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Heavy-mineral suites in unconsolidated Paleocene and younger sands, western Tennessee

Heavy-mineral suites from unconsolidated sands of Wilcox and Claiborne age (Eocene) in the subsurface of western Tennessee were tabulated and compared with heavy-mineral suites obtained from outcropping sands known to be of Midway (Paleocene) and Wilcox age and younger. In the subsurface at Memphis, both pink and colorless garnet are relatively abundant in the Claiborne but rare in the Wilcox. Garnet, however, is very rare in both the Claiborne and the Wilcox in the subsurface 35 miles northeast of Memphis. The mineral is very rare also in the terrace sands of western Tennessee and in samples of the Pliocene(?) and Pleistocene deposits of the Tennessee River in eastern and western Tennessee. It is possible, therefore, that the relative abundance of the mineral garnet is related to the quantity of sediment received from differing source areas in Wilcox and Claiborne times, but that, owing to the shifting of the axis of the embayment, no one source area furnished all the sediment for any formation. Heavy-mineral suites from Pliocene(?) and Pleistocene terrace deposits of the Tennessee River in both eastern and western Tennessee, and heavy-mineral suites from Pliocene(?) deposits of the Mississippi River are much alike, and the only isotropic mineral noted in these sediments was a very rare green mineral. Heavy-mineral suites from Recent deposits of the Mississippi River at Memphis and reported heavy-mineral suites from Cambrian sandstones of Wisconsin and Minnesota differ greatly from heavy-mineral suites of Pliocene(?) terrace deposits of the Tennessee and Mississippi Rivers and include much pink and colorless garnet. The possibility, therefore, is suggested that the Pliocene(?) terrace deposits of the Mississippi River in western Tennessee were derived largely from the basin of the Tennessee River.

Tennessee↗

Bottom sediments of Saginaw Bay, Michigan

Saginaw Bay is a southwest extension of Lake Huron on the east shore of the Southern Peninsula of Michigan. It is a shallow-water derivative of the Pleistocene Lake Saginaw. Sixty-one bottom samples were collected on a semigrid pattern and analyzed physically. Findings were treated statistically. Sediments range in size from large pebbles to clay. Medium- to fine-grained clear quartz sand is common to all parts of the bay. Currents and wave action are primarily responsible for both median diameter and sorting distribution patterns. Only a very general correlation can be established between depth and median diameter. Heavy minerals occur in abundance locally and show an affinity to shallow-water areas subject to prevailing currents. Shape also locally determines heavy mineral concentrations. Only general conclusions can be established from roundness and sphericity and acid-soluble content. Increased organic content is correlative with quiet water environments. The shallow-water, heterogeneous nature of Saginaw Bay is not conducive to the recognition of sedimentary criteria suitable for correlations in other than a local environment.

Journal of Sedimentary Petrology↗

Color variations within glacial till, east-central North Dakota--A preliminary investigation

Color variations (orange zones within buff-colored till) in drift in east-central North Dakota are believed to represent two tills of separate origin. Mean size, standard deviation, and number and type of pebbles show greater difference between the two tills than do skewness, kurtosis, and partial chemical analyses. Probably blocks of older till were moved by the last glacier crossing the area and were redeposited in a matrix of younger till.

Journal of Sedimentary Petrology↗

Modern sedimentary environments in Boston Harbor, Massachusetts

Analyses of sidescan-sonar records supplemented by available bathymetric, sedimentary, subbottom, and bottom-current data reveal the distributions of the following three categories of sedimentary environments within the glaciated, topographically complex Boston Harbor estuary in Massachusetts. 1) Environments of erosion appear on the sonographs either as patterns with isolated strong reflections or as uniform patterns of strong reflectivity. These patterns define outcrops of bedrock or till and coarse lag deposits that are being scoured and winnowed by tidal- and wave-induced currents. Erosional areas are located primarily along mainland and insular shores, within large channels that have strong tidal currents, atop submerged ridges and knolls, and across much of the harbor entrance. 2) Environments of deposition are depicted on the sidescan-sonar records as smooth, featureless surfaces that have low to moderate reflectivity. Depositional environments are found predominantly over shallow subtidal flats and in broad bathymetric lows where tidal currents are weak. Sediments within depositional areas are organic-rich sandy and clayey silts that are accumulating at rates ranging from 0.01 to 0.11 g/cm 2 /yr or 4000 to 46,100 metric tons/yr. The cumulative mass of modern mud in harbor depocenters is 24.3 million metric tons. 3) Environments of sediment reworking constitute areas affected by a combination of erosional and depositional processes. They are characterized on the sonographs by mosaics of light and dark patches produced by relatively subtle and gradational changes in reflectivity. Reworked sediments have diverse grain sizes that overlap and are transitional between those of the other two sedimentary environments, and they are indicative of highly variable bottom currents.

Massachusetts↗

Porosity trends of the Lower Cretaceous J Sandstone, Denver Basin, Colorado

This study examines relationships between porosity and time-temperature history, and the influence of rock properties upon porosity, for the Lower Cretaceous J Sandstone in the Colorado portion of the Denver basin. The J Sandstone is classified as a quartzarenite to litharenite and was deposited in nearshore-marine, deltaic, and fluvial-estuarine (valley-fill) settings. Principal elements of its paragenetic sequence include quartz cementation and pressure solution, carbonate cementation and dissolution, dissolution of feldspar and rock fragments, and formation of authigenic clays. Porosity versus vitrinite reflectance (R 0 ) regression lines of the form Phi = A(R 0 )Beta (where B is a negative number) depicting the 10th, 25th, 50th, 75th, and 90th porosity percentiles of the J Sandstone were derived from 963 core-plug measurements representing 31 wells. The data span a thermal maturity range of R 0 = 0.41%-1.14%. Porosity distributions at different locations within the basin can be estimated as a function of thermal maturity on the basis of these regression lines. Porosity trends of the J Sandstone, if considered as a function of R 0 , are similar to those of broad, composite data sets representing sandstones in general. The petrographic factors that most affect J Sandstone porosity variability at a given level of thermal maturity are carbonate cementation and clay content. Carbonate cement, where present, reduces porosity. If previously more widespread, carbonate cement could also introduce porosity heterogeneity by temporarily preserving the pore network relative to uncemented intervals. Abundant detrital and authigenic clay reduces porosity by occupying pores. Low clay content indirectly reduces porosity because the inhibiting effects of clay upon quartz cementation and pressure solution are largely absent.

Journal of Sedimentary Petrology↗

Role of microbial processes in linking sandstone diagenesis with organic-rich clays

Numerous chemical reactions within clay sequences have been proposed to produce dissolved material for diagenesis of deeply-buried sands. However, reactions responsible for solubilizing inorganic and organic constituents in clays at intermediate depths, and their importance to sandstone diagenesis, have not been evaluated. Results from this study show that the processes of microbial organic-acid production (via fermentation) in clays and microbial organic-acid consumption (via sulfate reduction) in sands effectively link organic-rich clays to sandstone diagenesis in the Black Creek Formation of South Carolina. Diagenetic processes have resulted in the formation of 10 volume percent calcite cement, 0.1 volume percent authigenic pyrite, and 1.5 volume percent secondary porosity in Black Creek sands. However, the distribution of these diagenetic processes is not uniform, resulting in net destruction of porosity in some parts of the sand and net porosity enhancement in other parts. Mass balance-derived rates of microbial organic-acid production (10 (super -5) mmole carbon 1 (super -1) yr (super -1) ) and microbial CO 2 production (4 x 10 (super -5) mmole l (super -1) yr (super -1) ) show that microbial processes can account for all organic carbon in the calcite cements (at least 11% of carbonate carbon based on isotope-balance calculations), all observed authigenic pyrite, and all observed secondary porosity. These findings show that microbial processes can serve to link organic-rich clays with sandstone diagenesis at intermediate depths.

Journal of Sedimentary Petrology↗

Lithofacies analysis of colluvial sediments - an aid in interpreting the recent history of Quaternary normal faults in the Basin and Range Province, western United States

Inferring the frequency and magnitude of past earthquakes from the stratigraphy in exposures of normal-faulted sediments is difficult because colluvial lithofacies assemblages adjacent to faults are complex. Similarities in facies assemblages adjacent to young fault scarps in arid to semiarid areas, such as the Basin and Range province, allow lithofacies to be grouped into two genetic architectural elements: debris and wash elements. Upper and lower facies associations can commonly be recognized within each element. A lithofacies code scheme, similar to those used in the analysis of fluvial and glacial lithofacies sequences, provides a concise way of illustrating lithofacies relations in fault exposures. The source lithology of colluvial lithofacies is shown in the code, and soil-horizon symbols can be included. The architecture of lithofacies assemblages near fault scarps in semiarid areas is explained by a model of colluvial sedimentation in response to a single surface faulting event. Analysis of lithofacies assemblages exposed in three trenches across normal faults in the eastern Basin and Range shows how the model can be used to interpret fault histories. Similar facies analysis methods may be useful in interpreting colluvial sequences formed by non-tectonic processes.

Journal of Sedimentary Petrology↗

Measured reflectance suppressed by thin-film interference of crude oil smeared on glass - as on vitrinite in coal or petroliferous rocks

The tool of measuring "vitrinite reflectance" under a microscope has great value in petroleum exploration and coal utilization, and the reflectance is a simple number, such as 1.4% Ro, with some slight variations depending on technique. Sample collection, preparation and measurement are simple and many sedimentary rocks yield vitrinite. However, the reported number can lead one astray if its origin and quality are not fully understood. I analyze here just one factor, "smear" of crude oil on the polished surface (from the sample), which may reduce reflectance because of thin-film interference. Some other causes of error are listed in an addendum to this note.

Society for Organic Petrology Newsletter↗

Anorogenic nature of magmatism in the Northern Baikal volcanic belt: Evidence from geochemical, geochronological (U-Pb), and isotopic (Pb, Nd) data

The Northern Baikal volcanic belt has an age of 1.82-1.87 Ga and extends along the boundary between the Siberian Platform and the Baikal foldbelt. The volcanic belt is composed of volcanics of the Akitkan Group and granitic rocks of the Irel and Primorsk complexes. The geochemistry of the rocks points to the intraplate anorogenic nature of the belt. U-Pb zircon dating of the Chuya granitoids revealed that they are older (2020-2060 Ma) than the Northern Baikal volcanic belt and, thus, cannot be regarded as its component. Data on the Pb isotopic system of feldspars from the granitoids confirm the contemporaneity of all volcanic rocks of the belt except the volcanics of the upper portion of the Akitkan Group (Chaya Formation). Our data suggest its possibly younger (???1.3 Ga) age. The isotopic Nd and Pb compositions of the acid volcanic rocks provide evidence of the heterogeneity of their crustal protoliths. The volcanics of the Malaya Kosa Formation have ??Nd(T) = -6.1, ??2 = 9.36, and were most probably produced with the participation of the U-depleted lower continental crust of Archean age. Other rocks of the complex show ??Nd(T) from -0.1 to -2.4, ??2 = 9.78, and could have been formed by the recycling of the juvenile crust. The depletion of the Malaya Kosa volcanics in most LILEs and HFSEs compared with other acid igneous rocks of the belt possibly reflects compositional differences between the Late Archean and Early Proterozoic crustal sources. The basaltic rocks of the Malaya Kosa Formation (??Nd varies from -4.6 to -5.4) were produced by either the melting of the enriched lithospheric mantle or the contamination of derivatives of the depleted mantle by Early Archean lower crustal rocks, which are not exposed within the area. Copyright ?? 1998 by MAEe Cyrillic signK Hay??a/Interperiodica Publishing.

Petrology↗