Dissimilatory Fe(III) and Mn(IV) reduction.

Fresh fracture surfaces of the martian meteorite ALH84001 contain abundant polycyclic aromatic hydrocarbons (PAHs). These fresh fracture surfaces also display carbonate globules. Contamination studies suggest that the PAHs are indigenous to the meteorite. High-resolution scanning and transmission electron microscopy study of surface textures and internal structures of selected carbonate globules show that the globules contain fine-grained, secondary phases of single-domain magnetite and Fe-sulfides. The carbonate globules are similar in texture and size to some terrestrial bacterially induced carbonate precipitates. Although inorganic formation is possible, formation of the globules by biogenic processes could explain many of the observed features, including the PAHs. The PAHs, the carbonate globules, and their associated secondary mineral phases and textures could thus be fossil remains of a past martian biota.

https://science.sciencemag.org/content/sci/273/5277/924.full.pdf

Search for past life on Mars: possible relic biogenic activity in martian meteorite ALH84001.

Detrital modes of sandstone suites primarily reflect the different tectonic settings of provenance terranes, although various other sedimentological factors also influence sandstone compositions. Comparisons of sandstone compositions are aided by grouping diverse grain types into a few operational categories having broad genetic significance. Compositional fields associated with different provenances can then be displayed on standard triangular diagrams.

Interpreting Provenance Relations from Detrital Modes of Sandstones

The determination of detrital modes in graywackes and arkoses, here grouped as subquartzose sandstones, requires special attention to the identification of detrital grain types, as defined operationally, and to the recognition of detrital textures, as opposed to textural elements of diagenetic origin. Accurate detrital modes can yield specific information on provenance that can be gained in no other way. Reserving the terms graywacke and arkose for imprecise field descriptions and abandoning the proportion of matrix as a prime means of classification, subquartzose sandstones can be described adequately using six numerical grain parameters. Three primary parameters summing to 100 permit quantitative serial designation of rock types in the form QxFyLz, w ere Q is total quartzose grains, F is total feldspar grains, and L is total unstable lithic fragments in the framework. Three secondary parameters in the form of ratios permit desirable refinement within each primary parameter: C/Q where C is total polycrystalline quartzose grains, P/F where P is total plagioclase grains, and V/L where V is total volcanic lithic grains. Stable grains, whose sum is the parameter Q, are essentially pure silica in mineralogy, and include both monacrystalline quartz and polycrystalline lithic fragments. The latter are gradational to less quartzose types and difficult to distinguish from felsite grains, which display relict textures of volcanic origin and internal relief owing to their polyminerallic nature. Lithic fragments are subdivided at two levels into our main categories within which are various subcategories: (a) volcanic fragments include felsitic, microlitic, lathwork, and vitric types; (b) clastic fragments include silty-sandy and argillaceous types; (c) tectonite fragments include metasedimentary quartzose types and metavolcanic feldspathic or ferromagnesian types; and (d) microgranular fragments include hypabyssal, hornfelsic, and indurated sedimentary types. Interstitial materials include (a) exotic cements like calcite or zeolite; (b) homogeneous, monominerallic phyllosilicate cement displaying textures indicative of porefilling; (c) clayey detrital lutum called protomatrix; (d) recrystallized lutum or protomatrix called orthomatrix with relict detrital texture| (e) murky, polyminerallic, diagenetic pore-filling called epimatr x, whose growth is accompanied by alteration of framework grains; and (f) deformed and recrystallized lithic fragments called pseudomatrix. Extensive albitization and other alterations complicate the interpretation of detrital modes in many rocks, but potential errors commonly can be avoided by close attention to mineralogy and relict texture except where tectonite fabrics disrupt or transpose the original detrital framework. Most subquartzose sandstones were derived from one. or a mixture, of three salient provenance types: (a) volcanic terranes, yielding feldspatholithic rocks nearly free of quartz, (b) plutonic terranes, yielding feldspathic rocks with few lithic fragments, and (c) uplifted sedimentary and metasedimentary "tectonic" terranes, yielding "chert-grain" lithic rocks with f w quartz and feldspar grains except where recycled volcanic or plutonic detritus is abundant. Many voluminous accumulations of subquartzose sandstones near continental margins had as their provenance complex volcano-plutonic orogens representing ancient magmatic arcs analogous to modern arcs associated with trenches.

/pdf/interpreting-detrital-modes-of-graywacke-and-arkose-45atewvig8.pdf

Interpreting detrital modes of graywacke and arkose

Bathymetry of trace fossils

A descriptive classification of sandstones (exclusive of carbonate and volcaniclastic sandstones) based on the composition of framework grains is proposed that can be used equally well in the field and laboratory. The framework grains are grouped into (1) quartz plus chert and quartzite, (2) feldspar, and (3) rock fragments which constitute the end-members of the quartzarenite, arkose, and litharenite clans respectively. The clans are subdivided to make eight major rock types. Graywacke is defined on the basis of both texture and framework composition and is treated as a special rock type instead of as a clan or subclan type. Because the texture of a sandstone is as important an attribute as framework composition, a polynomial system of sandstone nomenclature should be used in which at least grain size and clan name are cited (i.e., muddy coarse arkose).

A Classification of Common Sandstones

Sedimentary attributes of the Martinsburg (Ordovician) Formation were studied between Staunton, Virginia, and Kingston, New York, Graded bedding, sole marks, intraformational shale fragments, convolute and small-scale cross-lamination of the flysch-type Martinsburg graywackes suggest a turbidity current origin. These turbidites are interbedded with gray pelagic shales, in places graptolite-bearing, and locally with radiolarian cherts. The composition of the graywacke indicates that the chief source was sedimentary and lowgrade metamorphic rocks; acid plutonic rocks were a secondary source. Many rock fragments were derived from the Martinsburg itself. Sole marks, chiefly groove and flute casts, show paleocurrent trends to be parallel (longitudinal), oblique, and perpendicular (transverse) to the present strike. The transverse currents flowed generally toward the northwest. Most longitudinal currents as far north as New Jersey flowed northeast whereas southwestward flowing currents predominated farther north. Graywackes in the north have much more quartz and slightly more feldspar and metamorphic rock fragments than those in the south. This difference is not accompanied by a northward decrease in grain size and cannot be due solely to sorting during transport; hence lateral supply is required even for the northeast flowing longitudinal currents. The turbidity currents flowed down the sub-sea slope of a landmass ("Appalachia") locate to the southeast. Many currents which reached the bathymetric axis of the basin changed course and flowed longitudinally down the regional plunge.

Flysch and associated beds of the Martinsburg Formation (Ordovician), central Appalachians

Diagenetic processes that alter the depositional composition of sands must be considered when making provenance interpretations. These modifying processes operate from the zone of weathering to the deep subsurface where diagenesis grades into metamorphism. Of greatest importance is the total dissolution of grains of feldspar, rock fragments, and heavy minerals by corrosive meteoric or subsurface waters to severely modify original sand composition. Some “diagenetic quartzarenites” have been produced by essentially wholesale dissolution of non-quartz grains under severe weathering. Even partial dissolution of some rock fragments can alter them beyond identification. Of great importance also is the replace­ment of detrital grains by authigenic carbonates, clays, zeolites, and many other minerals. Although the identity of pseudomorphous grains of distinctive size and shape can logically be deduced for some sandstones, the identity of most totally replaced grains cannot be determined with confidence. The albitization of plagioclase more calcic than An10 and of K-feldspar should be expected for all deeply buried sandstones. In the Texas Gulf Coast basin, albitization is important for sandstones that reach burial temperatures in excess of 120° C.

Diagenetic Processes That Affect Provenance Determinations in Sandstone

The Parras and La Popa basins of Coahuila and Nuevo Leon, Mexico, have a composite thickness of about 5.4 km of paralic sedimentary rock (Parras Formation and Difunta Group) deposited during Late Cretaceous to Paleocene time. Detritus of sedimentary, volcanic, and hypabyssal rocks eroded from a highland to the west and southwest of the Parras basin and northwest of the La Popa basin was deposited in deltaic, delta-flank, strand-plain, and marine-shelf environments. Progradation and retrogradation of deltas in each basin in response to basin subsidence and variable sediment influx resulted in a complicated intertonguing of rock units. Using wedge-shaped red-bed bodies and sandstone marker-beds, the Difunta Group is divided into nine formations in the Parras basin and five formations in the La Popa basin. The correlation of stratigraphic units between the basins is uncertain, because only one formation (Muerto Formation) can be mapped across the basin margins, and diagnostic fossils are absent. Fence diagrams and isopach maps document the geometry of the major stratigraphic units.

The clastic basin-fill was deformed during a post-Paleocene (Laramide) event that formed the main folds of the Sierra Madre Oriental. Both tight and broad folds, thrust faults of small displacement, and minor tear faults characterize the deformation within the study area.

The relations of the stratigraphic units, the geometry of the units, and the internal anatomy of the units suggest that the Parras and La Popa basins acted as subsiding elements on the craton and were filled by deltaic and shelf sediments. The basins were not sites of linear geosynclinal deposition as suggested by some workers, and characterization of the basins as foredeeps is undesirable because of the ambiguous meaning of the term.

Earle F. McBride

Papers

A Classification of Common Sandstones

Flysch and associated beds of the Martinsburg Formation (Ordovician), central Appalachians

Diagenetic Processes That Affect Provenance Determinations in Sandstone

Stratigraphy and Structure of the Parras and La Popa Basins, Northeastern Mexico

Diagenetic evolution of Cenozoic sandstones, Gulf of Mexico sedimentary basin