The Mississippian Barnett Formation of the Fort Worth Basin is a classic shale-gas system in which the rock is the source, reservoir, and seal. Barnett strata were deposited in a deeper water foreland basin that had poor circulation with the open ocean. For most of the basin's history, bottom waters were euxinic, preserving organic matter and, thus, creating a rich source rock, along with abundant framboidal pyrite. The Barnett interval comprises a variety of facies but is dominated by fine-grained (clay- to silt-size) particles. Three general lithofacies are recognized on the basis of mineralogy, fabric, biota, and texture: (1) laminated siliceous mudstone; (2) laminated argillaceous lime mudstone (marl); and (3) skeletal, argillaceous lime packstone. Each facies contains abundant pyrite and phosphate (apatite), which are especially common at hardgrounds. Carbonate concretions, a product of early diagenesis, are also common. The entire Barnett biota is composed of debris transported to the basin from the shelf or upper oxygenated slope by hemipelagic mud plumes, dilute turbidites, and debris flows. Biogenic sediment was also sourced from the shallower, better oxygenated water column. Barnett deposition is estimated to have occurred over a 25-m.y. period, and despite the variations in sublithofacies, sedimentation style remained remarkably similar throughout this span of time.

Mississippian Barnett Shale: Lithofacies and depositional setting of a deep-water shale-gas succession in the Fort Worth Basin, Texas

The Neoproterozoic assembly of Gondwana and its relationship to the Ediacaran–Cambrian radiation

From synthetic drugs to biodegradable plastics to the origin of life, the chiral selection of molecules presents both daunting challenges and significant opportunities in materials science. Among the most promising, yet little explored, avenues for chiral molecular discrimination is adsorption on chiral crystalline surfaces - periodic environments that can select, concentrate and possibly even organize molecules into polymers and other macromolecular structures. Here we review experimental and theoretical approaches to chiral selection on inorganic crystalline surfaces - research that is poised to open this new frontier in understanding and exploiting surface-molecule interactions.

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Chiral selection on inorganic crystalline surfaces

The emergence of biochemical homochirality was a key step in the origin of life, yet prebiotic mechanisms for chiral separation are not well constrained. Here we demonstrate a geochemically plausible scenario for chiral separation of amino acids by adsorption on mineral surfaces. Crystals of the common rock-forming mineral calcite (CaCO3), when immersed in a racemic aspartic acid solution, display significant adsorption and chiral selectivity of D- and L-enantiomers on pairs of mirror-related crystal-growth surfaces. This selective adsorption is greater on crystals with terraced surface textures, which indicates that D- and L-aspartic acid concentrate along step-like linear growth features. Thus, selective adsorption of linear arrays of D- and L-amino acids on calcite, with subsequent condensation polymerization, represents a plausible geochemical mechanism for the production of homochiral polypeptides on the prebiotic Earth.

Selective adsorption of L- and D-amino acids on calcite: Implications for biochemical

Major volcanoes of the Southern Washington Cascades (SWC) include the large Quaternary stratovolcanoes of Mount St. Helens (MSH) and Mount Adams (MA) and the Indian Heaven (IH) and Simcoe Mountain (SIM) volcanic fields. There are significant differences among these volcanic centers in terms of their composition and evolutionary history. The stratovolcanoes consist largely of andesitic to dacitic lavas and pyroclastics with minor basalt flows. IH consists dominantly of basaltic with minor andesite lavas, all erupted from monogenetic rift and cinder cone vents. SIM has a poorly exposed andesite to rhyolite core but mainly consists of basaltic lavas erupted from numerous widely dispersed vents; it has the morphology of a shield volcano. Distribution of mafic lavas across the SWC is related to north-northwest trending faults and fissure zones that indicate a significant component of east–west extension within the area. There is overlap in eruptive history for the areas studied, but it appears that peak activity was progressively older (MSH (<40 Ka), IH (mostly <0.5 Ma), MA (<0.5 Ma), SIM (1–4 Ma)) and more alkalic toward the east. A variety of compositionally distinct mafic magma types has been identified in the SWC, including low large ion lithophile element (LILE) tholeiitic basalts, moderate LILE calcalkalic basalts, basalts transitional between these two, LILE-enriched mildly alkalic basalts, and basaltic andesites. Compositional diversity among basaltic lavas, both within individual centers as well as across the arc, is an important characteristic of the SWC traverse. The fact that the basaltic magmas either show no correlation between isotopic and trace element components or show trends quite distinct from those of the associated evolved lavas, suggests that their compositional variability is attributable to subcrustal processes. Both the primitive nature of the erupted basalts and the fact that they are relatively common in the SWC sector also imply that such magmas had little residence time in the crust. A majority of the SWC basaltic samples studies are indistinguishable from oceanic island basalts (OIB) in terms of trace element and isotopic compositions, and more importantly, most do not display the typical high field strength element (HFSE) depletion seen in subduction-related magmas in volcanic arcs elsewhere. LILE enrichment and HRSE depletion characteristics of most arc magmas are generally attributed to the role of fluids released by dehydration of subducted oceanic lithosphere and to the effects of sediment subduction. Because most SWC basalts lack these compositional features, we conclude that subducted fluids and sediments do not play an essential role in producing these magmas. Rather, we infer that they formed by variable degree melting of a mixed mantle source consisting mainly of heterogeneously distributed OIB and mid-ocean ridge basalt source domains. Relatively minor occurrences of HFSE-depleted arclike basalts may reflect the presence of a small proportion of slab-metasomatized subarc mantle. The juxtaposition of such different mantle domains within the lithospheric mantle is viewed as a consequence of (1) tectonic mixing associated with accretion of oceanic and island arc terranes along the Pacific margin of North America prior to Neogene time, and possibly (2) a seaward jump in the locus of subduction at about 40 Ma. The Cascades arc is unusual in that the subducting oceanic plate is very young and hot. We suggest that slab dehydration outboard of the volcanic front resulted in a diminished role of aqueous fluids in generating or subsequently modifying SWC magmas compared to the situation at most convergent margins. Furthermore, with low fluid flux conditions, basalt generation is presumably triggered by other processes that increase the temperature of the mantle wedge (e.g., convective mantle flow, shear heating, etc.).

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Cornelis Klein

Papers

The geology of North America—An overview

Lunar materials: Their mineralogy, petrology and chemistry