Geosciences

The Lower Cambrian Chilhowee Group of northeastern Tennessee consists of the Unicoi, Hampton and Erwin Formations, and is divided into four facies. The conglomerate facies occurs only within the lower 200 m of measured section (the Unicoi Formation) and consists of fine-grained to pebbly quartz wacke with rare thin beds of laminated siltstone. Low-angle to horizontally laminated, fine-grained sandstone with laminae and lenses of granules and pebbles represents upper flow-regime, overbank deposition within a braided stream system that was close to a coastline. Medium-scale, planar-tabular cross-stratified conglomerate in which megaripple bedforms are preserved is interpreted as representing deposition in interbar pools of braided channels, as flood stage waned and larger bedforms ceased to migrate. Large-scale, planar-tabular cross-stratified conglomerate beds represent migration of large transverse bars within a broad braided stream channel during high flood stage.The sandstone facies occurs throughout the Chilhowee Group, and is therefore interbedded with all other facies. It consists of mainly medium- to very coarse-grained, subarkosic to arkosic arenite. Thinly interbedded, laminated siltstone and sandstone, which may exhibit wavy or lenticular bedding, represents deposition during slack water periods between ebb and flood tides. Large-scale planar-tabular and trough cross-stratification reflects deposition within the deepest areas of subtidal channels, whereas medium-scale cross-stratification represents deposition in shallower water on shoals separating channels. Fining- and thinning-upward sequences most likely resulted from the longshore migration of channels and shoals.The hummocky facies occurs only in the Erwin Formation and consists of horizontally laminated to hummocky stratified, fine-grained arkosic to subarkosic arenite interbedded with equal amounts of bioturbated mudstone. It represents deposition between storm and fairweather wave-base by combined-flow storm currents.The quartz arenite facies is characterized by an absence of fine-grained units and lithologically consists of a super-mature, medium- to coarse-grained quartz arenite. Large-scale planar-tabular cross-stratification and abundant low-angle cross-stratification with rare symmetrical ripples (lower quartz arenite facies) occurs interbedded with the braided fluvial conglomerate facies, and was deposited within either a ridge-and-runnel system or a system of nearshore bars. Large-scale, planar-tabular cross-stratification (upper quartz arenite facies), which forms the top of two 40 m-thick coarsening-upward sequences of the type: hummocky faciessandstone faciesquartz arenite facies, probably represents deposition on sand ridges that formed on a sand-starved shelf as transgression caused the detachment and reworking of shoreface channel-shoal couplets.Palaeocurrent data for the Chilhowee Group are unimodal but widely dispersed from 0° to 180°, and exhibit a minor mode to the west. The data are interpreted to reflect the influence of longshore, tidal and storm currents. The ichnofossil assemblage changes upsection from one characterized only by Paleophycus to a Skolithos ichnofacies and finally to a Cruziana ichnofacies. The facies sequence, biogenic and palaeocurrent data reflect the interaction through time of (I) non-marine and marine processes; and (2) transgression coupled with shoreline progradation. The Chilhowee Group represents an overall deepening from terrestrial deposition to a marine shoreface that experienced both longshore and tidal currents, and finally to a storm shelf environment that periodically shoaled upward.

Burial compaction is one of several major obstacles to estimating palaeoprecipitation from depth to pedogenic carbonate in favourably preserved palaeosols. Palaeosols must be decompacted and the preburial depth to the pedogenic carbonate obtained. Vertic palaeosols may be particularly good candidates for palaeoprecipitation estimates, because of their increased likelihood of preserving clastic dykes, one of the best features for estimating burial compaction. Compaction estimates from clastic dykes and literature-based depth of burial estimates suggest vertic palaeosols undergo significantly less burial compaction than may be commonly assumed. Late Carboniferous vertic palaeosols, buried to 2·5-3·0 km, compacted to 93% of their original thickness. In contrast, clastic dykes in a nonpedogenic shale directly underlying one of the Late Carboniferous palaeosols records compaction to 70% of original thickness. Similarly obtained burial compaction and burial depth estimates for Early Carboniferous, Ordovician, and Proterozoic vertic palaeosols were used to test a burial compaction curve and equation specific to vertic palaeosols. Results suggest this 'vertic-calibrated' curve and equation can be used to estimate burial compaction for vertic palaeosols lacking clastic dykes, but additional testing is needed. Naturally high bulk densities may have limited the compactibility of vertic palaeosols. Likewise, high initial bulk density and an abundance of swelling clays may have severely limited the transmissivity of some vertic palaeosols as they passed from pedogenic to burial environments. Upon burial these vertic palaeosols may have behaved as closed systems, which has implications for understanding their diagenetic modification. Additional efforts to understand burial compaction of vertic palaeosols also promises to improve our understanding of aquifer/aquiclude and hydrocarbon reservoir/seal relationships in sedimentary basins containing intercalated palaeosols.

The occurrences of titanium (Ti) and zirconium (Zr) within eight Vertisols formed in a climosequence on the Upper Beaumont Formation of the Texas Gulf Coastal Plain were investigated in order to determine processes responsible for Ti and Zr redistribution during pedogenesis. Discontinuities defined by significant shifts in particle size distribution and the content (in volume percent) of Zr are present at 160 to 260 cm depth in each pedon. The discontinuities are interpreted to be functional boundaries, i.e., physico-chemical expressions of pedogenic domains, between an upper soil domain dominated by open-system pedogenesis and a lower, more closed-system domain dominated by chemical weathering. The depth at which the functional boundary occurs is dependent on physical and hydrogeochemical influences, which are largely a function of available water. Soil materials above the discontinuities are slightly coarser textured and enriched in Zr, whereas below the sediments are finer textured and have lower and more constant Zr contents. The Zr is associated almost exclusively with zircon and Zr contents correlate positively to the weight percent sand + coarse silt, with negligible Zr present in the < 20 Am size fraction. By comparison, Ti occurs preferentially in the < 20 Am size fraction and there are no marked discontinuities in Ti contents with depth. Scanning electron microscopy (SEM) of individual zircon and rutile grains show predominantly physical damage to zircons, whereas rutile grains appear to have been affected predominantly by chemical weathering. Thus, different processes dictate Ti and Zr content and distribution in these Vertisols, although both elements are often considered immobile in weathering profiles.

The Late Neogene represents warm Earth conditions immediately prior to the development of extensive northern hemisphere glaciation, and this period in Earth history may therefore provide the best available analog for the projected outcome of continued global warming. There are few interior continental sites of Late Neogene age from the eastern half of North America and subsequently very little is known about the conditions characterizing climate. The Early Pliocene (~ 5 Ma) Pipe Creek Sinkhole (PCS) includes the sediment fill of a complex karst environment that developed in north-central Indiana, USA (Lat. 40° 27′ 25.4″, Long. 85° 47′ 37.2″). The site includes more than 3 m of high-chroma, red-colored silty-clay sediment interpreted to be terra rossa. The terra rossa δ13C values average − 20 ± 0.7‰ PDB and are interpreted to represent sediment deposited in a closed cave system under high summer temperatures and with well-drained soils. An in-situ paleosol at the top of the terra rossa represents a transition from a closed cave to an open environment that eventually flooded, thereby becoming a small pond. δ13C values from lacustrine sediments with organic matter derived dominantly from algae average − 20.6‰ and suggest the pond was stagnant and enriched with bicarbonate from the underlying limestones or via aquifers. Pond sediments include abundant vertebrate fossils, which are broadly consistent with those inhabiting an open ecosystem such as a savannah or parkland. However, the PCS pollen includes low taxonomic diversity that is dominated by pine with some hickory and flowering plants, but no grass pollen. We propose two hypotheses to explain the PCS stratigraphic record: (1) The pollen assemblage may represent a local pine dominated ecosystem associated with the pond paleoenvironment, such as a riparian community, and that the greater landscape was drier and open; (2) Alternatively, the climate may have became wetter raising the elevation of the groundwater table and initiating the formation of the pond. Then in response to the wetter conditions an early succession forest ecosystem developed.

Identification of paleosol parent material can be difficult; a common simplistic assumption is that nonpedogenic strata directly beneath paleosols are representative of the parent material. Field relationships, petrography, and geochemical mass-balance were used to test the simple hypothesis that the parent material of both an Early Carboniferous (Late Mississippian) vertic (Vertisol-like) paleosol and a modern analog soil (Vertisol) was immediately subjacent marine carbonate rock. An alternative hypothesis also tested was that the paleosol formed primarily from siliciclastic sediment deposited on top of the carbonate. The modern-ancient analog comparison is valid because both the paleosol and the modern soil have high clay contents, extensive shrink–swell features, and rest directly on top of marine carbonate substrates.Field and thin-section observations indicate minor assimilation of marine carbonate into both the paleosol and soil matrices. Mass-balance of both the paleosol and soil, calculated assuming that the subjacent carbonate was the parent material and that Ti was immobile during weathering, indicated 25–75% net volume loss during weathering and 25–99% net losses of many alkali, alkaline earth, and redox-sensitive elements. However, calculations based on an alternative hypothesis of clayshale parent material for the paleosol indicated only 10–25% volume loss and lower translocation losses for weathering of this material. The insoluble residue content of the dolostone would require that a fourfold volume reduction during weathering occurred, if the dolostone were the only parent material. If the paleosol formed from clayshale, then significantly less volume loss characterised weathering. These results collectively illustrate that the simplest interpretation of paleosol parent material may not always be correct, and that paleosols can be derived from weathering of more than one parent material.

Previous publications combining the properties of multiple soil orders show that depth to carbonate (DTC) increases systematically between 350 and 1000 mm of mean annual precipitation (MAP). We hypothesize that carbonate in Vertisols (clay-rich, shrink-swell soils) respond differently to water flux than other soil orders because of lower permeability. To test this hypothesis, we compiled soil description and characterization data from multiple published sources across a late Pleistocene climosequence of the coast prairie of Texas to assess the relationship between MAP (700-1400 mm) and DTC. The DTC of carbonate nodules represents an index of accumulation and the DTC of calcium carbonate equivalent (total carbonate !2.0 mm diam.) an index of leaching. The DTC for 1%, 2%, and 5% abundances were assessed using regression analysis. The R 2 values were highest for the DTC of 2% nodules and of 1% calcium carbonate equivalent in Vertisol microlows. Surprisingly, relatively high R 2 values were calculated for regression between MAP and DTC in Vertisol microhighs, whereby the relationship is expressed as a parabolic curve and DTC is shallowest in the central part of the climosequence where gilgai expression is greatest. When compared with previous MAP-DTC relationships, it is clear that Vertisols retain carbonate into rainfall isohyets exceeding 1400 mm, 1400 mm higher than the preservation of carbonate in other soil orders. When replotted, the use of DTC to estimate paleoprecipitation with previous equations underestimates MAP in a Mississippian paleo-Vertisol microlow by approximately 32% at a DTC of 100 cm for 5% nodules. Other paleosol proxies also project greater rainfall than previous DTC equations in this paleo-Vertisol.

Vertisols occurring in the Coastal Prairie of Texas preserve distinctive patterns of carbon isotopic values with depth for both soil organic matter as well as pedogenic carbonate. These isotopic values may be used to reconstruct past climate and ecosystems (C3 versus C4 vegetation). Some soils contain large carbonate nodules (> 2 cm diameter) that exhibit δ13C isotopic gradients of up to 2–3‰ across the nodule and have an internal structure that resembles concentric growth rings. These isotopic gradients are used to potentially track relative nodule movement in soil profiles. Some nodules possibly move within and even across microenvironments displaced by several centimeters. The isotopic gradients of the nodules may also track climate and ecosystem changes associated with relative changes in soil depth caused by soil movement. In order to make accurate climate and ecosystem interpretations, soil organic matter and pedogenic carbonate should be demonstrated to have formed in isotopic equilibrium at their respective soil depth.

Plant and root traces from the Fort Prével Member of the Battery Point Formation (late Early Devonian, Emsian), Gaspé Bay, Québec (Canada), are larger and more complex than previously postulated for land plants of this time. The traces are preserved as clay- and silt-lined casts in or near growth position and provide evidence that early vascular land plants achieved substantial stature (2 3 m) and were capable of deep rooting (to nearly 1 m). The root traces and alluvial deposits in which they occur suggest increased landscape stabilization and root system and paleosol morphologies that were influenced by a water-stressed, episodically energetic environment. Early Devonian plants of such large stature may have been partly responsible for initiation of a steep decline in atmospheric p</em>(CO2), through organic carbon burial and accelerated terrestrial weathering.