacing), (2) mouth bars (up to 15 ft thick, ranging in size from 40 to 640 acres in area, commonly 5,000 ft wide), and (5) washover fans (up to 10 ft thick, and 7,000 ft wide). Many reservoir compartments, including the prolific 21-B interval, contain a significant degree of stratigraphic trapping caused by updip pinchout of delta front or washover sandstones or convex-updip segments of meandering distributary channel sandstones. The methodology and results of this Study are directly applicable to other Gulf Coast fluvial-deltaic reservoirs in the Frio Formation and Wilcox Group, as well as to deltaic reservoirs throughout the U.S. The general methodology should be applied to develop remaining reserves in mature fields where geophysical data are lacking. Further study of T-C-B reservoirs may include stratigraphic analysis of 3-D seismic data and infill drilling to confirm between-well interpretations. End_of_Record - Last_Page 475-------

Episodic Hydrocarbon Migration during Deposition of the Frio Clastic Wedge, Gulf of Mexico: Its Expression and Causes: ABSTRACT

A petrographic and geochemical studies of the Oligocene Frio Formation show that the carbon isotopic composition of calcite varies systematically with stratigraphy, due to varying hydrocarbon migration to the sea bed during Frio deposition. In the St. Charles Ranch fields, Aransas County, Texas, compositions in the Lower Frio are relatively heavy ([delta][sup 13]C = [approximately]-2[per thousand]), decreasing stratigraphically upward to -5.5[per thousand] in the Middle Frio, then increasing to -2[per thousand] in the Upper Frio. The average calcite composition is identical in sandstones and mudstones at specific stratigraphic levels. This pattern contrasts with [delta][sup 18]O, in which sandstone and mudstone calcite has different compositions and no stratigraphic control is evident. Two limestone beds have very light [delta][sup 13]C compositions ([approximately] -20[per thousand]); they are analogous to modern carbonate buildups in the Gulf of Mexico forming around methane vents. The carbon isotopic composition of calcite reflects the flux of hydrocarbons to the sea bed: low [delta][sup 13]C values indicate greater incorporation of hydrocarbon into calcite and therefore a large hydrocarbon flux, and vice versa. The data suggest that a pulse of hydrocarbon migration occurred during Frio deposition, peaking during Middle Frio deposition. Two mechanisms for this migration pulse are considered: first,more » deposition of the Frio resulted in sudden hydrocarbon maturation and generation in underlying source rocks; and second, growth faulting, associated with deposition of the Frio wedge, opened migration pathways. As sedimentation rates slowed during deposition of the Upper Frio, movement on growth faults decreased and migration pathways closed. These mechanisms are tested with numerical models.« less

Predicting Seal Efficiency and Trapped Hydrocarbon Type in Gulf Coast Hydrocarbon Systems: Lessons Learned From West Fulton Beach Field, Mid-Texas Gulf Coast: ABSTRACT

ABSTRACT Many Gulf Coast fields consist of multiple vertically stacked sandstones in which oil and gas are seemingly randomly distributed stratigraphically. Hydrocarbon entrapment is strongly affected by seal competency and possibly by formation pressure, and these factors are in turn controlled by the characteristics of the interbedded shales. In West Fulton Beach field, Aransas County, Texas, the Oligocene, Frio Formation shales were deposited as shelf mudstones and represent flooding events in a barrier bar/strandplain and inner shelf setting. Reservoirs were placed into a high-frequency genetic stratigraphic framework to test the theory that hydrocarbon entrapment is controlled by a hierarchy of maximum flooding surfaces. Cumulative oil and gas production and gas-to-oil ratios were tabulated for individual reservoirs, as well as for all reservoirs in each 5th-, 4th-, and 3rd-order genetic unit, as measures of entrapment. Total producible hydrocarbons (oil plus gas, in barrels of oil equivalent) increase in volume in successively shallower 3rd-order Frio units (each 800 to 1,000 ft thick), capped by the 600-ft-thick Miocene Anahuac Shale. Likewise, successively shallower 4th-order units (120 to 200 ft thick) within each 3rd-order unit contain greater volumes of total hydrocarbons, as well as greater percentages of gas. This pattern exists independent of shale thickness or reservoir porosity, and it is repeated at the 5th- and 6th-order levels, when viewed on a per-gross-ft-of-sandstone basis. Thus, although appearing random, when evaluated carefully within a stratigraphic framework, total hydrocarbon volumes and oil versus gas distributions follow a systematic pattern tied to their position within a stratigraphic hierarchy. This finding can be used to more accurately constrain seal risk in exploration or deeper pool drilling and to evaluate hydrocarbon type ahead of the drill bit. Further study is needed to model reservoir filling, incorporating reservoir pressures, to better understand the petrophysical controls on hydrocarbon entrapment. Additionally, observations of lateral changes in the observed cyclic patterns may produce a greater understanding of the effects that facies changes and genetic unit stacking patterns have on seal competency. End_of_Record - Last_Page 476-------

Accommodation-Based Controls on Fluvial-Deltaic Reservoir Compartmentalization: Examples from the Oligocene Frio Formation, South Texas: ABSTRACT

Accurate prediction of compartment architecture and intracompartment heterogeneity is necessary to locate and recover the estimated 15 billion barrels of mobile oil remaining in U.S. fluvial-dominated deltaic reservoirs. To improve this prediction, facies-specific relationships between accommodation trends and sand-body architecture that were established by outcrop studies in the western interior were Successfully applied in a study of Oligocene Frio Formation reservoirs along the Vicksburg Fault Zone of South Texas. Compartment architecture and internal heterogeneity were determined for mature upper and lower delta-plain reservoirs within two 4th-order genetic units to document reserve growth potential. In lower delta-plain settings, low-accommodation conditions at the base of genetic units produce narrow distributary channels that feed many small mouth bars, whereas high-accommodation conditions near the top of genetic units yield broader distributaries arid an expansive wave-dominated delta front. In proximal upper delta-plain settings, low accommodation produces few narrow fluvial channels that are commonly internally trough cross-strata predominate, with the finer grained silty upper point-bar deposits being removed by successive scour events during which base level has not risen substantially. In contrast, high accommodation yields many broad internally heterogeneous channels in which high and erosion of fine-grained channel-top facies. This study documents the profound effect thatmore » accommodation conditions can have suiting production behavior. Similar studies in other settings are needed to identify these relationships for the entire spectrum of depositional systems.« less

Diagenesis of Calcite Cement in Frio Formation Sandstones and its Relationship to Formation Water Chemistry

ABSTRACT Calcite cement is one of the most volumetrically important diagenetic minerals formed during burial of Frio Formation sandstones from the Corpus Christi area of Texas. Syndepositional calcite is restricted to shore-zone sandstones, whereas later, post-quartz-overgrowth, burial-diagenetic calcite is present in both shore-zone and shelf sandstones. The d18O of burial-diagenetic calcite becomes depleted with depth. Chemical and textural evidence favors partial dissolution and reprecipitation (recrystallization) with consequent isotopic resetting as being responsible for the change in calcite d18O with progressive burial. The change in calcite 87Sr/86Sr with depth also supports progressive recrystallization of calcite. There is a strong relationship between the 87Sr86Sr and trace element composition of calcite and formation water from individual growth-fault blocks. Significant differences in strontium isotopic and trace element composition exist between adjacent fault blocks, implying that each fault block has behaved as a chemically separate system since the time of calcite precipitation.

On: “A 3-D seismic case history evaluating fluvially deposited thin‐bed reservoirs in a gas‐producing property,” by B. A. Hardage, R. A. Levey, V. Pendleton, J. Simmons, and R. Edsen (November 1994 GEOPHYSICS, 59, p. 1650–1665).

Hardage et al. (1994) conducted a study at Stratton Field with the purpose of detecting thin‐bed compartmented reservoirs in a fluvially deposited system (Oligocene Frio Formation), and rightly concluded that, in order to determine which seismically imaged stratigraphic changes are compartment boundaries, it is necessary to incorporate geologic and reservoir engineering data (particularly reservoir pressure data) into seismic interpretation. Their interpretation philosophy consisted of defining depositional stratal surfaces (I would rather say paleodepositional surfaces), and in analyzing seismic reflection amplitude behavior on such surfaces.

Na-Ca-Cl saline formation waters, Frio Formation (Oligocene), south Texas, USA: Products of diagenesis

Saline formation waters from reservoirs in Oligocene deltaic clastic sedimentary rocks (Frio Formation), south Texas, are derived from three sources. Mixtures characterize most samples, accounting for the wide range in chemistry. Low salinity water is derived from Cenozoic shales. In contrast, saline water having variable Br/Cl ratios is allochthonous, and is derived from at least two sources in Mesozoic strata which underly the Cenozoic units. Saline water in this area, regardless of Br/Cl ratio, contains variable, high concentrations of Ca. Calcium is derived primarily from dissolution of plagioclase, as demonstrated by petrography, 87Sr/86Sr analyses, and chemical modelling. The McCl2 or Carpenter function of saline formation waters in this area thus can be derived from water-rock interaction, and cannot be used to identify the ultimate source of solutes. Reverse osmosis is an unimportant processes in this area, and δ18O and δD values converge on rock-buffered values of approximately +8%o and −30%o, respectively.

Reservoir Architecture and Permeability Characteristics of Fluvial-Deltaic Sandstone Reservoirs, Frio Formation, Rincon Field, South Texas: ABSTRACT

Reservoir Architecture and Permeability Characteristics of Fluvial-Deltaic Sandstone Reservoirs, Frio Formation, Rincon Field, South Texas: ABSTRACT

High-Resolution Sequence Stratigraphy: The Key to Identifying Compartment Styles in Frio Formation Fluvial-Deltaic Reservoirs, T-C-B Field, Jim Wells County, Texas: ABSTRACT

High-Resolution Sequence Stratigraphy: The Key to Identifying Compartment Styles in Frio Formation Fluvial-Deltaic Reservoirs, T-C-B Field, Jim Wells County, Texas: ABSTRACT

Sea Level Changes Reflected in the Carbon Isotopic Composition of Sandstone Cements Frio Formation, Texas: ABSTRACT

Sea Level Changes Reflected in the Carbon Isotopic Composition of Sandstone Cements Frio Formation, Texas: ABSTRACT

Application of Sequence Stratigraphy to the Prioritization of Incremental Growth Opportunities in Mature Reservoirs: An Example from Frio Fluvial-Deltaic Sandstones, T-C-B Field, South Texas: ABSTRACT

ABSTRACT The U. S. Department of Energy has identified mature fluvial-deltaic reservoirs as being the highest priority reservoir type for near-term domestic reserve-growth potential. Detailed characterization studies to locate the estimated 15 billion barrels of mobile oil remaining in these reservoirs must focus first on those reservoirs with the greatest potential if they are to be economically successful. In quick-look analyses that estimate reserve-growth potential, stratigraphic heterogeneity is the most difficult factor to evaluate. Through detailed study of Frio Formation upper delta-plain fluvial reservoirs in T-C-B field, south Texas, and by using outcrop observations from the Ferron Sandstone, central Utah, a model has been developed that relates stratigraphic heterogeneity to rate of accommodation during deposition, a feature that varies predictably throughout a depositional cycle. Upper delta-plain fluvial channelbelts deposited at the base of a depositional cycle, during periods of low accommodation, are narrow, few in number, and relatively internally homogeneous. As a result, they are not effectively contacted by rigid well patterns but can drain large reservoir volumes through individual completions. Because they are commonly isolated within floodplain mudstones, they possess the potential for stratigraphically trapped accumulations away from the structural crest. In contrast, channelbelts deposited at the top of a depositional cycle, during periods of high accommodation, are broad and internally heterogeneous. Despite appearing laterally continuous, they are extensively compartmented by high volumes of low-permeability siltstones and shales, as well as mudclast-rich lag deposits that drape channel boundaries and limit fluid flow at channel-on-channel contacts. AU other factors being equal, these channelbelt types possess the greatest reserve-growth potential because past completions have only contacted small reservoir volumes. This accommodation-based model for reservoir heterogeneity in upper delta-plain fluvial settings represents the first in a new generation of reservoir models. Similar models developed for reservoirs deposited in the spectrum of depositional settings will improve the effectiveness and increase the use of detailed reservoir characterization studies, resulting in the improved identification and recovery of oil and gas remaining in mature reservoirs throughout the United States.

Elemental mobility in sandstones during burial; whole-rock chemical and isotopic data, Frio Formation, South Texas

ABSTRACT The deepest (> 3.5 km) and most altered Oligocene Frio sandstones from south Texas typically contain authigenic minerals that exceed 30 volume percent of the rock. Elemental and isotopic analyses of whole rocks confirm conclusions based on petrography that authigenesis is accompanied by substantial material transfer involving both import to and export from the sandstone. From petrographic observations, Zr is apparently less mobile than other immobile elements such as Al, Ti, and REE, but nonuniform initial distribution of Zr due to sorting limits its usefulness for normalizing elemental variations. If Zr is assumed to be strictly immobile, and Zr was once uniformly distributed, then volume loss of approximately 38% from the silicate fraction of the deepest sandstones is suggested. Over the range of burial depths for samples used in this study (0.9-4.3 km), elemental trends controlled by feldspar reactions are the most readily documented. Dissolution and replacement of detrital K-feldspar is accompanied by loss of 2-3 weight percent of K2O from the whole rock. Loss of more than half of the initial Rb, Sr, and Pb also correlate with alteration of detrital feldspars. 87Sr/86Sr ratios for whole-rock and silicate fractions increase with depth as loss of nonradiogenic strontium characteristic of the volcanic-rich detrital fraction is overwhelmed by more radiogenic strontium derived from deeper in the basin. Relatively constant Na is compatible with feldspar alteration in which dissolution exceeds albitization. Changes in Nd concentrat ons and Nd also closely track the progress of alteration of detrital feldspar. Magnesium gain accompanies precipitation of authigenic chlorite and very minor ferroan dolomite. Whole-rock calcium values are dominated by calcite that includes both detrital and authigenic components. Significant dissolution of detrital carbonate grains does not occur, and calcium in authigenic carbonate cement exceeds by about 5 times the calcium released by dissolution of detrital silicates, thus requiring considerable import of calcium and CO2. Material transfer on the observed demonstrates that burial diagenesis is an open-system process for several major elements, and introduces a major obstacle to use of elemental data for provenance interpretation. Defining the sources of materials transported into sandstones, and the fate of materials exported, is key to documenting the scale of system closure in sedimentary basins and requires a better understanding of diagenetic processes in mudrocks.

Cathodoluminescent textures and the origin of quartz silt in Oligocene mudrocks, South Texas

ABSTRACT In subsurface mudrocks of the Oligocene Frio Formation in South Texas, a substantial proportion of silt-size quartz (10-62 µm) manifests cathodoluminescence (CL) that varies greatly in intensity within each particle, giving rise to textures that differ greatly from grain to grain. In contrast, most quartz grains of sand size (>= 62 µm) have CL that is relatively homogeneous within individual grains. A substantial percentage of the quartz within silt grains is very weakly luminescent. No depth trend in the occurrence of luminescent textures in detrital quartz is observed, suggesting that the component of dark, possibly low-temperature silt-size quartz is not related to chemical processes in the present burial setting. Paleogene rhyolites of West Texas are a partial model for the source rocks that supplied sediment to the Frio Formation. Phenocrystic quartz in these volcanics has uniform CL similar to that observed in the Frio sand fraction, whereas complex CL structure seen in the associated groundmass bears many similarities to the CL textures observed in Frio silts. This observation further supports the idea that CL structure of detrital quartz in the Frio is most likely an inherited feature. If the quartz with dark CL is of low-temperature origin, it would help to elucidate the relatively 18O-enriched isotopic values reported for mudrock quartz in the Frio.