Occurrence Of Chert Research Articles

Pervasive accumulations of chert in the Equatorial Pacific during the early Eocene climatic optimum

Biosiliceous cherts hosting the transformation of biogenic silica to paracrystalline opal-CT, deposited in the low-latitude realms of Jurassic to Neogene, were recently interpreted as developing during episodes of elevated primary productivity along upwelling areas in equatorial and sub-equatorial paleo-latitudes. Geographically widespread distribution of chert and other rock types of biosiliceous origin, including porcellanite and siliceous claystones throughout the Eocene however indicates these diagenetic deposits are not in any case the dependable markers of latitude-driven upwelling regimes. This study aims to explore the dominance of early Eocene chert occurrences in the Equatorial Pacific (EP) sediments, and their contribution to the Cenozoic global climate change. Our new assessment of the age-abundance dispersal of EP Cenozoic chert sections accommodated by Deep Sea Drilling Project (DSDP)- and Ocean Drilling Program (ODP)-cored deposits realized that the cherts transformed from deposited opal-A sediments most commonly in the early Eocene, with a peak in abundance at ∼50 Ma. This age interval of significant chert occurrences coincided with the period of seawater-temperature maxima of the early Eocene climatic optimum (EECO), during which the world ocean was supposedly marked by highly reduced upwelling intensity and siliceous bioproduction. The spatio-temporal distribution model of the EP Cenozoic cherts with the peak occurrences during the EECO presented in this study bears close similarities to the global distribution patterns of chert during the Cenozoic, which strongly correspond to a global paleoclimatic driver rather than upwelling regimes. Following our survey, cherts occurred less frequently in the post-early Eocene, which is characterized by cooling bottom waters and amplified upwelling and bioproductivity. This model suggests the paleo-seawater rich in dissolved silicon (DSi) would have been imperative for peak incidences of early Eocene chert production, despite extreme surface warmth and greenhouse conditions of the EECO, making DSi available to opal-secreting phytoplankton via deep-water ventilation. These imply that the paleo-climate was more impactful to diagenetic chert production than latitudinally mediated ocean upwelling. The chert distribution events during the early Eocene reduced siliceous bioproductivity appear to have been insufficient to compensate for the significant silica input to the basin mainly from climate-controlled continental weathering. The DSi uptake by precipitation of authigenic clays which accompanies opal-A diagenesis in many depositional systems has been operative, with higher rates in carbonate- than clay-dominated sediments, so as to sustain balance between DSi released to the ocean from terrestrial sources and the silica expelled in sediments during the EECO weakened upwelling.

Petrographic and geochemical characterization of chert artifacts from Middle, Upper, and Epi‐Paleolithic assemblages in the Jebel Qalkha area, southern Jordan

AbstractThis study conducted petrographic and geochemical analyses of chert artifacts from the Late Middle Paleolithic, the Initial Upper Paleolithic, the Early Upper Paleolithic, and the Epi‐Paleolithic assemblages in the Jebel Qalkha area, southern Jordan, to examine their correlations with the visual attributes and diachronic variability. The results revealed two different aspects of the petrographic and geochemical signatures. The first aspect showed some correlations with the visual chert types that were characterized by the abundance/preservation of fossils, the enrichment of several elements (i.e., Ca, Sr, and Ba), and the quartz crystallite size.The second aspect of geochemical signatures, such as Fe, Mn, Ni, Cr, and Co, showed no correlation with the visual types but was correlated with the chrono‐cultural groups. Given the chert occurrences and variability in and around the study area, the first aspect of the chert variability likely represents the variations of different chert outcrops as well as the internal variations within the same sources. Whether the second aspect represents chrono‐cultural changes in the use of chert sources needs to be clarified in future by our ongoing examination of geological chert samples in the study area.

Geochemistry and petrology of striped chert as a provenance tool for artefacts from the KRZEMIONKI NEOLITHIC mining area (Poland)

A prehistoric flint mine in Krzemionki with a neighboring area covers at least 2,300-year history of striped chert extraction and dressing, spanning the age bracket of 3900 through 1600 B.C. The striped cherts occur in the form of concretions (nodules) varying from centimeters to at least 2 m across. This paper presents updated results of petrological and geochemical study of these siliceous rocks using optical and scanning electron microscopy, electron microprobe analysis (major/trace elements), and laser ablation-inductively coupled plasma-quadrupole mass spectrometry (rare earth elements). Striped chert concretions show strong light REENASC enrichments in relation to heavy REENASC; distinctly positive Eu, Pr, Tb, Ho, and Tm anomalies (≥1.2); as well as predominantly low (<0.6) Al2O3/Al2O3 + Fe2O3 + Mn2O3 ratios. These features discriminate these siliceous rocks from most chert occurrences in Europe. Microanalysis of individual concretions has confirmed multiphase provenance of striped chert concretions as evidenced by variations in concentrations of REE and other minor and trace elements. These characteristic markers may be used as geochemical proxies for studying the chert geologic/geographic origin and for sourcing of chert artefacts.

Siliceous rocks from the southern part of the Kraków-Częstochowa Upland (Southern Poland) as potential raw materials in the manufacture of stone tools – A characterization and possibilities of identification

The Upper Jurassic sediments from the Kraków-Częstochowa Upland developed as carbonates, mostly limestones, which represent the microbial-sponge megafacies that were typical of the northern margin of the Tethys Ocean. Outcrops of these rocks are scattered along an extended belt ranging from Portugal to the Caucasus Mts. One of the common features of the bedded limestones belonging to this megafacies is the local occurrence of chert concretions.The siliceous rocks embedded within the Upper Jurassic sequences from the southern part of the Kraków-Częstochowa Upland originated from a broad spectrum of limestone silicification processes and are represented by chert concretions, bedded cherts and epigenetic siliceous rocks. These rocks served as raw materials in the production of stone tools from the Middle Palaeolithic onwards. These tools have been studied extensively by archaeologists.This paper presents the identification of the above mentioned types of siliceous rocks used in the manufacture of tools based upon (i) microscopic observations of thin sections which reveal the primary microfacies of limestones subjected to silicification, and (ii) X-ray diffraction analyses, including the determination of the crystallinity index of SiO2. Unfortunately, the available research methods do not permit the identification of the varieties of chert concretions to an extent that would permit them to be even roughly connected with particular outcrops or, at least, with particular regions of the occurrence of siliceous raw materials on a Pan-European scale. Macroscopic criteria have recently been adopted in archaeological classifications aimed at determining the origin of stone tools and drawing conclusions as to the source outcrops or regions. However, regrettably, these have proven groundless from the geological point of view.

Early Cretaceous sedimentary evolution of the northern Lhasa terrane and the timing of initial Lhasa-Qiangtang collision

Lower Cretaceous strata in the Baingoin basin of the northern Lhasa terrane record initial collision between the Lhasa and Qiangtang blocks, followed by the early uplift of central Tibet. North-south traverses across the Baingoin basin highlight major differences between the Duba Formation in the north and the quasi-coeval Duoni Formation in the south. The Duba Formation documents upward transition from shallow shelf and deltaic environments to coarse-grained siliciclastic fluvial sedimentation. Abundance of detrital zircons yielding Jurassic-Cretaceous ages with εHf(t) values mainly between −2 and +10, occurrence of chert, Cr-spinel, and pyroxene grains, together with southward paleocurrent directions indicate that the Duba Formation was sourced from the southern Qiangtang terrane and Bangong-Nujiang suture zone to the north. The Duoni Formation in the south was deposited in shelfal to fan-delta and fluvial environments. Abundant volcanic clasts, detrital zircons yielding Cretaceous ages with mainly negative εHf(t) values, and northward paleocurrents indicate an active volcanic source located in the central Lhasa terrane to the south, with minor input from the northern Lhasa terrane. Only the northern part of the Baingoin basin was directly controlled by the Lhasa-Qiangtang collision and may thus be considered a peripheral foreland basin, whereas the southern part was mainly influenced by tectonic processes related to the northward subduction of Neotethyan lithosphere, and may thus be comparable to a retroarc foreland basin. But these sedimentary features and the 139–79 Ma Baingoin plutonic intrusion do not fit well with classical foreland-basin models. Zircon chronostratigraphy constrains the final consumption of Bangong-Nujiang oceanic lithosphere and initial collision between the Lhasa and Qiangtang microcontinents to have taken place by 122 Ma, which has major implications for paleotectonic reconstructions of the Tibetan Plateau.

Chert in the Pacific: Biogenic silica and hydrothermal circulation

Data on sections drilled by DSDP and ODP in the Pacific have been compiled for 187 sites, 105 of which contain chert. The spatial pattern of siliceous deposits in the Pacific generally follows the pattern of biologic productivity. There is some evidence that enhanced vertical diffusivity in the ocean during the Eocene warm climate gave rise to siliceous deposits on and near bathymetric highs and basin margins. The most common occurrence of chert is in Eocene and older sections; however, cherts do occur in sections as young as 7 Ma in the sites investigated. Over half of the youngest chert occurrences in sedimentary sections are within 150 m of basement. Nearly all the chert occurrences on crust younger than 60 Ma are within 150 m of basement. The deficit in heat flow over ocean crust younger than 60 Ma is explained as the result of heat advection by hydrothermal waters circulating in the upper part of the ocean crust. These waters appear to dissolve biogenic silica and deposit it as chert in the lower parts of sedimentary sections. Diffusion of these waters into the lower sediment section is enhanced by small-scale faulting. This faulting may be associated with intra-plate stresses that develop during ridge jumps and reorientations of plate motion. The relatively warm bottom waters of the early Cenozoic may have also enhanced the dissolution of biogenic silica during hydrothermal circulation in the upper oceanic crust and basal sediment section.

Biogenic silica and chert in the Pacific Ocean

Evidence for the dissolution of biogenic silica at the base of pelagic sections supports the hypothesis that much of the chert formed in the Pacific derives from the dissolution and reprecipitation of this silica by hydrothermal waters. As ocean bottom waters flow into and through the crust, they become warmer. Initially they remain less saturated with respect to dissolved silica than pore water in the overlying sediments. With the diffusion of heat, dissolved ions, and to some extent the advection of water itself, biogenic silica in the basal part of the sedimentary section is dissolved. Upon conductively cooling, these pore waters precipitate chert layers. The most common thickness for the basal silica-free zone (20 m) lies below the most common height of the top of the chert interval above basement (50 m). This mode of chert formation explains the frequent occurrence of chert layers at very shallow subbottom depths in pelagic sections of the Pacific. It is also consistent with the common occurrence of cherts ≤150 m above basement.

Widespread formation of cherts during the early Eocene climate optimum

Radiolarian cherts in the Tethyan realm of Jurassic age were recently interpreted as resulting from high biosiliceous productivity along upwelling zones in subequatorial paleolatitudes the locations of which were confirmed by revised paleomagnetic estimates. However, the widespread occurrence of cherts in the Eocene suggests that cherts may not always be reliable proxies of latitude and upwelling zones. In a new survey of the global spatio-temporal distribution of Cenozoic cherts in Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sediment cores, we found that cherts occur most frequently in the Paleocene and early Eocene, with a peak in occurrences at ∼ 50 Ma that is coincident with the time of highest bottom water temperatures of the early Eocene climatic optimum (EECO) when the global ocean was presumably characterized by reduced upwelling efficiency and biosiliceous productivity. Cherts occur less commonly during the subsequent Eocene global cooling trend. Primary paleoclimatic factors rather than secondary diagenetic processes seem therefore to control chert formation. This timing of peak Eocene chert occurrence, which is supported by detailed stratigraphic correlations, contradicts currently accepted models that involve an initial loading of large amounts of dissolved silica from enhanced weathering and/or volcanism in a supposedly sluggish ocean of the EECO, followed during the subsequent middle Eocene global cooling by more vigorous oceanic circulation and consequent upwelling that made this silica reservoir available for enhanced biosilicification, with the formation of chert as a result of biosilica transformation during diagenesis. Instead, we suggest that basin–basin fractionation by deep-sea circulation could have raised the concentration of EECO dissolved silica especially in the North Atlantic, where an alternative mode of silica burial involving widespread direct precipitation and/or absorption of silica by clay minerals could have been operative in order to maintain balance between silica input and output during the upwelling-deficient conditions of the EECO. Cherts may therefore not always be proxies of biosiliceous productivity associated with latitudinally focused upwelling zones.

Geochemistry of Metabasalts and Associated Metasedimentary Rocks from the Lushan Formation of the Upthrust Slate Belt, South-Central Taiwan

Pillow lavas and massive flow associated with metasedimentary rocks of the Lushan Formation near Baolai and Fangshan represent Miocene intraplate volcanism in the Upthrust Slate Belt of central Taiwan. The lavas are alkali-olivine basalts that have undergone variable degrees of seafloor hydrothermal alteration. Trace-element and isotopic compositions (Th/Ce = 0.07 to 0.08, (La/Yb)n = 10.5 to 13.0, εNd(15Ma) = +4.4 to +5.8, (87Sr/86Sr)i = ~0.7040), are consistent with derivation from asthenospheric and continental mantle sources and similar to those of Late Cenozoic volcanism in the Penghu Islands and northern Taiwan. Variable calcite in-filling of vesicles and the occurrence of chert within the volcanic pile indicates eruption around the calcite-compensation depth, placing the volcanism near the base of the continental slope along the northern margin of the South China Sea.

THE PRE-OROGENIC VOLCANO-SEDIMENTARY COVERS OF THE WESTERN TETHYS OCEANIC BASIN: A REVIEW

The records of the Jurassic Western Tethys Ocean are the ophiolitic rocks now scattered in the Tertiary orogenic belts of the Alps, Apennines and Betic Cordillera. These ophiolites, involved in a convergent margin environment, are affected I) by HP/LT metamorphism derived from a subduction process or II) by very low grade overprint, corresponding to the tectonic prism at the margin of the overriding plate. On the whole, they share common characteristics: a- The MORB geochemical signature. b- The ophiolitic successions often “reduced” and thin. c- The volcano-sedimentary covers often directly overlying the serpentinised peridotites. d- The widespread occurrence of cherts as Jurassic pelagic sediment. In the thickest “complete” ophiolitic successions, basalt flows, generally thin, are preceded and followed by ophiolitic breccias. Only the basal portion of the breccias on top of the serpentinites (Levanto Breccias) has a tectonic origin, all other levels have a sedimentary origin. These breccia-basalt assemblages are overlain by thick sequences of Mt. Alpe Cherts and Calpionella Limestones, followed by Palombini Shales. In the reduced (or incomplete) successions, thin breccias and cherts were directly deposited, above the Levanto Breccias (ophicalcites pro parte), and followed by Palombini Shales. This stratigraphic pattern seems to be widespread in the whole Western Tethys ocean. In some sequences, transitional mid-ocean ridge (T-MOR) basalts are present and the ophiolitic rocks are associated with Variscan continental slices and debris, as in the Err-Platta succession (Central Alps) and in some exotic blocks in the flysch of the External Ligurides (Northern Apennines). In the Balagne (Corsica) T-MOR basalts are associated with quartzarenites. These occurrences show that an unroofed mantle and sections of oceanic crust evolved very near to a continental margin. The different radiolarian ages of the cherts deposited before, within, or on top of the MOR basalts allow to infer a minimum time interval for the Western Tethys oceanisation. This interval can be considered between 16 and 21 Ma (from Late Bajocian to Kimmeridgian/Tithonian). If we assume 1cm/yr spreading rate during this time, the basin would have reached about 150-200 km width. The same ages suggest that the ocean opening was diachronous along the Western Tethys basin. Mainly on the basis of the Northern Apennines and Corsica data, it is possible to reconstruct the following evolutive geodynamic, paleogeographic and sedimentary evolution of the Western Tethys ocean basin: 1- Bajocian/Bathonian stage: opening of the Ligurian Northern Apennines oceanic segment and, perhaps, also of the Ligurian, Western and Central Alps ones. 2- Bathonian/Callovian stage: opening of all the segments of the Western Tethys ocean basin. The volcano-sedimentary covers formed during these two stages are constituted by breccias, basalts and siliceous pelagites (cherts). 3- Tithonian/Berriasian: end of the ocean spreading (Tithonian) and beginning of the quiescent stage in the whole basin, marked by the lack of any tectonic activity and by the sedimentation of the Calpionella Limestones and, locally, of mixed siliceous-calcareous deposits (Nisportino-Murlo Fm.). 4- Hauterivian/Santhonian: this is the longest quiescent stage of the basin, dominated by the sedimentation of the Palombini Shales and Limestones. Some siliciclastic deposits are shed from both passive continental margin sides. During the Early Cretaceous, there is also evidence of a rare intraplate magmatism in Southern Tuscany. The Western Tethys ophiolitic successions are similar to those of present day, slow spreading oceans, in particular to those of the Atlantic Ocean (Mutter and Karson, 1992; Tucholke and Linn, 1994). The Galician North Atlantic margin provides a model for the process of mantle denudation. For the oceanic evolution, the model of Tucholke and Linn (1994) is particularly taken in consideration. According to this model, tectonic extension was one major process in the Western Tethys oceanic development.

The effect of paleotopography on the late Albian and Cenomanian sea-level record of the Canadian Cretaceous interior seaway

In western Canada, a major paleoenvironmental change at the Albian-Cenomanian boundary is related to a eustatic sea-level rise overprinted by a relative sea-level fall in conjunction with preexisting topography within the basin. This paper shows the lateral variability of sedimentology, paleoecology, and biostratigraphy of the latest Albian to Cenomanian interval along the Canadian western margin of the Cretaceous interior seaway. This paper (1) provides an integrated depositional and paleoecological examination of the Albian and Cenomanian lithologic units in the northern Western Interior Seaway; (2) demonstrates the effect of antecedent paleotopography within the basin at the time of the transgression; (3) documents the variability in sedimentary facies and paleoecology controlled by paleorelief; and (4) demonstrates the diachroneity, lateral extent, and extremity of the multiple unconformities controlled by this paleorelief. An enigmatic aspect of the lowermost Cenomanian Fish Scales Formation has been the regional occurrence of chert, quartz, and bioclastic pebbles associated with black anoxic shale. A regional north-south traverse across Alberta provides insight into this problem. In southwestern Alberta, chert- and quartzite-pebble conglomerate and sandstone that are equivalent to the Fish Scales Formation—known as Barons Sandstone (subsurface) and Blairmore Grits (outcrop)—represent proximal shelf sedimentation. This coarse-grained sediment was flushed out during sea-level lowstand and then reworked by a subsequent transgression associated with the Belle Fourche Formation. A large paleohigh existed in southwesternmost Alberta at this time. Northward, the Fish Scales Formation is bounded top and bottom by unconformities and conglomerate, indicating multiple sea-level fluctuations and deposition in a wave-influenced shelf environment. In west-central Alberta, preserved deposits of the Fish Scales Formation indicate deposition in a nearshore setting. Farther to the north and, most distally, in northwestern Alberta and elsewhere to the east, the coarse component of the Fish Scales Formation is predominantly gravel-sized fish and other vertebrate debris with significantly less siliciclastic detritus. Regionally, the unconformity at the base of Fish Scales–Barons becomes more pronounced southward and westward where the underlying shale of the Westgate Formation (or Westgate Member) has been eroded or was never deposited. The regional paleogeographic setting for the Barons Sandstone and Fish Scales Formation indicates greater amounts of erosion and coarser-grained deposition in the southwest associated with the paleohigh. Northward and eastward, there was greater accommodation space, less erosion associated with the unconformity, and finer-grained sediment.

Chert in the Iberian Pyrite Belt

Since lenses of chert are common within the volcano-sedimentary succession hosting the massive sulphide deposits of the Iberian Pyrite Belt (Spain and Portugal), we examined numerous chert occurrences, both petrographically and geochemically, to test their possible value for massive sulphide exploration. The chert is found at two main lithostratigraphic levels (upper and lower) that are also interpreted as massive-sulphide bearing. In both cases the chert is located at the top of acidic volcanic sequences or in the associated sediments; we have not been able to observe the relationships between massive sulphides and chert, but some of the large orebodies of the Province (Lousal, La Zarza, Tharsis, Planes-San Antonio body of Rio Tinto, Neves) are described as being locally capped by chert facies. Four main types are recognized among the chert and associated facies: (1) red hematitic chert ± magnetite; (2) radiolarian and/or sedimentary-textured (conglomeratic) chert with hematite and/or Mn oxides; (3) pale sulphidic chert; (4) rhodonite and/or Mn carbonate ± magnetite facies. In the Spanish part of the Province the radiolarian chert is confined to the upper level; the distribution of the other types appears to be haphazard. The hydrothermal origin of the South Iberian chert is shown by its high Fe-Mn and low Co-Ni-Cu contents. The presence of small positive Ce anomalies indicates a shallow marine environment (shelf or epicontinental sea), which is consistent with the volcanological and sedimentological data. The chert was emplaced below the sea floor through chemical precipitation and/or through alteration and replacement of the country rock, residual traces of which are ghost phenocrysts and high Al, Ti and rare earth contents. Macro- and microscopic relationships indicate that the oxide facies (hematite ± magnetite) formed first, probably providing a protective insulating cover against the marine environment and enabling an evolution towards sulphide facies; a phase of Mn carbonate and silicate + quartz ± chlorite + sulphides appears to be even later. It was not possible, through discrimination, to isolate a chert that could be considered as representing a lateral marker of massive sulphides; moreover, both field observations and geochemical data seem to indicate a relative independence of this siliceous sulphide hydrothermal activity from the hydrothermal activity giving rise to the massive sulphides. Such is also indicated by the lead isotopic signature of the chert, which is appreciably more radiogenic than that of the massive sulphides; the lead enrichment in the sulphidic chert facies indicates the participation of a different source (sediments, sea water) from that of the massive sulphides. The hypothesis of an independent hydrothermal “chert” event can thus be envisaged, wherein the chert reflects submarine low-temperature hydrothermal activity that is most apparent during a “break” within the volcano-sedimentary succession and which may locally have competed with the high-temperature hydrothermal activity giving rise to the massive sulphides. The interest of the chert thus rests in its palaeodynamic significance, as a marker of periods of volcanic quiescence, and in its possible role as a protective insulating cap favourable to the deposition of massive sulphides.

Chert occurrences in carbonate turbidites: examples from the Upper Jurassic of the Betic Mountains (southern Spain)

ABSTRACTIn Upper Jurassic carbonate turbidites of the Betic mountains (southern Spain), chert occurs in three morphologies: bedded chert, nodular chert and mottled chert. The last refers to a weak dispersed and selective silification which gives a speckled appearance to the rock. The three types of chert are formed by replacement of limestones and are associated with different calcareous facies. Turbidite packstones of Saccocoma and peloids, and turbidite lime mudstones of pelagic material contain bedded and nodular cherts. The silicification textures are mainly micro‐ and cryptocrystalline quartz, with local chalcedonic quartz (both length‐fast and length‐slow) which is more common in the packstones. Only mottled chert is produced where calcareous breccia beds are silicified. Mottled chert consists of micro‐ and cryptocrystalline quartz, length‐slow chalcedonic quartz and mosaics or individual crystals of euhedral megaquartz. Beds and nodules are the result of early diagenetic silicification, with silica derived from the calcitization and dissolution of radiolarians and, subordinately, sponge spicules, whereas mottled chert is the consequence of later silicification in a probably Mg‐rich environment.Early silicification is mainly confined to turbidite beds and only rarely occurs in the interbedded pelagic limestones. Turbidite sedimentation favours silicification because rapid burial of the transported siliceous tests prevents silica from the dissolution of tests passing into overlying sea water. A silica‐rich interstitial fluid develops in the turbidite layer and this migrates to more permeable zones giving rise to bedded and nodular chert.

Paleoenvironmental Significance of Chert in the Franciscan Formation of Western California: Discussion Concerning the Significance of Chert in Timor

In his recent paper, Chipping (1971), in support of his argument that cherts may be developed in a particular setting relative to an ocean ridge and a subduction zone, discussed the mode of occurrence of chert in some Asian mountain belts. He referred to the occurrence of chert in Timor where, following Grunau (1965), he said it is an “important constituent” of a melange. Chipping went on to claim that there has been subduction of oceanic crust beneath continental crust in the Timor region, although he cited no evidence in support of this interpretation. This reply challenges both of these statements on the basis of field evidence that a relatively insignificant amount of chert is associated with the melange in Timor, and in the absence of any evidence to support his suggestion that subduction of oceanic crust below continental crust has occurred in the Timor region since the early Permian. Another model is proposed, which relates the occurrence of chert in Timor in part to lack of supply of coarse terrigenous detritus rather than to any physical barrier to turbidity currents as Chipping (1971) suggested. The present model proposes that Chert formed in Timor above a sedimentary sequence on continental crust close to the outer margin of the continental slope, and it involves the northward subduction of oceanic crust below oceanic crust away from the northern continental margin of Australia.

Jurassic Stratigraphy of Sweetgrass Arch--Manitoba Section: ABSTRACT

In southern Saskatchewan and Manitoba the interval between the late Paleozoic and late Jurassic epeirogenies is represented in stratigraphic order by continental redbeds, evaporites, carbonates, and clastics. The sequence is in excess of 1,400 feet thick in south-central Saskatchewan and thins north, west, and east. Depositional and erosional thinning are complementary and the northern limits are determined by late to post-Jurassic truncation. The system was divided (Milner and Thomas, 1954) into the Watrous, Gravelbourg, Shaunavon, and Vanguard formations, each representing widely recognizable lithologic units. Proof of Jurassic age is available for the Gravelbourg, the Shaunavon, and Vanguard formations. These formations together appear to represent the Bajocian to Kimmeridgian time interval. The evaporites of the Watrous formation possibly represent the lower Bajocian or earlier Jurassic stages. Recent findings, still unpublished (Peterson, 1955), indicate that rocks of Triassic age may be included in the lowermost continental sediments of the Watrous. A minor unconformity of Middle Jurassic age is indicated by the occurrence of chert on top of marine carbonates of the Gravelbourg. The evidence for this unconformity is strongest in central and southeast Saskatchewan. There the anhydrite of the Watrous formation thickness markedly and the evaporite conditions appear to have lasted into a higher stratigraphic level than in western Saskatchewan. In the same area the marine sediments above the unconformity thin and gradually change facies. The carbonates to the west are substituted by clastics, and coarse clastics appear in increasing End_Page 415------------------------------ amounts in an easterly direction. It would appear that a separate basinal area existed toward the end of Middle Jurassic time in Manitoba, receiving clastic sediments from the north and east. The Shaunavon formation can not be traced lithologically in this facies district but approximately equivalent picks can be made from electric and radiation logs. The lithologic character of the uppermost Gravelbourg, Shaunavon, and Vanguard formations in Manitoba resembles closely that of the Sundance formation in North and South Dakota. Callovian time is represented by the lower part of the Vanguard formation. During this time uniform basinal conditions were established across the entire area. A slight unconformity, recognizable in western Saskatchewan, marks a change from marine to brackish conditions which must have taken place in middle Oxfordian time. In the center of the basin 150 feet of marine middle Vanguard represent the Oxfordian interval. The upper Vanguard is partly marine, partly brackish and contains Kimmeridgian fauna. The lithologic character of the upper Vanguard, and the fact that it contains reworked fragments of older Jurassic faunas, indicate redeposition of material from the truncated basin flanks into the center. The hiatus between Kimmeridgian and Lower Cretaceous is represented by a major unconformity which marks the top of the Jurassic system in Saskatchewan. End_of_Article - Last_Page 416------------