Earle F. McBride

A Classification of Common Sandstones

ABSTRACT A descriptive classification of sandstones (exclusive of carbonate and volcaniclastic sandstones) based on the composition of framework grains is proposed that can be used equally well in the field and laboratory. The framework grains are grouped into (1) quartz plus chert and quartzite, (2) feldspar, and (3) rock fragments which constitute the end-members of the quartzarenite, arkose, and litharenite clans respectively. The clans are subdivided to make eight major rock types. Graywacke is defined on the basis of both texture and framework composition and is treated as a special rock type instead of as a clan or subclan type. Because the texture of a sandstone is as important an attribute as framework composition, a polynomial system of sandstone nomenclature should be used in which at least grain size and clan name are cited (i.e., muddy coarse arkose).

Stratigraphy and Structure of the Parras and La Popa Basins, Northeastern Mexico

The Parras and La Popa basins of Coahuila and Nuevo Leon, Mexico, have a composite thickness of about 5.4 km of paralic sedimentary rock (Parras Formation and Difunta Group) deposited during Late Cretaceous to Paleocene time. Detritus of sedimentary, volcanic, and hypabyssal rocks eroded from a highland to the west and southwest of the Parras basin and northwest of the La Popa basin was deposited in deltaic, delta-flank, strand-plain, and marine-shelf environments. Progradation and retrogradation of deltas in each basin in response to basin subsidence and variable sediment influx resulted in a complicated intertonguing of rock units. Using wedge-shaped red-bed bodies and sandstone marker-beds, the Difunta Group is divided into nine formations in the Parras basin and five formations in the La Popa basin. The correlation of stratigraphic units between the basins is uncertain, because only one formation (Muerto Formation) can be mapped across the basin margins, and diagnostic fossils are absent. Fence diagrams and isopach maps document the geometry of the major stratigraphic units. The clastic basin-fill was deformed during a post-Paleocene (Laramide) event that formed the main folds of the Sierra Madre Oriental. Both tight and broad folds, thrust faults of small displacement, and minor tear faults characterize the deformation within the study area. The relations of the stratigraphic units, the geometry of the units, and the internal anatomy of the units suggest that the Parras and La Popa basins acted as subsiding elements on the craton and were filled by deltaic and shelf sediments. The basins were not sites of linear geosynclinal deposition as suggested by some workers, and characterization of the basins as foredeeps is undesirable because of the ambiguous meaning of the term.

Diagenetic evolution of Cenozoic sandstones, Gulf of Mexico sedimentary basin

Abstract The Gulf of Mexico sedimentary basin is a natural laboratory for the study of on-going diagenetic and incipient metamorphic processes. Sediments and rocks of Eocene through Pleistocene age have been studied from the surface to depths in excess of 6 km. Sediments heated to temperatures above 100°C have been massively transformed by mechanical compaction, cementation, and extensive alteration of detrital components. Grain dissolution, albitization, and clay-mineral transformations have reduced an initially complex detrital assemblage to quartz, albite, illite and minor carbonate at temperatures above 100°C. Volumetrically significant diagenetic processes observed in the basin include cementation by quartz, carbonate and kaolinite, grain dissolution (affecting mainly potassium feldspar, heavy minerals, and plagioclase), albitization, and the transformation of smectite to illite. Excepting carbonate cementation which shows essentially no depth-related variation, these processes occur shallower in older units, most likely in response to variations in the geothermal gradient, which is higher in the older Cenozoic depocenters. The magnitudes of the principal diagenetic processes all support the view that basinal diagenesis operates as an open system on a very large scale. Strontium isotopic data for authigenic carbonates document vertical transport on the scale of kilometers. The extent to which metamorphic processes below 6 km have effected the course of diagenesis in shallower rocks is still unproven, but current data suggest that burial diagenesis must be studied in such a context.

Numerical Assessment of Dissolution Versus Replacement in the Subsurface Destruction of Detrital Feldspars, Oligocene Frio Formation, South Texas

ABSTRACT Dissolution is the main process responsible for modifying detrital feldspar assemblages in the Oligocene Frio Formation of South Texas, accounting for up to 80% of feldspar modification in completely altered assemblages. This finding is supported through two independent calculations, one based on an estimate of initial feldspar content of the sandstones, the other on estimated initial composition of the feldspars themselves. Volumetric loss of detrital feldspar on the observed scale over such a short time span has major implications for provenance interpretations and for understanding the basin-wide mass balance of several major diagenetic components. The wide variation of feldspar content and composition at any given depth (temperature) attests to the premier significance of fluid co position and flux in driving the reactions that bring about dissolution of feldspars in these rocks.

Calcite-Cemented Concretions in Cretaceous Sandstone, Wyoming and Utah, U.S.A.

ABSTRACT Spheroidal (cannon ball) calcite-cemented concretions, some of gigantic size, were studied from three Cretaceous shelf sandstone units: Ferron, Frontier, and Second Frontier. The concretions have diameters between 2 cm and 6 m; those larger than 40 cm in diameter in the Ferron and Frontier have septarian structure. Rare concretions in the Second Frontier with cone-in-cone also have septaria. The pattern of carbon and oxygen isotopic data and intergranular volume (IGV) data across concretions indicate that concretion growth was complex; not all concretions grew progressively from their centers toward their edges. In many concretions displacive fascicular-optic high-Mg calcite (FOC) precipitated simultaneously throughout the concretions (pervasive growth pattern), although the earliest cementation was concentrated in and near their centers. Remaining pores were subsequently filled by poikilotopic calcite of lower Mg content. Displacive FOC cement in the bodies of concretions and in cone-in-cone structure, and the large IGV values, indicate that these carbonate phases formed at burial depths no greater than tens of meters. IGV values indicate that spar cement continued to depths of at least 50 m and possibly deeper. The association of organic matter and microbial structures, or microbial byproducts, with displacive FOC and cone-in-cone calcite raises the possibility that biogenic processes were responsible for the apparently rapid growth of the displacive calcite. The oxygen isotope composition of pore waters was fairly uniform (in the range of 18O of -4 to -6‰ SMOW) during 80% of concretion growth but became even more depleted in 18O during the last stage of growth and during septarian fracture filling, ranging into values as depleted as -14‰ SMOW. 13C in calcite is negative (-4 to -18‰ V-PDB) but variable, reflecting a range of microbial processes under both oxidizing and reducing conditions. Earlier calcites fall mostly in the range of -10 to -16‰ V-PDB; relatively later calcite, in veins and on concretion margins, is more enriched in 13C. Fe and Mn concentrations in calcites have a rough positive correlation with higher 13C values. The concentration of Mg, derived from seawater and altered volcanic rock fragments, covaries positively with the degree of meteoric influence: lower Mg contents correlate positively with relatively depleted 18O values. 87Sr/86Sr values are typical of Sr derived from coeval seawater or marine skeletal debris. Growth of concretions 4 to 6 m in diameter, apparently in < 5 My, requires some spatial localization of growth sites by a self-organization process, and probably a large supply of carbonate shells in the original sediment. Elongation of larger concretions in the plane of bedding suggests that advective rather than diffusive supply of cement components dominated late in concretion growth history. Concretions formed close to the tops of parasequence boundaries.

Significance of Color in Red, Green, Purple, Olive, Brown, and Gray Beds of Difunta Group, Northeastern Mexico

ABSTRACT Varicolored rocks of the Difunta Group (Upper Cretaceous-Paleocene) are composed of detritus derived from a relatively uniform terrane of volcanic rocks and deposited in fluvial, deltaic, and shelf environments. Red, green and purple rocks are restricted to delta-plain facies, whereas the dark colors are present in all facies. The color of claystone is a function of color mixing of red hematite, green illite and chlorite, and black organic matter; and possibly of grain size of hematite (purple color). Red and purple rocks owe their color to pervasive hematite grain coatings and crystals intergrown with clay; brown rocks owe their color to faint or localized iron-oxide grain coatings; and gray rocks to organic matter and authigenic iron sulfide. Green rocks owe their color to chlorite and illite and to the absence of hematite, organic matter and sulfides. Olive and yellow claystone colors are imparted by color mixing of green clay and black organic matter. Field relations and petrographic studies indicate that red and purple colors originated through post-depositional reddening of sediment, in part in soil zones on the delta plain, in a sub-humid to semi-arid climate that had seasonal wet and dry periods. Reddening occurred both by aging of hydrous ferric oxides plus staining of grains by hematite pigment formed by oxidation of detrital iron oxide and mafic grains. Some brown siltstone beds were pigmented in a manner similar to red beds, but other siltstone beds developed brown color upon weathering. Green beds formed by bleaching of red (or proto-red) beds by interstratal percolation of reducing water derived largely from fluvial channels overlying the green beds. Olive and gray claystone are present predominantly in marine facies that contain abundant organic matter and in some delta-plain facies where destruction of organic matter was incomplete. Total Fe content of claystone samples is essentially the same regardless of color, except that gray claystone has significantly less total Fe than other colors; 67% of the samples have total Fe between 3 and 4%. Iron reduced in red beds was not removed in solution but resides in chlorite in green strata, and some iron reduced in gray beds resides in sulfides.

Features and Origin of Italian Jurassic Radiolarites Deposited on Continental Crust

ABSTRACT Jurassic radiolarites that were deposited on continental crust and which are overlain and underlain by pelagic limestones were examined in four basins. Radiolarites studied range from bedded chert, where chert occurs in even, continuous beds separated by conscpicuous rhythmic shale interbeds, to nodular-lumpy chert, where chert occurs as isolated nodules in limestone at one extreme to lumpy beds of irregular thickness at the other extreme. Bedded chert formations are composed of microcrystalline quartz, clay minerals, and minor amounts of either hematite or organic matter pigment; nodular cherts contain in addition considerable micrite (altered coccolith ooze). Chert nodules, lenses, and beds all formed by the diagenetic reorganization of silica almost entirely of biogenic origin, chiefly from Radiolaria. Nodular-lumpy chert beds and even some bedded chert formed by partial to complete replacement of limestone. Lumpy beds formed because compaction, dissolution of calcite, and replacement of calcite were not uniform in either time or place within a bed. Many even-bedded chert layers were originally silts or sands composed almost entirely of current-deposited Radiolaria. The rhythmic bedding of radiolarite is the product of 1) episodes of rapid current-deposition (e.g., turbidity currents) of radiolarian sediment alternating with slow deposition of hemipelagic mud and 2) episodic growth of Radiolaria. Bioturbation blurs the evidence of these two different origins, but the following features indicate that many chert-precursor beds were current-deposited: sharp chert-shale bedding contacts, flute casts, graded beds, clay clasts, laminations and rapidly buried animal tracks and trails. Water depths during radiolarite deposition are uncertain. Regional stratigraphic data, evidence of redeposited radiolarian sand, and acceptance of the Bosellini-Winterer model of carbonate dissolution surfaces during Late Jurassic time leads EFM to conclude that well-bedded radiolarite was deposited at depths close to the CCD (2500 m) and that lumpy-bedded chert was deposited between the ACD and the CCD (1500 to 2000 m). Features indicating the presence of evaporites (evaporite pseudomorphs, breccias formed by crystal growth, quartzine, lutecite), paleosoil fabrics, stalactite-stalagmite fracture fillings, and disbelief in the validity of the CCD in Late Jurassic time in the Tethys leads RLF to conclude that the Lombardy radiolarites were deposited in environments that were, in part, shallow intertidal mudflats where local precipitation and solution of evaporite minerals and occasional subaerial exposure took place.

Heterogeneous Distribution of Calcite Cement at the Outcrop Scale in Tertiary Sandstones, Northern Apennines, Italy

Calcite cement derived intraformationally in seven stratigraphic units of marine origin (five submarine-fan deposits and two shelf deposits) is distributed heterogeneously at the outcrop scale. Sandstone beds intercalated with calcareous shale older than Pliocene tend to be completely cemented, whereas stacked sandstone beds that lack shale interbeds have calcite cement in the form of tightly cemented concretions that make up only 10-30% of a bed. The abundance and distribution of concretions, with few exceptions, are irregular and unpredictable. Concretion shapes include spheres (<1 m diameter), oblate and prolate spheroids (<1.5 m), tabular forms (to 8 m long), and irregular forms. Patterns of concretions within beds are remarkably varied and include both random and uniform spacing; preference for either the top, middle, or bottom of beds; preference for faults that cut bedding at a high angle; and localization around shale rip-up clasts. There is no preference of concretions for shell-rich layers. Some formations have cement patterns specific to that formation, whereas other formations have different patterns at different outcrops. Most formations have more than one cement pattern in an outcrop. The lack of strong textural (grain size, graded bedding) or compositional controls on the localization of calcite cement suggests the preeminence of highly localized hydrologic factors in determining the spatial distribution of authigenic pore-filling calcite. Spherical concretions grew by diffusive supply of intraformationally derived components, whereas prolate and elongate concretions grew chiefly under the influence of advective supply. Faults apparently served as fluid conduits and were selectively cemented. In general, only sandstones intercalated with shale are totally cemented. This indicates that shales were a major source of cement components for these sandstones at least.

Oriented Concretions, Ionian Coast, Italy: Evidence of Groundwater Flow Direction

ABSTRACT Highly elongate, aligned, calcite-cemented concretions are present in exposed shallow-marine Pleistocene sands and gravels along the Basilicata coast of the Ionian Sea, southern Italy. Concretions are shaped like rods (pencils and cigars, 0.5-3 cm in diameter) and thin blades (031 cm thick and 1-5 cm wide). Most have aspect ratios greater than 5 and some reach 30. Most concretions are in parallel-laminated beds, but a few are in cross-stratified and ripple-cross-laminated beds. Concretions cut across foresets and inclined laminae. The standard deviations of long axes at each of six localities with more than eight measurements are less than 7° the standard deviation of the orientation of concretions from 30 localities over a 400 km2 area is less than 17°. Previous interpretations for elongate carbonate concretions in sandstones include growth in the direction of groundwater flow, growth parallel with current-oriented grains and/or paleocurrents, and selective cementation in the troughs of wave ripples. The Basilicata concretions are oriented perpendicular to the local coastline and parallel with the direction of present-day groundwater flow. The groundwater flow direction was probably the same during the Pleistocene. The evidence indicates that the concretions grew parallel with groundwater flow and were not influenced by sedimentary structures or grain fabric. The isotopic values (13C and 18O) range from 10 to -1 [PDB] and from -5 to -3 [PDB], respectively, and textures of calcite cement in the concretions indicate that the calcite was precipitated from meteoric groundwater. Nannobacteria may have mediated precipitation of calcite.

Diagenesis of the Maxon Sandstone (Early Cretaceous), Marathon region, Texas; a diagenetic quartzarenite

ABSTRACT Environments of deposition, climate, detrital mineral composition, and stratigraphic position within a thick sequence of carbonate rocks were major controls on the diagenesis of Maxon sandstones. Fluvial-deltaic sands, which were subjected to calichification and other pedogenic processes, lost all macroporosity prior to burial. Most sands underwent compaction (average of 14% porosity loss), minor cementation by quartz and kaolinite, possibly during invasion of meteoric water introduced during a low stand of sea level, followed by extensive cementation and grain replacement by calcite. C and O isotopic values of calcite suggest that carbonate was introduced by formation waters derived from adjacent limestones. Mass-balance calculations on the loss and gain of chemical components during diagenesis show there was a major loss of silica (from loss of feldspars) and major gains in CaO and CO2 (in calcite cement). The overlap of calcite cement on detrital quartz grains gives the impression in thin section that the replacement of quartz by calcite was extensive. However, thin-section and SEM views of samples leached in HC1 show that even partial replacement of quartz is rare and is limited largely to the edges of tiny quartz overgrowths. Ignoring intraformational clasts, most Maxon sandstones now are quartzarenites (Q86F1R3). However, compensating conservatively for feldspar grains replaced by calcite, sands originally were subarkoses (Q86F10R3).