Sedimentary Petrography

The Paleoproterozoic (∼2.15 Ga) Franceville Basin, South Eastern Gabon, is a continental sedimentarybasin that host unmetamorphosed sediments. This study involve detailed mineralogy, sedimentology,and petrography of the basal sedimentary units of FA and lower FB (FB1) Formations, from the basinmargin to centre in relation to mineral paragenesis and fluid flow. The FA Formation conglomerate, sand-stone, and mudstone consists lithofacies of mixed fluvial and fluvio-deltaic transitional origin, while theoverlying FB1 Formation includes alternating organic rich black shale and sandstone of marine deposit.The medium- to coarse grained fluvio-deltaic quartz arenite in upper part of FA Formation is charac-terized by pervasive authigenic quartz cementation that reduced the porosity and permeability in theearly stage of burial history. This provides a resistant framework for subsequent diagenetic modificationand also inhibits fluid flow during burial diagenesis. In contrast, the clay and unstable detrital grainsrich fluvial arkosic to sub-arkosic sandstones that escaped early quartz cementation show considerablepressure solution at grain contacts. These arkosics were less porous and permeable when depositedbut transformed to diagenetic aquifers as a result of dissolution of detrital grains during diagenesis andsubsequent precipitation of authigenic mineral cements in the resulting secondary porosities.From the proximal to distal basin and within sample suites, there is no considerable chemical variationin the petrographic distinct generations of the precipitated illite and chlorite suggesting their precipita-tion from a near equilibrium homogenous pore-fluid. The predominance of 1Mtillite polytype in mostlithologies reflects precipitation of the clay minerals and probably other cements in an environmentwith high fluid/rock ratio. Dolomite, anhydrite, barite, and Fe-oxides are the main crystallized authigenicminerals aside illite and Fe-rich chlorite clay minerals. The mineralogical assemblages and textural occur-rences of the rocks suggest that diagenesis and fluid flow in the FA Formation in the Franceville Basin aremainly controlled by depositional facies.

Archaeologists lack a protocol for systematically attributing quartzite artifacts to particular geologic sources of the material. This paper, in an effort to begin to remedy that situation, reports the preliminary results of a petrographic study of quartzite samples from the Upper Gunnison Basin (UGB), Colorado. In that region, the overwhelming predominance of quartzite (often > 90 per cent) at most archaeological sites has hampered efforts to ascertain with any certainty the mobility strategies of Paleoamerican (and later) residents. In this study, qualitative and quantitative characterization of texture and grain composition of 50 UGB quartzite specimens led to the identification of six statistically distinct groups of samples. The groups are not arbitrary divisions of the data set; rather, they are meaningful from geologic, geospatial, chronological, and human-behavioral perspectives.

For the first time, the characterisation and the ceramic properties of thirty clays from southern Tuscany are discussed on the basis of their particle size distribution, Atterberg limits, geochemical (XRF) and mineralogical (XRD) composition and microfossils content. Very little information was available for the Albegna Basin, therefore, sampling included all outcroppings of Neogene–Quaternary clays. The results allowed the distinction of Miocene, Pleistocene and Holocene Ca-poor clays, suitable for the production of red stonewares and dense and frost resistant masonry materials, from Pliocene Ca-rich clays, used for the production not only of red earthenware but also of hard and porous masonry materials.

On the territory Transcarpathian Ukraine, about 100 Palaeolithic localities are known up to our days. Most of them are surface finds. In spite of the rich archaeological heritage, the elaboration of the material, especially its petroarchaeological evaluation supported by professional scientific analytical methods, is in the initial phase as yet. The aim of the present study is to supply information on the lithic raw materials of the Palaeolithic settlements in Transcarpathian Ukraine, the detailed survey and description of the primary raw materials, their identification, description and terminology, as well as the outlining of the local raw material provinces and study of the distribution of the raw materials on archaeological sites. In the archaeological literature of Transcarpathian Ukraine, lithic raw materials are still described under incorrect petro-graphical terms. For example, for the raw material of Korolevo Palaeolithic site is, correctly speaking, hyaline dacite, and the " flints " of Beregovo region are indeed rocks of volcanic origin which have undergone metasomatic processes. Field survey for collecting geological samples localized 19 different raw material sources all of which yielded hard rocks with conchoidal fracture that are suitable for tool making with knapping. Out of the 19 raw material types 11 were actually found in archaeological assemblages of the studied area. The most popular raw materials of Transcarpathian Ukraine are the Korolevo hyaline dacite, Rokosovo obsidian, (Carpathian 3 type) and sili-ceous rhyolite tuff varieties (type I and II), siliceous tuffite (type I and II), siliceous and opalised rhyolite (type I and II) from the Beregovo Hills area, as well as silicified sandstone (type II) and the siliceous argillite. Certain types of potential raw materials were found in archaeological assemblages as yet. These are the Kriva limno-chalcedonite and limnoopalite, radiolarite of Svalyava type I, II and III, the siliceous limestone of Svalyava and Priborzhavske, and the hornfels of Suskovo. The paper also points out patterns in lithic raw material circulation in the prehistoric period of Transcarpathia. In the Palaeolithic, the settlement system and location of sites was largely dependent on the lithic sources. Altogether 9 types of rocks played important role: Korolevo hyaline dacite, the Carpathian 3 type obsidian from Rokosovo, 6 types of metasomatites of Beregovo Hills, and the silicified sandstone (type II). Upper Palaeolithic communities settled close to the outcrops of primary and secondary geological positions and this phenomenon is observable at each important Palaeolithic settlement. On the basis of the principal raw material circulation of the Palaeolithic three territorial groups have been formulated. These are named after the most abundant and used rock types of the given region. Three raw material regions are recognized in Transcarpathia: volcanic, metasomatic, and sedimentary. Furthermore, sub-regions were also established in the volcanic region (Rokosovo-Maliy Rakovets and Korolevo-Veryatsa sub-regions) and in the metasomatic region (Beregovo, Muzhiyevo and Bene-Kvasovo sub-regions).

Petrographic, isotopic and geochemical data are reported for the Cerro Victoria Formation of the Arroyo del Soldado Group (Uruguay). It is divided into four informal lithostratigraphic units (A to D), which are mainly composed of an alternation of stromatolitic, oolitic and clastic dolostones. The genesis of dolomite as a primary or penecontemporaneous precipitate is discussed, considering excellent preservation of textural details. Whereas 87 Sr/ 86 Sr ratios in the least altered samples are too high (0.7106-0.7109) to represent seawater values, δ 13 C determinations are interpreted as reflecting near-seawater composition. The curve obtained for the Cerro Victoria Formation is characterized by negative values at its base (−3.5‰ PDB ), passing into a slightly positive plateau up section. Peak values of + 0.64‰ PDB are encountered in the upper unit C. The upper Cerro Victoria Formation is characterized by a return to moderately negative values. The shape of the δ 13 C curve is consistent with the data reported for the Nemakit-Daldyn stage of the Lower Cambrian. We suggest that the Precambrian/Cambrian boundary is located at the Cerros San Francisco-Cerro Victoria transition or slightly lower in the succession. Thalassinoides-bearing dolostones of the upper Cerro Victoria Formation might reach the Tommotian or Atdabanian. In the Arroyo del Soldado Group, δ 13 C excursions parallel sea-level oscillations, transgressive events being characterized by positive δ 13 C values. Falling amplitudes of coupled δ 13 C/sea-level fluctuations recorded in these carbonates are in accordance with the trend observed in Ediacaran-Lower Cambrian successions worldwide. Covariance of δ 13 C and sea-level might be explained by bioproductivity fluctuations affecting the carbon cycle, atmospheric CO 2 concentrations, climate, ice-volume and ultimately sea-level. While these oscillations were of large magnitude in the late Neoproterozoic, they progressively faded away in the Cambrian, giving place to a more stable environment. The cause of the amplitude decrease in biogeochemical, climatic and sealevel perturbations might be the decreasing rates of nutrient input at mid-ocean ridges, as reflected by rising 87 Sr/ 86 Sr values between 800 and 500 Ma, punctuated by at least three events roughly coincident with rifting episodes of Rodinia.

This paper presents the results of a study of the linear hydric dilatation of some sandstones and meta-sandstones and its relations with their mineralogy, texture and physical properties. The ultimate aim is to identify the main factors and mechanisms responsible for hydric dilatation in silicoclastic rocks. The study has been conducted on various rock types selected for their different mineralogical, textural and physical properties: Macigno, Monte Senario and Manciano sandstones, and Pietra del Cardoso and Monte Pisano Quartzite meta-sandstones. In the selected samples, linear hydric dilatation measured perpendicular to rock bedding or foliation ranges from 0.15 to 1.01 mm/m. The results herein obtained indicate that the main factors influencing the degree and kinetics of dilatation upon water uptake are the amount, size and size distribution of the open pores, the pore shape and spatial distribution, the preferential orientation of phyllosilicates and the presence of significant amounts of expansible clayey components. These factors combine and interact in various ways to generate the different hydric behaviours observable in these stones. In particular, the linear hydric dilatation depends at first on the pore dimensions in respect to the absolute water absorption.

Three basic paleosol morphologies, named Type A, Type B and Type C, are described from the middle-upper Permian strata of the Moradi Formation, Tim Mersoi Basin, northern Niger. The Moradi Formation is a typical alluvial redbed succession dominated by red mudrocks with fine to coarse-grained pebbly channel sandstones and matrix-breccias. Type A paleosols are hosted by well-sorted fine to medium grained trough cross bedded and massive sandstones and preserve abundant vertical to horizontal micritic and microspar calcite tubules, interpreted as rhizoliths. Lateral variability of rhizoliths in Type A paleosols, and their close association with fluvial channel-fill sediments suggests they are the roots of grove stands of phreatophytic vegetation that grew within unstable anabranching stream systems. Type B paleosols are hosted by mudrocks and preserve well-developed ped structure, abundant micritic calcite nodules and vertically-stacked micritic calcite nodular bodies, as well as rare calcite with satin-spar texture interpreted as a pseudomorphic replacement of pedogenic gypsum. The morphology of Type B paleosols suggests they were formed in well-drained floodplain deposits on stable landforms. Type C paleosols are similar to Type B but preserve pedogenic structures indicative of soil volume expansion and contraction, as well as more abundant Stage II pedogenic carbonate nodules. The morphology of Type C paleosols suggests that they developed periodically rather than seasonally in poorly-drained deposits that nevertheless occupied a relatively stable part of the landscape such as the plains flanking ephemeral lakes or sabkhas. X-ray diffraction analysis of the b2 μm fraction from the Moradi Formation strata indicates that paleosol phyllosilicates are composed of illite, smectite, and occasionally kaolinite and talc. Illite is likely a detrital mineral, whereas smectite and kaolinite are likely pedogenic weathering products. The presence of talc in the Moradi Formation paleosols is unusual. It is limited to paleosol horizons that also preserve evidence for pedogenic gypsum accumulation and is therefore most likely related to a pedogenic weathering process. It is possible that this talc is a relatively low-temperature (~50-100°C) diagenetic alteration product of pedogenic Mg-phyllosilicates such as sepiolite. The range of morphologies, petrographic textures and mineralogy of the paleosol profiles indicates semi-arid to hyper-arid climatic setting. This paleoclimatic reconstruction is in agreement with Middle and Late Permian conceptual paleoclimate models and quantitative general circulation models. Nevertheless, and in spite of an arid climate, Moradi paleosols and their host strata also indicate a relatively shallow groundwater table. Importantly, this shallow groundwater resource undoubtedly helped to support the moderately diverse fossil vertebrate assemblage and large-stature macrophytes preserved in the Moradi Formation.