John Bloch

Silica activity and the smectite-illite reaction

Early silicate diagenesis is characterized by the precipitation of minerals with high silica activity, such as opaline silica, smectite, and clinoptilolite, and by pore-water silica activities exceeding quartz saturation. With burial, this early, high-silica activity diagenesic facies is succeeded by a low-silica activity facies distinguished by quartz precipitation and progressive development of illite as the nonexpandable component of mixed-layer clays. We suggest that the smectite-illite reaction occurs as a consequence of the reduction of silica activity at the onset of quartz precipitation. Data from oceanic and sedimentary basins confirm that the smectite-illite reaction does not control aqueous silica activities. Where the smectite-illite reaction occurs at higher temperatures, aqueous silica activities likewise reach quartz saturation at higher temperatures.

LPNORM: a linear programming normative analysis code

Abstract The computer code LPNORM implements the mathematical method of linear programming to calculate the mineralogical makeup of mineral mixtures, such as rock, sediment, or soil samples, from their bulk geochemical composition and from the mineralogical (or geochemical) composition of the contained minerals. This method simultaneously solves the set of linear equations governing the distribution of oxides into these minerals, subject to an objective function and a set of basic constraints. LPNORM allows the user to specify what minerals will be considered for normative analysis, what their composition is (in terms of mineral formula or geochemical composition), and whether to maximize mineral abundances, minimize slack variables (oxides that can not be accounted for), or do both at once in the objective function. Independent knowledge about the abundance of one or several of the minerals in the sample can be entered as additional equality or inequality constraints. Trial-and-error approach enables the user to “optimize” the composition of one or a few of the contained minerals. Results of comparative tests, highlighting the efficiency, as well as the shortcomings, of LPNORM are presented.

Life in a temperate Polar sea: a unique taphonomic window on the structure of a Late Cretaceous Arctic marine ecosystem

As the earth faces a warming climate, the rock record reminds us that comparable climatic scenarios have occurred before. In the Late Cretaceous, Arctic marine organisms were not subject to frigid temperatures but still contended with seasonal extremes in photoperiod. Here, we describe an unusual fossil assemblage from Devon Island, Arctic Canada, that offers a snapshot of a ca 75 Myr ago marine palaeoecosystem adapted to such conditions. Thick siliceous biogenic sediments and glaucony sands reveal remarkably persistent high primary productivity along a high-latitude Late Cretaceous coastline. Abundant fossil faeces demonstrate that this planktonic bounty supported benthic invertebrates and large, possibly seasonal, vertebrates in short food chains. These ancient organisms filled trophic roles comparable to those of extant Arctic species, but there were fundamental differences in resource dynamics. Whereas most of the modern Arctic is oligotrophic and structured by resources from melting sea ice, we suggest that forested terrestrial landscapes helped support the ancient marine community through high levels of terrigenous organic input.

Emplacement and reworking of Cretaceous, diamond-bearing, crater facies kimberlite of central Saskatchewan, Canada

In central Saskatchewan, Canada, kimberlites were emplaced into Cretaceous marine and nonmarine clastic sediments. Core recovered from one drill hole that intersects kimberlite (Smeaton FAC/UK core 169/8) was selected for an integrated study involving sedimentology, volcanology, mineralogy, geochemistry, palynology, micropaleontology, organic petrology, and radiometric age determination. Only crater facies kimberlite has been observed; there is no indication of the locations of feeder dikes. Four varieties of kimberlite occur, all originating from subaerial volcanism: (1) fluvial-reworked kimberlite; (2) diamondiferous kimberlite lapillistone air-fall deposits; (3) kimberlite olivine crystal-tuff air-fall deposits; and (4) diamondiferous marine wave-reworked kimberlite. Within the multiple primary eruptive phases of the kimberlite air-fall deposits, the volcanic style changed upward with time, from violent Strombolian to more explosive volcanism. The bulk of the volcanism formed conformable, air-fall deposits on terrestrial sediments of the Cantuar Formation, resulting in the development of positive-relief tephra cones. Subsequent marine transgression associated with the Westgate Formation partially beveled the top of the cone. The kimberlite air-fall deposits contain microdiamonds, 5 to 25 μm in diameter. The maximum temperature and vitrinite reflectance values of coaly matter in the kimberlites indicate that these deposits, although originally derived from magma at high temperatures, did not thermally affect entrained surficially derived clasts or the country rock during emplacement. The chemical content of intrakimberlite shale clasts is markedly different from the marine and nonmarine shales and indicates significant synemplacement and postemplacement fluid movement through the volcanic pile. At least two episodes of kimberlite volcanism occurred in the middle and late Albian (paleontologically assigned). A U-Pb perovskite radiometric age of 101.1 ± 2.2 Ma from a kimberlite lapillistone from the younger episode of volcanism is internally consistent with biostratigraphic studies that constrain the kimberlite volcanism as post–middle Albian and pre–late Albian to late Albian.

Late Cretaceous (Cenomanian to Campanian) paleoenvironmental history of the Eastern Canadian margin of the Western Interior Seaway: bonebeds and anoxic events

Abstract Upper Cretaceous strata in the Pasquia Hills of the northern Manitoba Escarpment, eastern Saskatchewan, Canada provide a detailed paleoenvironmental and sea-level record of the eastern margin of the Western Interior Seaway. Sediments deposited during the Cenomanian/Turonian Greenhorn marine cycle are dominantly black mudstones deposited in a stratified water column, with bottom-water anoxia recurrently reaching into the photic zone. A middle Cenomanian sea-level lowstand event followed by transgression left a series of bonebeds within the Belle Fourche Member of the Ashville Formation, indicating a sedimentary environment starved of coarse siliciclastics. Maximum sea level resulted in the formation of limestone beds within the Favel Formation, further favoured by reduced terrigenous sediment input compared to the western margin. Limestone sedimentation was followed by a phase of increased freshwater input under lower sea level conditions, and reducing zoo- and phytoplankton diversities. During final Greenhorn regression, eastern Saskatchewan probably turned into a restricted basin severely limiting marine circulation. Poor or absent benthic foraminiferal assemblages and biomarker analysis suggest prevailing watermass stratification throughout the Cenomanian/Turonian transgressive/regressive cycle. This was caused either by a freshwater lid, stratification of Boreal and Tethyan-derived watermasses, or both, to various intensities affected by changing sea level. Basin oxygenation during Niobrara time varies between localities along the eastern margin as documented by presence/absence of benthic and planktic foraminifera.

An anoxic event at the Albian-Cenomanian boundary: the Fish Scale Marker Bed, northern Alberta, Canada

Abstract The Fish Scale Marker Bed (FSMB) of the Shaftesbury Formation, which marks the Albian-Cenomanian boundary, is a regional stratigraphic marker in the Western Interior of Canada. At the outcrop studied on the Smoky River in northwestern Alberta, three major shale units can be distinguished in the FSMB and contiguous strata. The lowermost shale (Unit 1) is bioturbated and contains high-diversity dinoflagellate and moderate-diversity foraminiferal assemblages. It has dominantly Type III (terrestrial) organic matter (OM) and low total organic carbon content (TOC). The unit was deposited in an open-marine, neritic environment of normal salinity. The FSMB (Unit 2) represents a zone of condensed bioclastic accumulation composed of abundant fish remains. The dinoflagellate species diversity is drastically reduced in this unit and it lacks benthonic foraminifera and bioturbation. Unit 2 is characterized by mainly Type II (marine) OM and high TOC values. Unit 2A contains rippled sandstone related to either a shallowing or to deeper water currents. A fish-hash conglomerate making up Unit 2B can alternatively be interpreted as a bioclastic, condensed and winnowed deposit or as a transgressive lag. Unit 2C consists of black, platy shale with abundant fish remains and represents a minor marine trangression during the deposition of FSMB, when the bottom waters were dominantly anoxic. Collectively, the features of Unit 2 suggest deposition under a stratified water column with moderate productivity of planktonic and nektonic organisms in the upper oxygenated layers but with anoxic bottom waters. Unit 3, overlying the FSMB, consists of blocky shale with reduced concentration of fish remains. Due to increased rate sedimentation during its deposition, the organic-rich sediment of Unit 3 was progressively diluted by clastic material and there was an increase in the dissolved oxygen content of the bottom waters. The anoxic event at the FSMB is related to a relative rise in sea level and possibly to the mixing of waters different salinities and temperatures from the Arctic Ocean and the Gulf of Mexico in the Western Interior seaway.

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

The majority of Tertiary volcanic rocks of the western United States and Mexico are alkaline in composition and may contain as much as 50 wt% equivalent K-feldspar. Emplacement of these volcanic strata is coeval with Tertiary shale deposition in the Texas Gulf Coast, and they previously have been identified as likely sources of sediment for Gulf Coast shales. Evaluation of chemical trends in Gulf Coast shales, particularly K 2 O, indicates changes in sediment composition in the lower Eocene, Oligocene, and near the Oligocene-Miocene boundary. In particular, there is a 250% increase in K 2 O content from ∼2 wt% to ∼5 wt% from the late Eocene to the early Oligocene. Gulf Coast shale bulk-rock compositions are consistent with a Tertiary volcanic source. Estimates of erosion and mass balance calculations suggest that in the south Texas Gulf Coast, the Oligocene Frio Formation may contain between 60% and 85% volcanic detritus, and coeval Frio shales to the north contain ∼25%. Vertical and lateral compositional variations highlight variable abundances of source detritus and the effects of weathering and depositional processes on Gulf Coast shale composition. Trends of increasing K 2 O content with depth in Gulf Coast shales previously have been interpreted to result from open-system diagenesis and K-metasomatism at depth. The data presented herein suggest instead that these trends result from variable provenance and the influx of large volumes of Tertiary alkaline volcanic material. Therefore, diagenetic models that invoke a homogeneous initial shale composition and open-system behavior may be invalid.

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.

COMPARISON OF THE MINERALOGICAL AND CHEMICAL COMPOSITION OF 2 SHALES FROM THE WESTERN CANADA SEDIMENTARY BASIN AND THE UNITED STATES GULF COAST

The mineralogy and geochemistry of shales reflect the composition of the initially deposited precursor mud, subsequently modified by diagenetic processes. To see if significant geochemical differences exist between shales that mainly owe their present-day composition to either deposition or diagenesis, we compare the published mineralogical, bulk and clay-fraction geochemical, and clay-fraction O-isotopic compositions of 2 shales. One shale is from the Western Canada Sedimentary Basin (WCSB), and its composition mainly reflects primary (depositional) chemical and mineralogical variations (smectitic to illitic illite/smectite) within this unit. The other shale is from the United States Gulf Coast (USGC), and its composition mainly reflects mixed-layer illite/smectite (I/S) diagenesis of deposited smectitic clay material. The chemical and mineralogical trends of WCSB and USGC shales, including one of increasing illite content in I/S with depth or maturity, are essentially indistinguishable, in both bulk shale and clay fraction, despite the contrasting genetic interpretations for the origin of the contained I/S. Thus, similar mineralogical and chemical trends with depth or temperature can result either from inherited depositional compositional heterogeneity of the sediment, from burial metamorphism of shale or a combination of both. Interestingly, the O-isotopic compositions of the clay fractions from the WCSB and USGC are significantly different, a fact that reflects original clay formation from source material and water of quite different isotopic compositions. The discrimination between depositional and diagenetic contributions to shale composition continues to pose challenges, but a combination of bentonite, illite polytype, clay isotopic and trace and rare earth elemental analyses together with illite age analysis holds promise for future work.