Primary Sedimentary Fabric Research Articles

A first paleomagnetic and rock magnetic investigation of calcareous nodules from the Chinese Loess Plateau

Abstract A detailed paleomagnetic and rock magnetic investigation was carried out on calcareous nodule layers embedded in the loess-paleosol sequences near Baoji city, southern Chinese Loess Plateau. The anisotropy of low-field magnetic susceptibility confirms that calcareous nodules retain the primary sedimentary fabric of original deposits. Rock magnetic properties demonstrate that calcareous nodules have the same mineralogy and grain sizes as loess-paleosol deposits. Characteristic remanent magnetizations of calcareous nodules have directions consistent with those of adjacent loess and paleosol deposits. The Punaruu event, previously found from loess deposits in the upper part of loess unit L13 in Weinan, southern Chinese Loess Plateau, is observed at the corresponding stratigraphic level in the calcareous nodule layer. These observations indicate that the calcareous nodule layers preserve primary remanent magnetizations of original deposits before calcification, and that they can contribute to paleomagnetic studies in the Chinese Loess Plateau.

Trace fossils from Carboniferous floodplain deposits in western Argentina: implications for ichnofacies models of continental environments

Floodplain deposits from the Carboniferous Tupe Formation of western Argentina contain a low-diversity ichnofauna of invertebrates and plants. The assemblage consists of Archaeonassa fossulata, Didymaulichnus lyelli, Helminthoidichnites tenuis, Palaeophycus tubularis, Planolites isp., and root traces. This ichnofauna is characterized by the dominance of very simple forms, superficial or very shallow trace fossils, combination of locomotion, grazing, and dwelling structures, and production by arthropods and vermiform organisms. Primary sedimentary fabrics are disturbed only by plant trace fossils while animal trace fossils are mostly restricted to bedding surfaces, resulting in the virtual absence of bioturbation. The poorly-defined morphology of the ichnofossils, and the absence of backfilled striated trace fossils and structures indicative of subaerial exposure suggest subaqueous production. The envisaged depositional environment is a freshwater body developed in a floodplain and overfilled by overbank deposits. A review of floodplain ichnofaunas documented from the stratigraphic record indicates that two recurrent assemblages may be recognized. The first assemblage is characterized by low to rarely moderate diversity of invertebrate trace fossils, moderate to high diversity of vertebrate structures, and common presence of meniscate backfilled structures, bilobate trace fossils with scratch marks, arthropod trackways and tetrapod trackways. It is regarded as an example of the Scoyenia ichnofacies developed in desiccated floodplains. The second type of floodplain assemblage, illustrated by the example documented in this paper, is characterized by grazing trails, locomotion trails and dwelling burrows, low to rarely moderate ichnodiversity, simple trace fossils, and superficial to very shallow structures. Although formed in floodplain environments also, this type of assemblage is remarkably different from the Scoyenia ichnofacies. It is here regarded as an example of the Mermia ichnofacies. Basic features of these ichnofaunas reflect subaqueous production of the assemblage. The lower ichnodiversity of these floodplain assemblages in comparison with their equivalents from lacustrine basins is probably an expression of the less stable conditions and temporary nature of floodplain water bodies. Our study supports the view that ichnofacies are not indicators of sedimentary environments but reflect sets of environmental factors.

Paleomagnetic evidence for rapid vertical‐axis rotations during thrusting in an active collision zone, northeastern Papua New Guinea

A paleomagnetic study of three thrust sheets of the fold and thrust belt north of the Ramu‐Markham Fault Zone (RMFZ) indicates very rapid vertical‐axis rotations, with differential declination anomalies related to tectonic transport of thrust units. Data from this investigation indicate depositional ages straddling the Brunhes‐Matuyama reversal (780 ka) for the Leron Formation in Erap Valley. Net counterclockwise, vertical‐axis rotations as great as 90° since 1 Ma have occurred locally in the Erap Valley area. These rotations appear to be kinematically related to shear across a tear fault within the foreland fold and thrust belt of the colliding Finisterre Arc, which in turn is aligned with and may be structurally controlled by a major fault in the lower plate. These data indicate that vertical‐axis rotations occurred during thrusting; consequently, the actual rotation rate is likely several times higher than the calculated minimum rate. Such very rapid rotations during thrust sheet emplacement may be more common in fold and thrust belts than is presently recognized. Anisotropy of magnetic susceptibility data yields foliated fabrics with subordinate, well‐grouped lineations that differ markedly in azimuth in the three thrust sheets. The susceptibility lineations are rendered parallel by the same bedding‐perpendicular rotations used to restore the paleomagnetic remanence to N‐S thus independently confirming the rapid rotations. The restored lineations are perpendicular to the direction of tectonic transport, and the minimum susceptibility axes are streaked perpendicular to the lineation. We interpret these anisotropy of magnetic susceptibility data as primary sedimentary fabrics modified by weak strain accompanying foreland thrusting.

Diagenetic Exaggeration of Primary Texural Variations in Incised Valley Fills, Cusiana Field, Colombia: ABSTRACT

Incised Valley Fills in the late Eocene Mirador Formation of the Cusiana Field are dominated by quartz arenite sandstones that vary in grain size from fine sandstones to sandy conglomerates. The average porosity of the valley sandstones is [approx]8% which reflects significant compaction during deep burial ([approximately]20,000feet) and abundant quartz cement ([approximately]14%) precipitated over a wide temperature range (85-140[degrees]C). Surprisingly, the present permeability structure of the valley fill sandstones still reflects grain size variations. Indeed, the effect of the large amounts of quartz cement has been to exaggerate the primary textural variation in permeability. Very coarse, well sorted sandstones at the valley bases possess two orders of magnitude higher permeability than fine-grained equivalents of the same porosity. This permeability contrast is more than original depositional textures in sandstones would predict and is consistent with predictions from simple sphere pack models which show that at low porosity (<12%), the contrast in permeability due to grain size is greatly enhanced. As a result the reservoir characterization has been based on the primary sedimentary fabric, not on diagenesis. Production logging results show that the coarse grained bases of the valley fills are indeed the major contributors to flow. Thus the impact of diagenesismore » on the flow structure of these valley fills has been to selectively reduce the permeability of finer-grained sandstones, while maintaining high permeability in the coarsest grained bases of the valley fills.« less

Description and analysis of bioturbation and ichnofabric

A new scheme for the description and analysis of bioturbation and the resultant ichnofabrics is proposed. This system can be used in core and field-based studies and consists of two parts. (1) A bioturbation index in which a descriptive grade is assigned to the degree of bioturbation. This integrates the sedimentology and ichnology, where the higher grades of bioturbation result from increased burrow overlap and the subsequent loss of the primary sedimentary fabric. (2) An ichnofabric constituent diagram which records the detail of the ichnofabric by graphically plotting the dimensional data of the ichnotaxa and their order of emplacement against their coverage. This scheme differs from previous attempts to characterize bioturbation using semi-quantatitive methods and visually portrays the ichnofabric so that comparative studies can be carried out. This is of particular use in facies analysis, the establishment of the ichnocoenoses and tier diagrams, and within sequence stratigraphic studies so that hiatal surfaces can be recognized.

Halite saltern in the Canning Basin, Western Australia: a sedimentological analysis of drill core from the Ordovician‐Silurian Mallowa Salt

ABSTRACTThe Late Ordovician‐Early Silurian Mallowa Salt of the Carribuddy Group, Canning Basin, north‐west Australia, is the largest halite deposit known in Australia, attaining thicknesses of 800 m or more within an area of approximately 200 000 km2. Study of 675 m of drill core from BHP‐Utah Minerals’ Brooke No. 1 well in the Willara Sub‐basin indicates that the Mallowa Salt accumulated within a saltern (dominantly subaqueous evaporite water body) that was subject to recurrent freshening, desiccation and exposure. Textures and bromine signatures imply a shallow water to ephemeral hypersaline environment typified by increasing salinity and shallowing into evaporitic mudflat conditions toward the top of halite‐mudstone cycles (Type 2) and the less common dolomite/anhydrite‐halite‐mudstone cycles (Type 1). The borate mineral priceite occurs in the capping mudstones of some cycles, reinforcing the idea of an increasing continental influence toward the top of mudstone‐capped halite cycles.The rock salt in both Type 1 and Type 2 cycles typically comprises a mosaic of large, randomly orientated, interlocking halite crystals that formed during early diagenesis. It only partially preserves a primary sedimentary fabric of vertically elongate crystals, some with remnant aligned chevrons. Intraformational hiati, halite karst tubes and solution pits attest to episodic dissolution. Stacked Type 2 cycles dominate; occasional major recharges of less saline, perhaps marine, waters in the same area produced Type I cycles.The envisaged saltern conditions were comparable in many ways to those prevailing during the deposition of halite cycles of the Permian Salado Formation in New Mexico and the Permian San Andres Formation of the Palo Duro Basin area in Texas. However, in the Canning Basin the cycles are characterized by a much lower proportion of anhydrite, implying perhaps a greater degree of continental restriction to the basin. The moderately high level of bromine in the Mallowa Salt (156·5 ± 43·5 ppm Br for primary halite, 146·1 ± 54·7 ppm Br for secondary halite) accords with evolved continental brines, although highly evaporative minerals such as polyhalite and magnesite are absent. The bromine levels suggest little or no dissolution/reprecipitation of primary halite and yet, paradoxically, there is little preservation of the primary depositional fabric. The preservation of early halite cements and replacement textures supports the idea of an early shutdown of brine flow paths, probably at burial depths of no more than a few metres, and the resultant preservation of primary bromine values in the secondary halite.

New paleomagnetic data from the Huanan Block, China, and Cretaceous tectonics in Eastern China

A total of 72 grey-green fine-grained sandstone and siltstone core samples (from 10 sites) were collected from the Early Cretaceous Jieshan and Hekou Formations at Yongan, in the Huanan Block of southeastern China (Fujian Province). After removal of an erratic soft magnetic component in fields of 10–20 mT, stable magnetizations were retrieved from both rock units which are carried mainly by single-domain/pseudo-single-domain (SD/PSD) magnetite or titanomagnetite, as suggested by isothermal remanent magnetization, coercivity measurements and microscopic observations. The Hekou Formation yields a tilt-corrected pole at 66.9°N, 221.4°E ( n=5 sites, K=203.3, A 95=5.4°). A fold test on this result suggests that the magnetization was acquired before tilting which occurred shortly after deposition (between Early and Late Cretaceous). This magnetization is related to a primary sedimentary fabric, as indicated by the low-field anisotropy of magnetic susceptibility (AMS) measurements. The positive fold test and occurrence of a primary sedimentary fabric suggest a depositional origin for the stable magnetization. The Jieshan Formation gives a pole at 70.8°N, 228.7°E ( n=5 sites, K=47.7, A 95=11.2°) in geographic coordinates, which is statistically indistinguishable from the tilted pole for the Hekou Formation. The associated stable magnetization corresponds to a secondary fabric. These features indicate that the Jieshan Formation has been remagnetized during Cretaceous time. The predominance of SD/PSD magnetite or titanomagnetite and igneous activities in eastern Fujian during the Cretaceous suggest a viscous thermal remanent magnetization origin for the secondary magnetization. The Early Cretaceous pole from the Hekou Formation is similar to those for the Yangzi Block, suggesting that the Huanan Block has been accreted to the Yangzi Block at least since Early Cretaceous time. A high consistency of Cretaceous poles from the Sino-Korean, Yangzi and Huanan Blocks as well as Hainan Island indicates that eastern China has been a coherent tectonic unit since Cretaceous time, although there might have been a small amount of northward displacement of the sampling area with respect to the Yangzi Block during the Early-Late Cretaceous period as a result of the overthrust of the Huanan Block onto the Yangzi carbonate platform. Comparison of Cretaceous poles for eastern China with the Eurasian apparent polar wander path (APWP) suggests that no paleomagnetically detectable relative motion has taken place since the Cretaceous. The Cretaceous poles of eastern China, however, display a paleomagnetically significant westward motion (about 9° in angular distance), which is not observed in the Eurasian APWP. This difference may be indicative of a younger age corresponding to the Cretaceous-Tertiary boundary for all Late Cretaceous poles from eastern China, or more probably may imply a minor error in the definition of the Eurasian APWP.

R f/ø f strain analysis using an orientation net

Since the classical work of Cloos, deformed distributions of elliptical objects such as ooids or pebbles have been recognized as an extremely important category of geological strain marker. However, elliptical objects are not easily analyzed, especially where primary sedimentary fabrics are tectonically imbricated. This paper demonstrates that previously published analytical techniques generally address only specific aspects of deformed ellipse distributions; as research tools, they are like a stereonet with great circles only or small circles only. All of the above methods can be combined with the aid of a new orientation net which is as convenient to use in the field as a standard stereonet. Uniform and imbricate fabrics are evaluated with equal ease and assumptions are subjected to statistical testing.

Micromorphological and SEM analysis of cretaceous-paleogene petrosols from Eastern Montana and western North Dakota

Microscopy is used to distinguish pedogenic features from those associated with primary sedimentary fabric and non-pedogenic diagenesis in a suite of 65 million year old sediments. Sand-to-clay-sized material was deposited in meandering river systems over a broad coastal alluvial plain, and pedogenesis occurred in all lithologies. A variety of petrosol (lithified soil) horizons which exhibit both syndepositional bedding and pedogenic and non-pedogenic diagenetic features remains today. Features attributable to the in-situ interaction between root and soil provide the most convincing evidence of pedogenesis. In addition, features such as illuvial clay and iron-oxide accumulations around voids, which are commonly but not exclusively associated with soil environments provide accessory evidence of pedogenesis. Stress-related void and ground-mass birefringence fabrics are evident in the petrosols and some purely sedimentary deposits, and thus cannot be used as diagnostic indicators of pedogenesis. In this fluvial setting, erosion and transport of soil material occurred, which is indicated by the presence of pedorelicts in petrosol horizons and purely sedimentary deposits. Detailed study both by petrographic and by scanning electron microscope elucidates pedogenic indicators not visible in hand samples. Such tools are useful in resolving ambiguities between pedogenesis and non-pedogenic diagenesis.

Porosity of the Miami Limestone (Late Pleistocene), Lower Florida Keys

ABSTRACT This study investigates the amount and variability of porosity in the approximately 130,000-year-old politic grainstones of the Miami Limestone in the lower Florida Keys. Porosity is derived from porosimeter measurements of 326 plugs and core pieces representing 11 locations spanning about 65 km. Because larger voids in our samples result primarily from scouring by circulating water used in coring and plugging, measurements of matrix porosity, rather than of bulk porosity, represent the formation better in situ. X-ray diffraction analyses show that the rocks have undergone substantial inversion of aragonite to calcite (mean aragonite content of the matrix is 40 percent), but are not yet approaching mineralogical stabilization. Porosity of these partially stabilized rocks averages 36 percent, a decrease of 5-10 percent from that of the initial sediments. A qualitative interpretation suggests that the reduction in pore volume correlates with the degree of inversion of aragonite to calcite, and that the porosity decrease associated with complete mineralogical stabilization could be on the order of 15-20 percent. Porosity is narrowly distributed. The standard deviation of porosity for the entire data set is only 4 percent, and much of this variability is on the relatively small scale of primary sedimentary fabric. Although the pre-burial, near-surface meteoric environment may set the stage for subsequent porosity evolution, results imply that significant porosity loss and porosity diversity in similar but much older rocks develop during burial diagenesis.

Magnetic fabric and remanence analyses of cores from the U.S. continental margin and the Vema Channel

Paleomagnetic analyses have been performed on 8 cc cubic samples from 20 gravity and piston cores (172 samples) taken at the base of the U.S. continental margin in the region of a Deep Western Boundary Undercurrent (DWBUC) and from 28 gravity and piston cores (484 samples) from the eastern flank on the Vema Channel. Downcore remanent magnetic (RM) and magnetic fabric measurements have been performed on these samples to examine the effects of depositional mechanisms on the magnetization of sediment at both sites. Vector diagrams after stepwise a.f. demagnetization to 50 mT for both suites of samples indicate variable RM stability above 8–10 cm depth. Below this depth, samples exhibit a stable, apparently primary RM after demagnetization to fields of 10 mT. These results indicate that if the RM is to be used to orient these deep-sea cores, samples should be chosen from greater than 10 cm depth in core and demagnetized to at least 10 mT. Fabric analyses, based on anisotropy of magnetic susceptibility (AMS) measurements, reveal primary sedimentary fabric in most cores from the U.S. continental margin, but anomalous fabric below 40–80 cm depth in some Vema Channel cores. This anomalous fabric is believed to be the result of soft-sediment deformation at the site. AMS long-axis alignments are essentially random for these Vema Channel cores. Under the DWBUC however, the dominant fabric appears to be normal to the current flow direction while cores from outside the DWBUC exhibit a long-axis AMS alignment which parallels bottom contours. The magnetic fabric parameter F s for core-top samples on the eastern flank of the Vema Channel exhibits high values directly under fast flowing Antarctic Bottom Water (AABW) and relatively low values under more sluggish water masses above AABW. These F s results are consistent with those reported for earlier Vema Channel studies and for results from DWBUC core-top samples.

Vertical sheet structures; a new indicator of sedimentary fabric

ABSTRACT Vertical or subvertical sheet-like structures occurring in a turbidite sequence are shown to be developed parallel to the direction of preferred orientation of the long axes of grains. They are interpreted as reflecting primary sedimentary fabric, and can be used as paleocurrent indicators. The sheets are thought to represent a matrix-deficient residue of larger grains formed in the upwelling portions of convection cells initiated by differential settling of grains during deposition of a bed. They may thus provide evidence of deposition from suspension of the sedimentary unit in which they occur.