Mixed-layer clay in the Pierre Shale and equivalent rocks, northern Great Plains region

132

Abstract--Clay samples from shales and bentonites in the Mancos Shale (Cretaceous) and its stratigraphic equivalents in the southern Rocky Mountain and Colorado Plateau have been analyzed by X-ray powder diffraction methods. The major clay in the shales is mixed layered illite/smectite, with 20-60% illite layers. The regional distribution of ordered vs. random interstratification in the illite/smectite is consistent with the concept of burial metamorphism in which smectite intedayers are converted to illite, resulting finally in ordered interstratification. The interstratification data correlate with other geologic information, including rank of coal and Laramide tectonic activity. In addition, contact metamorphism of the shale by Tertiary igneous intrusions produced a similar clay suite. Chemical variation within these shales (particularly the presence or absence of carbonate) affected the clay conversion reactions in the interbedded bentonites and the shale itself during the early stages of transformation. In extreme cases, shales and bentonites from a single outcrop may contain clays that range from pure smectite (calcareous shales) to ordered illite/smectite containing/>50% illite layers (noncalcareous shales). The use of mixed-layered illite/smectite compositions to infer thermal regimes, therefore, may be misleading unless allowance is made for local chemical controls.

Section: Copyright 9 1981 the Clay Minerals Societymentioning

confidence: 64%

Burial and Contact Metamorphism in the Mancos Shale

Nadeau

1981

132

“…Some detrital kaolinite is also present in the shale beds ( Figure 14A). High quartz content of the bulk rock and a heterogeneous clay mineral assemblage, particularly with a variety of I/S and significant amounts of discrete illite, are mineralogies consistent with those of detrital clay beds and shale of this age (Schultz, 1978;Hoffman and Hower, 1979;Pollastro, 1981).…”

Section: Comparison O F K / T Boundary Unit To Tonsteins and Detritalmentioning

confidence: 99%

“…Clay minerals also dominate the mineralogic components of the K/T boundary unit. Specifically, the K/T lower claystone layer contains mostly homogeneous types ofneoformed clay minerals, little quartz and feldspar, and no detrital illite or muscovite: a mineral assemblage very similar to those that make up altered, vitric, volcanic ash-fall beds and tufts (Minato and Utada, 1969;Schultz, 1978;Hoffman and Hower, 1979;Sudo et al, 1981;Bohor et aL, 1978;Pollastro and Scholle, 1986) and strongly implies a glass precursor for the KiT boundary layers. Furthermore, Alvarez et al (1992) recently discovered several fragments of impact-ejecta glass stringers, shards, droplets, and spherules preserved at the K/T boundary in DSDP Leg 77, Sites 536 and 540, southeast Gulf of Mexico.…”

Section: Comparison O F K / T Boundary Unit To Tonsteins and Detritalmentioning

confidence: 99%

Origin and Clay-Mineral Genesis of the Cretaceous/Tertiary Boundary Unit, Western Interior of North America

Pollastro

Bohor

1993

Abstraet--A 3-cm-thick, two-layered clay unit that records mineralogic 'and textural evidence of a catastrophic event that occurred at a time now marked as the end of the Cretaceous Period was preserved in ancient peat-forming environments of the Western Interior Basin of North America. The two layers of this unit consist of altered distal ejecta and are easily distinguished by their distinctive texture and impact components from other clay beds, mainly tonsteins and detrital shales, occurring within the sequence of rocks enclosing the Cretaceous/Tertiary (K/T) boundary interval.The lower claystone layer of the K/T boundary unit represents melted silicic target rock that has altered mainly to kaolin minerals. Impact components and signatures of this lower layer include a relict imbricate fabric of glass fragments, shards, bubbles, hollow spherules (altered microtektites), small amounts of shocked mineral grains, and a subdued iridium anomaly. These components and textures, combined with the layer's restricted areal distribution, indicate that this layer, called the "'melt ejecta layer," is the distal part of an ejecta blanket deposit. We interpret the melt ejecta layer to be an altered deposit of mostly impact-derived, shock-melted, silicic target material that traveled through the atmosphere within a detached ejecta curtain and on other ballistic trajectories.The upper laminated layer of the K/T boundary unit consists mostly of altered vitric dust and abundant shocked minerals whose size and amounts decrease away from the putative crater site in the Caribbean area. High-nickel magnesioferrite crystals, high iridium content, geochemical signature, and worldwide distribution all suggest this upper layer originated from a cloud of vaporized bolide and entrained targetrock materials ejected above the atmosphere. The components of this layer, called the "fireball layer," settled slowly by gravitational processes from an Earth-girdling vapor cloud and were deposited immediately on top of the already-emplaced melt ejecta layer.The clay minerals that formed in the two layers are largely a function of composition and the highly unstable, shock-modified state of the fallout materials altered in acidic, organic-rich waters of ancient peat swamps. The fireball layer is mostly altered to smectitic clay from a mafic glass condensed from the vaporized chondritic bolide, along with some kaolinite formed from blebs of melted silicic target material entrained in the vapor plume cloud during ejection. In contrast, the melt ejecta layer is mainly kaolinitic, derived from silicic glass formed from melted target rocks. In this layer, the glass rapidly altered to mostly disordered, micrometer-sized "cabbage-like" or submicrometer-sized embryonic forms of spherical halloysite, probably from an allophane precursor. These crystallization characteristics of the melt ejecta layer are much different than those which formed coarse vermicular aggregates and platy kaolinite crystals in tonsteins from outside the K/T boundary interval througho...

“…Composition and ordering of the I/S clay was determined on oriented, ethylene glycol-saturated specimens of both the < 2-/~m and < 0.5-urn fractions and by the methods of Reynolds and Hower (1970) and Schultz (1978). Ordering types were defined using the "Reichweite" (R) notation, as described by Reynolds (1980), where "R" signifies the most distant layer in an interstratified sequence that affects the probability of occurrence of the final layer.…”

Section: New Mexicomentioning

confidence: 99%

Mineralogical and Morphological Evidence for the Formation of Illite at the Expense of Illite/Smectite

Pollastro

1985

105

Abstract--The conversion of smectite to illite by way of a mixed-layer illite/smectite (I/S) series was found to be the major depth-related reaction in clay-mineral assemblages from two cored sedimentary sequences in the Rocky Mountains. The I/S reaction occurred in both interbedded sandstone and shale of Upper Cretaceous and lower Tertiary age in the Green River basin, Wyoming, and in chalk and chalky shale of the Upper Cretaceous Niobrara Formation, Denver basin, Colorado. As the proportion of illite layers in I/S increased with depth in these rocks, the amount of I/S in the clay fraction decreased, and the amount of discrete illite increased. Scanning electron microscopy revealed that the morphologies of highly expansible, randomly interstratified I/S clay (samples from shallow cores) exhibited no distinctive intergrowth or overgrowth textures. In deeply buried rocks containing highly illitic, ordered I/S and abundant discrete illite, however, fibers or laths of illite were formed on earlier I/S substrates. Less commonly, I/S of low expandability shows morphological features of both smectite and illite whereby rigid laths of illite appear to have formed diagenetically from the wall surfaces of I/S honeycombs. This combined morphology suggests some dissolution and reprecipitation (or some reorganization) of materials from the I/S substrate as the substrate was transformed into a more illitic mixed-layer day.These data suggest that some I/S clay was destroyed by the selective cannibalization of smectite layers in I/S to provide the components needed to make a more illitic I/S. Moreover, discrete illite and other minerals apparently were formed during the reaction. Also, coarser mineral phases, such as potassium feldspar and detrital micas, may not have been required to supply the chemical components in the reaction. The observations provide an explanation for late diagenetic I/S reactions that occurred in restricted or relatively closed geochemical systems, as in early cemented rocks having extremely low permeability and little or no potassium feldspar.