Calcite-cemented Sandstone Research Articles

Coeval biochemical and biophysical weathering processes on Quaternary sandstone terraces south of Rabat (Temara), northwest Morocco

AbstractDespite numerous investigations on substrate‐inhabiting microflora, especially lichens, very little is known about the colonization of coastal escarpments by lithobiontic micro‐organisms, inland of a retreating coastline in Africa. Reported herein are the results of a combined field observation and microscopy study focusing on the connection between microrelief of the substrate, colonies of lithobiontic micro‐organisms (in particular the lichen Xanthoria parietina) and microstructures of putative bacterial origin. The occurrence of weathering pits in which the early stages of the biotic development occurs, and the subsequent disintegration of the rock indicate that lichens, mosses and fungi act synergistically by alternating chemical and mechanical weathering. Penetration of grains by expansion and contraction of the hyphae depletes the rock matrix and contributes to the mechanical breakdown of the rock. Calcite rhombs on the weathered surfaces of the calcite‐cemented sandstones are severely etched with well‐developed rhomb‐shaped etch pits (‘spiky calcite’), holes, or has one or more of the faces removed, and their cores exposed and leached. Nanofilaments (c. 100–700 nm) and ‘nanomicrobial’ fruiting bodies (c. 250 nm) emanating from micropores appear to be common on affected crystalline structures. Weddellite present immediately below the thallus is a strong indicator of biomineralization.Quartz responds differently to chemical weathering by producing peeling structures and microbrecciation features. The dissolution of these crystals appears to be a surface reaction‐controlled process mediated by microbial microfilaments and nanofilaments. A model is proposed, firstly indicating early‐stage biochemical weathering, followed by biophysical weathering. Disintegration of the rock outcrops in due to a complex interplay of several events, probably beginning at the nanoscale with penetration of sites on crystal faces. Copyright © 2006 John Wiley & Sons, Ltd.

Sequence stratigraphy and sea level history of the upper Paleocene strata in the Kopet-Dagh basin, northeastern Iran

The intracontinental Kopet-Dagh basin formed after the Middle Triassic orogeny in northeastern Iran and southwestern Turkmenistan. This underexplored basin could provide significant petroleum reserves in the 21st century. The upper Paleocene Chehel-Kaman Formation is exposed along northwest-southeast-trending folds and is composed of carbonate strata and minor amounts of siliciclastic and evaporite beds. Six stratigraphic sections in the central and eastern parts of the basin have been used to divide the upper Paleocene (Thanetian) carbonate supersequence into four major carbonate lithofacies, each having multiple subfacies. These lithofacies represent open-marine, shoal, semirestricted lagoon, upper intertidal, and tidal-flat subenvironments that formed on a shallow carbonate ramp. In addition, there are two siliciclastic lithofacies consisting of calcareous shale (marl) and calcite-cemented sandstone. The upper Paleocene interval consists of three depositional sequences (DS1, DS2, and DS3), bounded by type 2 (within the top of the underlying Pestehleigh Formation), type 2, and type 1 sequence boundaries, respectively. Sea level changes during the Thanetian in the Kopet-Dagh basin are similar to global changes proposed by Haq et al. (1988), with differences related to local and regional geological events. Upper Paleocene strata were deposited in about 4 m.y., within the range of second-order cycles. Each depositional sequence was developed as a third-order cycle composed of several shallowing-upward parasequences (fourth- to fifth-order cycles). We estimate that sea level fluctuations in the study area were between 5 and 11 m during development of parasequences. (Begin page 840)

Distribution of the Bandera Shale of the Marmaton Group, Middle Pennsylvanian of Southeastern Kansas

In southeastern Kansas, the Middle Pennsylvanian (Desmoinesian) Bandera Shale consists of sandstone, shale, limestone, and coal deposited between two carbonate formations, the underlying Pawnee Limestone and the overlying Altamont Limestone. Isopach maps and cross sections indicate that the Bandera Shale thickens southeastward towards the Oklahoma and Missouri borders. Analysis of gamma-ray-log signatures, augmented by neutron-log signatures, indicates that the Bandera Shale is rich in mudstone, with sandstones limited to intervals ranging from 10 ft to 30 ft (3-9.1 m) in thickness. Comparisons with previously studied cored and logged siliciclastic portions of overlying Missourian lithologies suggest that the Bandera Shale consists of various proportions of sandstone, siltstone, clay-rich shale, and calcite-cemented sandstone.
 Exposures of the Bandera Shale in Bourbon County, Kansas, consist of interbedded shale and calcite-cemented, fine-grained sandstone. Sandstone beds, ranging from 3 cm to 20 cm (1.2-7.9 in) in thickness, are, in places, rhythmically laminated with organic-rich and organic-poor lamina forming 2-mm (0.8-in)-thick couplets. Many sandstone bedding surfaces in the lower and middle portion of the Bandera Shale are bioturbated with horizontal feeding trails and some vertical burrows that suggest marine environments. Thicker sandstone units are either trough cross-bedded, with sets up to 1.5 m (4.9 ft) thick, or amalgamated ripple cross-laminated and flaser-laminated.
 Outcrop observations coupled with subsurface analysis indicated that Bandera Shale in southeastern Kansas was deposited as a siliciclastic complex that prograded westward during a sea-level lowstand. Siliciclastic sediments may have been deposited in a clastic wedge or deltaic complex, but sedimentary characteristics observed in outcrops record marine influence at least along the margins of the complex. Rhythmic stratification within sandstone beds that are interbedded with shale resemble tidal features described elsewhere in the Pennsylvanian of North America and suggest that embayments were present where tidal cells were amplified along a morphologically irregular shoreline. Bioturbated sandstone units, interbedded with clay shale, record high-energy events that influenced sand distribution.

Authigenic Quartz Microfabrics in Cretaceous Turbidites: Evidence for Silica Transformation Processes in Sandstones

ABSTRACT Lower Cretaceous sandstone turbidites in the Scapa Field (North Sea) contain a complex variety of authigenic quartz microfabrics. Early diagenetic microfabrics predate burial calcite cements and compaction. They comprise abundant grain-rimming microquartz, intergranular aggregates of microquartz with authigenic illite-smectite, and localized isopachous cryptocrystalline quartz and chalcedonite fringes. These precipitates contain evidence of precursor opal-CT, in the form of dissolved, recrystallized, or pseudomorphed lepispheres and rim cements. Late diagenetic quartz microfabrics are postcompactional and either replace or overgrow burial calcite cements. They display a paragenetic trend from microporous, fibrous silica (quartzine or chalcedonite) to equant, syntaxial, and mosaic meso uartz cements. None of these contain evidence for opal-CT precursors, and they are likely to have precipitated directly from silica-rich pore fluids. Cathodoluminescence examination of calcite-cemented sandstone intervals reveals that siliceous sponge spicules were a major depositional component, and were the most likely silica source for opal-CT and subsequent quartz cementation. Dissolution of biogenic silica and formation of opal-CT began shortly after deposition, with replacement of opal-CT by quartz during burial of the Scapa Member to 1-2 km. The early diagenetic silica transformations are identical to those recorded from Cenozoic porcelanites and cherts. Thermodynamic models developed for the diagenesis of biogenic cherts can therefore be applied to the Scapa sandstones, and qualitatively explain the development of contrasting quartz microfabrics on a bed-to-bed (or smaller) scale. In contrast, direct precipitation of chalce onic quartz during late diagenesis required a resurgence of high supersaturations. This is difficult to account for without some import of silica to the sandstones and/or cooling of pore fluids. Precipitation most probably occurred from allochthonous, deep basinal brines associated with early stages of hydrocarbon migration.

Geochemistry of dissolved inorganic carbon in a Coastal Plain aquifer. 1. Sulfate from confining beds as an oxidant in microbial CO 2 production

A primary source of dissolved inorganic carbon (DIC) in the Black Creek aquifer of South Carolina is carbon dioxide produced by microbially mediated oxidation of sedimentary organic matter. Groundwater chemistry data indicate, however, that the available mass of inorganic electron acceptors (oxygen, Fe(III), and sulfate) and observed methane production is inadequate to account for observed CO 2 production. Although sulfate concentrations are low (approximately 0.05–0.10 mM) in aquifer water throughout the flow system, sulfate concentrations are greater in confining-bed pore water (0.4–20 mM). The distribution of culturable sulfate-reducing bacteria in these sediments suggests that this concentration gradient is maintained by greater sulfate-reducing activity in sands than in clays. Calculations based on Fick's Law indicate that possible rates of sulfate diffusion to aquifer sediments are sufficient to explain observed rates of CO 2 production (about 10 −5 mmoll −1 year −1), thus eliminating the apparent electron-acceptor deficit. Furthermore, concentrations of dissolved hydrogen in aquifer water are in the range characteristic of sulfate reduction (2–6 nM), which provides independent evidence that sulfate reduction is the predominant terminal electron-accepting process in this system. The observed accumulation of pyrite- and calcite-cemented sandstones at sand-clay interfaces is direct physical evidence that these processes have been continuing over the history of these sediments.

Sandstone Diagenesis above and below a Pressure Seal--Tuscaloosa Trend, Louisiana

Significant diagenetic differences are observed in the Lower Tuscaloosa sandstone sampled above and below a pressure seal. Additionally, textural evidence suggests that the high porosity (> 20%) in some of the sandstones is secondary. Samples are classified into two groups: normal' (< 12%) and high porosity (12-24%). The normal' porosity sandstones, situated predominantly above the seal, are cemented by either quartz or calcite. At some grain-quartz overgrowth contacts, dissolution of overgrowths and portions of grains is concurrent with precipitation of calcite; some calcite is partially replaced by saddle dolomite. Pore-lining chlorite is present but rare in the normal' porosity sandstones. The high porosity sandstones, situated predominantly below the seal, retain remnants of quartz and carbonate cements; virtually all pores are lined with chlorite. High porosity is attributed primarily to dissolution of carbonate cement. Evidence for previously existing carbonate cement includes: remnant cement, rhomb-shaped porosity, and partially dissolved quartz grains whose surfaces are similar to grain surfaces of the calcite-cemented sandstones. The characteristic robust pore-lining chlorite growth is attributed to accelerated dissolution of volcanic rock fragments with the increase in porosity and permeability after carbonate dissolution. The authors suggest that the pressure seal plays two roles in creating and preserving high more » porosity: (1) the seal impedes fluid flow between two zones of different fluid chemistry and therefore different paragenesis in rocks of essentially the same age and depositional environment, and (2) the seal creates a compartment in which fluid pressures support the rock, inhibiting further compaction after the dissolution of the cement. « less

Fe Substitution for Al in Glauconite with Increasing Diagenesis in the First Wilcox Sandstone (Lower Eocene), Livingston Parish, Louisiana

AbstractGlauconites in early ankerite concretions, ferroan calcite-cemented sandstones, and uncemented sandstones in the first Wilcox sandstone of the Lockhart Crossing field, Livingston Parish, Louisiana, show a progressive substitution of Fe for octahedral Al with increasing diagenesis. An octahedral Fe content of 0.50 atoms was calculated from glauconite located in early ankeritic concretions. Octahedral Fe averaged 0.60 and 0.90 atoms in later ferroan calcite-cemented sandstone and uncemented sandstone, respectively. Corresponding octahedral Al averages were 1.16, 1.03, and 0.67, respectively. A systematic increase in average interlayer K from 0.49 to 0.54 to 0.61 was also observed, with apparent increases in diagenesis. All element determinations were made with an electron microprobe and recast on an anion equivalent basis to structural formulae based on the O10(OH)2 unit. The clay preserved in the early ankerite concretions was found to be an illite/smectite containing about 20% expandable layers, and the mineral in the glauconite pellets from uncemented areas of the sandstone, an ordered glauconite. “Minus cement” porosities of the sandstone indicate that glauconitization may have taken place at burial depths greater than 0.6 to 1.8 km, but the mechanism for the incorporation of Fe3+ in the glauconite at that depth is not apparent.

Reservoir Heterogeneity Within Bartlesville Sandstone, Glenn Pool Oil Field, Creek County, Oklahoma: ABSTRACT

The extent of reservoir heterogeneity within the Middle Pennsylvanian (Desmoinesian) Bartlesville Glenn sandstone in the 160-ac William Berryhill unit was established using more than 70 modern well log suites and 18 cores. Reservoir characterization of genetic sandstone units within the Bartlesville is based on differences in lithologic characteristics, sedimentologic features, porosity, permeability, and log-response characteristics. The upper delta-plain depositional setting of the Bartlesville was such that short-distance changes in facies created many small-scale heterogeneities. Recognition of laterally continuous distinct units in the Bartlesville sandstone requires integration of well log signatures and rock properties. Although thin intervals of interbedded sandstone and shale, and calcite-cemented sandstone are discontinuous laterally, they tend to compartmentalize portions of the reservoir. The Bartlesville sandstone is presently a sublitharenite-litharenite, even after its composition has been influenced strongly by diagenetic processes. Porosity is mostly secondary owing to the dissolution of unstable framework grains. Distribution and general trends of porosity are affected by changes in composition and pore geometry in particular rock units as well as local changes in depositional trend.

The treatment of calcite-cemented sandstone with acrylic-siliconate mixture in limewater

The treatment of calcite-cemented sandstone with acrylic-siliconate mixture in limewater

Example of Inner-Shelf Sand Ridges from Upper Cretaceous Eagle Sandstone, Central Montana Uplift: ABSTRACT

The Upper Cretaceous Eagle Sandstone of central Montana was deposited during a general eastward progradation of the western shoreline of the narrow, north-south-trending Western Interior epicontinental seaway. Cordilleran highlands to the west were episodically uplifted, and provided the main source for sediments deposited in the seaway. On the Central Montana uplift, the lower member of the Eagle consists predominantly of very fine-grained sandstone, which is exposed as a thick (average 100 ft, 30 m), continuous topographic rim. This sandstone gradationally overlies shales of the Telegraph Creek Formation. Within the rim, resistant beds and concretions of calcite-cemented sandstone define several smaller units, which can be traced laterally over distances of several miles. These units maintain fairly uniform thicknesses from north to south. However, in an east-west direction, the units thin, are imbricated, and become younger to the west. Excellent exposures of these imbricated lenses occur along a rim that extends 12 mi (19 km) southwest from the town of Winnett. Although the sandstone of the lower member is very fine-grained throughout the rim, systematic changes occur within a single lens. These changes include: (1) thinning and grading into shale and siltstone to the southwest: (2) bioturbation decreasing upward and to the northeast; (3) oblique Asterosoma burrows predominating in the lower part of each lens and to the southwest, with horizontal Ophiomorpha burrows being more common in the middle part of each lens and to the northeast; (4) parallel bedding and hummocky cross-stratification successively occurring in the upper part of each lens and to the northeast; and (5) relatively straight-crested symmetrical ripples generally capping each lens. The sedimentary structures within each lens indicate increasing energy and shoaling upward, bu do not indicate subaerial exposure. The lenses in the lower member are interpreted as landward-prograding (westward) sand ridges that were deposited on the inner shelf at distances of tens of miles from the shoreline. Laterally equivalent coastal sandstones of the Virgelle Sandstone Member prograded seaward (eastward) at this same time. The lenses are elongated in a north-south direction, generally parallel to the coast. However, the exact geometry of individual ridges is unknown. After bypassing the zone, the sand probably was transported parallel to the shoreline by geostrophic currents driven by wind-forcing. Storm waves reworked the upper, seaward-facing slopes of the ridges, whereas landward-facing parts of the ridges were more protected and subjected to bioturbation. Ridges that occur on the Central Montana uplift are comparable in many aspects to sand ridges on the modern Atlantic inner shelf. However, the modern sand ridges differ from those of the Eagle in two ways: (1) they occur at angles oblique to the shoreline, and (2) they resulted from shoreface detachment during the Holocene transgression. End_of_Article - Last_Page 1352------------

Pleistocene Shoreline Deposition and Sea-level History at Swartklip, South Africa

ABSTRACT A 50-m-thick succession of poorly cemented sandstone is exposed along the northern shore of False Bay, South Africa, This Late Pleistocene succession is attributed to shorezone-estuarine deposition in the last interglacial and dune activity during the highstand and early in the succeeding sea level withdrawal. Facies analyses allow us to establish that the last interglacial sea level high (120± ka) locally reached an elevation between the upper and lower boundaries of a washover sequence that is 5.1 to 8.0 m above present mean sea level. Four depositional facies are recognized on the basis of distinct lithologies and assemblages of sedimentary structures. Beachrock is present intermittently in the present intertidal zone as a seaward-dipping unit of flatbedded sandstone capped by calcrete. We interpret this facies as a beach foreshore wedge which originated in warmer climates of the 135± ka sea level high, and which developed its calcrete crust in the lowstand which immediately followed. Subsequent sea level rise at 120± ka and shoreline progradation is reflected in a two meter-thick estuarine fill containing Tomichia ventricosta. A third facies comprises landward-dipping sheetsands which overlie the estuarine fill. On the basis of facies relationships and primary sedimentary structures, this unit is attributed to washover fan deposition. The last and dominant facies comprises calcite-cemented sandstone arranged in sets of very large-scale crossbeds. Pulmonate gastropods occur throughout. Poorly developed calcretes interrupt the succession. Deposition of this facies is believed to have occurred as parabolic dunes in a coastal foredune complex near the start of the Wurm (Wisconsinan) glacial climate when atmospheric circulation was more intense. Modern pedotubule and hardpan calcretes cap the succession.

Early Diagenesis, Atherton Formation (Quaternary), Northern Indiana: A Guide to Understanding Early Cement Distribution in Nonmarine Sandstones: ABSTRACT

In the area studied, the Atherton Formation accumulated primarily as outwash deposits dominated by trough and wedge cross-bedding. Within sand (86% of deposits; Q77F8L15) and polymictic pebble gravel (14%) units, local cements of calcite (99%) and limonite/hematite (1%) are present. The distribution of these cements was controlled by at least three factors: (1) position within the deposit relative to the land surface, (2) average grain size, and (3) primary stratification. Over 90% of cement zones are within 7 m (23 ft) of the present land surface. Meteoric waters made more acidic by decaying organic material locally dissolved carbonate framework grains. As pore fluids continued to move downward and laterally, cementation occurred. On a second level, higher permeability zones associated with sediment of larger grain sizes was an important factor influencing the location of cementation. Cemented horizons are present in 48% of pebble and sandy pebble gravels (contact and pore lining types) and 17% of sands and pebbly sands (contact, pore lining, and occluded types). Moreover, within sand units primary stratification was a parameter that influenced the location of cementation sites. In wedge cross-bed sets, cement zones parallel inclined laminations; in tr ugh cross-beds, 79% of the cement is concentrated in the lower one-third of bed sets near trough axes or immediately below a trough's basal erosion surface. Cement zones were preferentially developed along internal curved laminations within cross-bed sets. Higher permeability concordant with stratification, the result of excellent sorting and coarser grain sizes within individual sand laminations, primarily controlled cement distribution at this level. Carbonate cements in the Atherton range from a trace to 6 mole % MgCo3 (X = 3.6 mole %) with traces of Fe in 16% of the samples analyzed (205 total analyses). The calcite and Mg-calcite generally form a mosaic of anhedral crystals, although distinct rhombohedra are present in some pores. The source for Mg and Fe is believed related to the dissolution of dolomite and Fe-bearing silicate framework grains. The limonite/hematite cements are present as rare, pore-filling patches. A comparison of the percent of carbonate grains in uncemented sand (X = 10 ± 1%) and within cemented zones (X = 7 ± 1%) of similar grain size indicates an information loss during very early diagenesis on the order of 20 to 30% of the carbonate framework component. Therefore, the original proportion of carbonate framework grains in ancient calcite-cemented sandstones may be greatly underestimated due to grain destruction during diagenesis. End_of_Article - Last_Page 489------------

The nature of deformation in experimentally deformed calcite-cemented sandstone : Underhill, DH Thesis. Figs, Tabls, Refs. McMaster Univ. Canada, 1972

The nature of deformation in experimentally deformed calcite-cemented sandstone : Underhill, DH Thesis. Figs, Tabls, Refs. McMaster Univ. Canada, 1972

Petrology and Paleogeography of Mississippian Madison Formation in Western Saskatchewan: ABSTRACT

In western Saskatchewan the Mississippian carbonate rocks may be categorized into 5 lithofacies, which individually or collectively can be used to assist the interpretation of the various environments of deposition. The five categories are: (1) impoverished calcareous mudstone, (2) skeletal calcareous mudstone, (3) micritic skeletal or nonskeletal sandstone, (4) poorly washed skeletal sandstone, and (5) calcite-cemented skeletal or nonskeletal sandstone. The impoverished calcareous mudstones dominate the lower part of the Mississippian succession, particularly in the southern part of the study area, where they are composed of bituminous and argillaceous materials as well as nonargillaceous calcareous mudstones. The impoverished calcareous mudstones probably represent sediments deposited in water below wave base on the seaward side of the shelf edge. Collectively, the skeletal lithofacies probably represent shelf-edge deposits. Calcite-cemented crinoidal sandstones accumulated as thick, elongate banks. The poorly washed sandstones are generally at the top and bottom of the banks and the interbank areas show outward gradations from micrite skeletal sandstones on the flanks of the banks to skeletal calcareous mudstones in the centers of the interbank areas. Micritic skeletal sandstones also form blanket deposits that commonly act as platforms upon which the crinoidal banks were built. The nonskeletal sandstones (primarily composed of oolites) probably represent subtidal deposits on the shoreward side of the shelf edge. In the central part of western Saskatchewan they are present in the lower part of the succession and pass laterally southward into shelf-edge deposits. Farther south they are found at higher stratigraphic levels, and they are in lateral continuity with and overlie shelf-edge deposits. The relationships suggest a regression of open marine conditions from north to south. If the Mississippian sequence of southeastern Saskatchewan is used as an analogy, a dolomite-evaporite facies representing a supratidal environment probably existed north of the subtidal deposits, that is, beyond the present subcrop edge of the Mississippian in western Saskatchewan. This postulated distribution of facies suggests that possible hydrocarbon reservoirs similar to those of the Virden area of Manitoba are present in subtidal and shelf-edge deposits along the Mississippian subcrop in Saskatchewan. End_of_Article - Last_Page 959------------

Elongate Concretions as Paleochannel Indicators, Tongue River Formation, North Dakota: ABSTRACT

Two types of sand bodies occur in the nonmarine Tongue River Formation (Paleocene) of western North Dakota, linear and tabular. The tabular sand bodies have sharply scoured bases, become finer upward, and the vertical sequence of sedimentary structures shows a decrease in the flow regime upward. These bodies are overlain by gray lignitic silts and clays, and they are interpreted as high-sinuosity stream deposits. Linear sand bodies are the most common type in the Tongue River Formation. They are usually straight and have deeply channeled bases and flat tops. Vertical changes in grain size show no consistent pattern, but the upper parts become finer upward in many places. Sedimentary structures are about 60% planar (omikron) cross-stratification, 25% horizontal or low-angle planar laminae (plane bed) with parting lineation, and 15% small-scale ripple cross-stratification. Paleocurrent indicators parallel the axes of these bodies, which are interpreted as low-sinuosity stream deposits. Elongate concretions, up to about 15 ft wide and hundreds of feet long, occur only in the linear bodies. They consist of calcite-cemented sandstone, and they usually occur at or near the tops of the bodies. They help to protect the bodies from erosion so that in places the bodies exist as exhumed paleochannel systems that form an inverted topography. The axes of the concretions parallel both paleocurrent indicators and the axes of the sand bodies. The concretions are clearly visible on air photos and can easily be used to map paleochannel patterns either on the air photos or in the field. End_of_Article - Last_Page 958------------

Nature, Origin, and Significance of Cone-in-cone Structures in the Kiowa Formation (Early Cretaceous), North-Central Kansas

ABSTRACT The parts of the Lower Cretaceous (Albian) Kiowa Formation that crop out in north-central Kansas contain abundant calcareous cone-in-cone structures occurring as discontinuous lenses and ellipsoidal concretions in shale, and as irregular layers in calcite-cemented sandstone. The cone structures are of two petrographic types: (1) those in the concretions and calcite-cemented sandstone, which are composed of plumose aggregates of fibrous calcite, and (2) those in the lenses of cone-in-cone, which are composed of microgranular calcite. Formation of the various types of cone-in-cone is attributed to precipitation and growth of fibrous crystals of calcite during early diagenesis of the enclosing brackish-water Kiowa sediments. The microgranular texture of the calcite in the cone-in-cone le ses is attributed to diagenetic recrystallization of initially fibrous calcite. Recrystallization probably was induced by anaerobic decay of organic matter in fossiliferous strata underlying the lenticular beds. The consistent association of calcite, marcasite, and carbonaceous debris in the cone-in-cone implies that a unique set of physico-chemical conditions was essential to the development of the cone structures at some unspecified depth below the sediment-water interface while the sediments still were plastic. The diagenetic environment in which the calcareous cone-in-cone structures formed contrasts with that in which concretions of impure siderite grew in Kiowa shale chiefly in the availability of calcium ion and reduced sulfur. The environments in which the shale that now encloses calcareous cone-in-cone structures was deposited, although brackish, probably were less brackish than the environments in which shale that now encloses concretions of impure siderite was deposited.

Recent Calcite-Cemented Sandstone Generated by the Equatorial Tree Iroko (Chlorophora Excelsa), Daloa, Ivory Coast

In the region of Daloa, Ivory Coast, several Iroko trees ( Chlorophora excelsa , family of the Moraceae), a frequent species of the equatorial rain forest known for its calcareous excretions always related to wounds kept unhealed by insect activity, show an unusual lithogenetic aspect of this process. Between severely damaged roots, large-scale bleeding of the sap, after unsuccessful attempts to build crusts of fibrous carbonate over the wounds, has penetrated into the interstitial spaces of the adjacent granitic residual sand and cemented it by precipitation of calcium carbonate into large blocks of hard and massive calcite-cemented pure-quartz sandstone. The quartz and rare feldspar grains displayed a high amount of marginal corrosion by the cryptocrystalline calcite cement. The agents responsible for the severe damage to the trees cannot be determined with certainty but are assumed to be elephants, wild boars or jungle buffaloes. This recent sandstone demonstrates processes of intense corrosion of detrital quartz and feldspar grains and of general cementation both by calcite which have taken place at or near the surface before any decay, burial, overburden or compaction could occur and which, therefore, should be considered as geologically instantaneous.

Discriminatory Analysis of Calcite- and Silicate-Cemented Phases of the Mountain Park Sandstone

The Mountain Park Sandstone is a thick lens of non-marine arenaceous strata present locally near the top of the Lower Cretaceous Blairmore Group in the folded foothills region of western Alberta. The sandstone is composed mainly of quartz, plagioclase, and volcanic rock fragments cemented by authigenic chlorite, illite, quartz, and calcite. The cements are distributed in a locally patchy fashion, forming irregularly shaped, interlensing calcite-cemented and silicate-cemented zones up to several centimeters or more in maximum dimension. Variation in calcite content is especially noticeable, with individual samples containing from 0 to 57 per cent calcite distributed irregularly throughout the sampled stratigraphic interval. Calcite is also irregularly distributed within most thin sections as optically continuous poikilitic units or crystal clusters, which contrast with surrounding calcite-deficient portions of the sections. Individual calcite crystals range in size from a few microns to several centimeters, depending on the amount present, forming spherulitic fibrous aggregates in samples especially rich in calcite. Discriminatory analysis of calcite- and silicate-cemented phases on the basis of their compositional and textural properties indicates that grain-packing (pore size) is the only variable associated with the distribution of cements. Calcite-cemented phases are more loosely packed than adjacent silicate-cemented phases, but grain size, grain orientation, and detrital composition show no significant association with cement distribution independent of packing. However, calcite-cemented sandstones tend to be coarser-grained on the average than silicate-cemented ones because of the marked correlation between pore size and grain size. These results are compatible with the observed intergrowth relationships of the cements, which show that chlorite was deposited first, completely cementing the more tightly packed portions of the sandstones. Illite and quartz were then deposited in the remaining parts of the larger pores. The circulation of later carbonate-bearing solutions was determined mainly by the distribution of earlier cements, with calcite being deposited in the more permeable, loosely packed sandstones, replacing some of the earlier detrital and authigenic constituents.

Petrofabric Analysis of Experimentally Deformed Calcite-Cemented Sandstones

Cylinders of sand crystals, composed of single crystals of calcite that poikilitically enclose detrital grains, and calcite-cemented sandstones from the Tensleep (Pennsylvanian, Wyoming) and Supai (Permian, Nevada) formations were experimentally deformed dry at confining pressures of 1-5 kilobars and temperatures of 150°-300° C. Thin sections of the undeformed and deformed specimens were studied microscopically to gain a better understanding of the behavior of sandstones in simulated tectonic environments. The calcite and the detrital grains (quartz, feldspar, and others), which have radically different physical and mechanical properties, are shown statistically to have deformed with respect to the principal stresses across the boundaries of the whole specimens rather than with regard to local stress concentrations at grain contacts. The deformation mechanisms of calcite and quartz are the same in the sandstones as in monomineralic aggregates, such as marbles and quartz sands. Statistically, twin lamellae are developed in those calcite grains that are favorably oriented for twin gliding with respect to the load axes. The resolved shear-stress coefficient for these twin planes averages 0.27. Compression and extension axes deduced from the best developed set of twin lamellae in each calcite grain yield derived positions for the principal stress axes that are in excellent agreement with those known from the experiments. In addition, the number of lamellae per millimeter increases with increased strain of the specimens. Quartz, feldspar, rock fragments, and garnet grains comprise the bulk of the detrital material. These deform primarily by fracturing. The microfractures in the grains are nearly planar features which, because of their geometric relationship to the known principal stress directions, are recognized as extension and shear fractures. They develop independently of thegrain mineralogy and, in quartz grains, greatly overshadow a slight tendency for fractures to parallel r{} and z{}. The degree of fracturing in a specimen, expressed as a fracture index, tends to increase with increased strain of the specimen. In sand crystals loaded unfavorably for twin gliding in the calcite crystal, extension fracturing in the detrital grains causes local reorientation of the stresses and produces twin lamellae in the adjacent calcite. Twin lamellae in calcite and fractures in detrital grains are shown to be criteria for simulated tectonism. The development of lamellae and microfractures is directly related to the orientations of the principal stresses in heterogeneous, sedimentary rocks at the time of deformation.

The Dynamic Significance of Deformation Lamellae in Quartz of a Calcite-Cemented Sandstone

In 3 specimens of macroscopically folded Oriskany Sandstone, deformation lamellae are profusely developed in quartz grains, and the calcite cement contains numerous twin lamellae on {0112} = e. Following the method used by Turner, axes of compression and tension were deduced from a statistical study of the pattern of calcite twinning in each specimen. The stress systems so deduced are symmetrically related to the geometry of macroscopic folding. Triaxial stress fields were obtained from samples from the limbs of an anticline, and a uniaxial stress field was determined from a sample from the axial plane of the anticline. The maximum and minimum stress axes, sigma 1 and sigma 3 , so determined lie within 10 degrees of the bedding and are perpendicular to the local fold axis; the intermediate axis, sigma 2 , parallels the fold axis. Quartz lamellae show strong patterns of preferred orientation also symmetrically related to the stress systems deduced from calcite twinning; they tend preferentially to develop in planes of high shear stress inclined at less than 45 degrees to the inferred axis of maximum stress, sigma 1 .