Planar Laminae Research Articles

Interpreting pre-vegetation landscape dynamics: The Cambrian Lower Mount Simon Sandstone, Illinois, U.S.A.

ABSTRACTThe Cambrian Mount Simon Sandstone has been the subject of extensive study and multiple industrial-scale carbon storage demonstrations at Decatur, Illinois, USA. The development of a reliable paleoenvironmental model is critical to successful large-scale carbon dioxide (CO2) storage, but is complicated by the need to interpret pre-vegetation sedimentation processes. The present study presents a paleoenvironmental model of the Lower Mount Simon Sandstone, based on analysis of primary sedimentary structures in two cores and four complete high-resolution resistivity logs (FMI).The Lower Mount Simon Sandstone represents a vertical “drying-up” sequence composed of three associated depositional units: a north–south oriented coastal system at the base, an eastward-directed fluvial unit in the middle, and a westward-directed eolian system at the top that recycled medium- and fine-grained sand in the basin. Quantitative analysis of fluvial cross-strata indicates that the perennial river system was shallow (c. 1 m deep) with relatively narrow channel belts (c. 1 km). Adjacent sandy eolian-floodplain deposits contain abundant thin, crinkly planar laminae that are enriched in fines and are interpreted as cementation surfaces, likely of biological origin. Deflation lags and wind-ripple strata are commonly interbedded with the crinkly strata, suggesting that the recurrence of erosion and deposition that controlled sedimentary preservation on the floodplain were dominated by eolian transport, re-wetting, and (bio-) cementation. Such a prominent role of exposure to the wind, basin-scale sediment recycling, and eolian removal of fine-grained sediment would have ceased to exist for most climates after the development of vegetation on land, yet, may well be key to understanding the environmental context for early life on Earth.

‘Stromatolites’ built by sponges and microbes – a new type of Phanerozoic bioconstruction

Two ‘stromatolites’ from Carboniferous and Triassic carbonates previously regarded as microbial bioconstructions are analysed and reinterpreted as sponge-microbial build-ups. The automicritic aggregations in these build-ups are similar to the previously reported fossils of keratose demosponges in showing moulded anastomosing filamentous structures. All the studied columnar or domal constructions were formed in turbulent water with high sedimentation rate. The Carboniferous build-ups were constructed in the shallow subtidal zone of an open shelf or a ramp. The laminations within the stromatolite-like columns are composed of alternating dark micritic laminae of sponge fossils and pale laminae of neomorphic microspars. The accretion of these columns is probably related to the repeated cycles of sponge growth, rapid lithification after burial, re-exposure and erosion, and settlement of new generations. The Triassic rocks are presumed to have been precipitated in a slightly evaporitic environment based on lithological features. They show a transition from planar laminae, which were formed under the influence of microbial mats, to stromatolitic columnar or domal build-ups, which are dominated by stacked micritic clumps of probable sponge fossils. The sponge–microbe alternation may have been controlled by variation of salinity. Comparable with a recent study, this work shows that sponge-related bioconstructions can be morphologically similar to microbialites in the level of mega- and mesostructures.

Ichnological constraints on the depositional environment of the Sawahlunto Formation, Kandi, northwest Ombilin Basin, west Sumatra, Indonesia

A low diversity trace fossil assemblage is described from the Oligocene Sawahlunto Formation near Kandi, in the northwestern part of the Ombilin Basin in western Sumatra, Indonesia. This trace fossil assemblage includes six ichnogenera attributed to invertebrate infaunal and epifaunal activities ( Arenicolites, Diplocraterion, Planolites, Monocraterion/ Skolithos and Coenobichnus) and two ichnotaxa attributed to vertebrate activity (avian footprints: two species of Aquatilavipes). Arenicolites, Diplocraterion and Monocraterion/ Skolithos record the suspension feeding activities of either arthropods (most likely amphipods) or vermiform organisms. Planolites reflects the presence of an infaunal deposit feeder. Coenobichnus records the walking activities of hermit crabs. Both the Coenobichnus and the avian footprints record the surficial detritus scavenging of epifaunal organisms within a subaerial setting. These traces occur within a fine-grained sandstone succession characterized by planar laminae and low-relief, asymmetrical, commonly mud-draped (locally bidirectional) ripples. The presence of traces attributable to suspension feeders implies deposition in a subaqueous setting. Their occurrence (particularly the presence of Arenicolites and Diplocraterion) in a sandstone bed characterized by mud-draped and bidirectional ripples implies emplacement in a tidally-influenced marine to marginal marine setting. Co-occurrence of these traces with well-preserved avian footprints ( Aquatilavipes) further implies periodic subaerial exposure. Thus, it is most likely that the Sawahlunto Formation near Kandi records deposition within an intertidal flat setting. Definitive evidence of marine influences in the Oligocene interval of the Ombilin Basin implies a more complex tectono-stratigraphic history than has previously been implied.

Evidence of Bedform Superimposition and Flow Unsteadiness In Unit-Bar Deposits, South Saskatchewan River, Canada

Abstract Unit-bar deposits are ubiquitous components of river-channel deposits and strongly influence their hydrological properties, yet they are not easy to interpret. This paper concerns details of the internal structures of six unit bars from the South Saskatchewan River, Canada, that were investigated using trenches, epoxy resin peels, and 900 MHz ground-penetrating radar (GPR) profiles. The composition of unit bars depends on flow unsteadiness and superimposed bedforms. Flow unsteadiness causes changes in the mean grain size of the sediment in transport, but is expressed primarily as a change in the type and direction of migration of smaller ripple- and dune-scale bedforms superimposed on unit bars. Superimposed bedforms with heights that exceed 25% of the host bedform height reduce their host’s slope and generate inclined sets. Host bedforms with smaller superimposed bedforms form angle-of-repose cross strata with a visible pre-sorting pattern. The relationship between the formative host- and superimposed bedforms and such pre-sorted cross strata can be used to interpret numerous aspects of the three-dimensional geometries of the bedforms. Such detailed interpretations rely on: (i) regularly spaced fine-grained drapes deposited during the passage of troughs of superimposed bedforms, (ii) grain-size sorting patterns generated by sorting within the superimposed bedforms before deposition, (iii) grain-size sorting during deposition by grainfall and intermittent grainflows down the bar lee-slope, and (iv) the presence and nature of low-angle reactivation surfaces. The detailed interpretations revealed that the deposits of dam-related floods were significantly smaller than the deposits of individual unit bars in this study. The unsteady flow and sediment transport conditions are reflected by changes in the structural composition of the unit bars. Reactivation surfaces associated with flow unsteadiness, as opposed to those formed by large superimposed bedforms, were characterized by changes in mean grain size, the buildup of sediment at the base of the bar lee slope, and changes in structures formed by superimposed bedforms. Reactivation surfaces generated by large superimposed bedforms indicate that bedform preservation is likely increased in areas of flow deceleration. The trough deposits of exposed unit bars contained bubble sand, planar laminae due to upper-stage plane beds, and low-angle inclined laminae due to antidunes. These structures indicate that unit-bar troughs can act as ephemeral channels. The composition of the investigated unit bars thus emphasizes the interplay between flow unsteadiness and bed morphology in the formation of sedimentary structures in river channels. Sediment surfaces generate visible GPR reflections where the thickness and contrast in electrical properties of the sediment layers are sufficiently large. Most cross-stratified sets were represented only by reflections from the base of the set, and not by high-angle inclined reflections from the cross strata. The local absence of reflections from cross strata is attributed to the lack of contrast in electrical properties of the well-sorted cross strata and their limited thickness relative to the radar wavelength. In contrast to cross strata, reactivation surfaces formed by large superimposed bedforms and flow unsteadiness were commonly associated with distinct inclined reflections.

Setulfs, modern and ancient: Formative mechanism, preservation bias and palaeoenvironmental implications

Setulfs or inverted flutes, both in growth stage and in fossilized state, are addressed here. This account resolves the long-standing questions about their formative mechanism, depositional prerequisites, and preservation bias, as well as their palaeoenvironmental implications. These structures were observed to grow by accretion of wind deflated sand in the lee of miniscule obstructions, in the littoral–supralittoral transition zone, a little above the high-water mark on top of a partially-emerged bar at Chandipur, eastern India. Pseudopellets of crabs and scarce chips of microbial mat create the obstructions, as shells might also do in the case of other modern analogs reported from essentially similar environmental conditions. Moisture on the beach sand limited the lifting power of the wind, so that even miniscule obstructions could have a well-developed wake zone at their lee side, resulting in localized accumulation of a small ridge of sand extending downwind from the obstruction. Setulfs arrested under microbial mats were also found in Chandipur on the seaward fringe of a shallow marsh encircled by a spit. Like those in growth stage these mat-covered setulfs also had their tapering ends directed landward, as the wind was directed onshore during the period of observation. Fossilized setulfs in swarms are also described here from two Neoproterozoic formations in India, viz., the Sonia Sandstone and the Upper Bhander Sandstone. In both the cases, setulfs are preserved in well-sorted sandstones, and are associated with planar laminae, profound wave ripples, bifurcated parting lineations, infrequent rill marks and local swarms of current crescents on bedsurfaces. Additionally, recorded fossil setulfs are found in association with inversely graded translatent strata, crinkled adhesion laminae, patches of impact ripples and isolated small sets of alternate grain-flow and grain-fall cross-strata of aeolian origin. This suggests preservation in the high littoral–supralittoral zone as in case of all known modern setulf occurrences. The palaeowind direction derived from the ancient setulfs conforms in orientation to that recorded from other aeolian structures in the strata, supporting their wind deflation origin. Although shells and biogenic pellets were altogether absent from these Neoproterozoic rocks, microbial mat curls and fragments had presumably been abundantly present in the Proterozoic high littoral–supralittoral zone to facilitate formation of setulfs. Pertinently many microbial mat-related features are present within both the siliciclastic formations studied. In polished sections some of the studied Proterozoic setulfs reveal the existence of crinkled carbonaceous lamina covers that signify microbial mat growth on top of them. An a priori assumption is that microbial mats must have enhanced the preservation potential of these delicate structures, which would otherwise be extremely unlikely to be preserved. The seeming preference of fossil setulf occurrences from the Proterozoic–Early Palaeozoic time is most likely due to a lack of burrowers and grazers that would devastate the protective microbial mat later in the Phanerozoic. These observations suggest that swarms of setulfs are, in general, wind sculpted, and their presence in the rock record prefers palaeoenvironment interpretration as a high littoral–supralittoral transition zone, and also stands as proxy records of syn-depositional microbial mat growth.

Process-sedimentological challenges in distinguishing paleo-tsunami deposits

There has been a lively debate since the 1980s on distinguishing between paleo-tsunami deposits and paleo-cyclone deposits using sedimentological criteria. Tsunami waves not only cause erosion and deposition during inundation of coastlines in subaerial environments, but also trigger backwash flows in submarine environments. These incoming waves and outgoing flows emplace sediment in a wide range of environments, which include coastal lake, beach, marsh, lagoon, bay, open shelf, slope and basin. Holocene deposits of tsunami-related processes from these environments exhibit a multitude of physical, biological and geochemical features. These features include basal erosional surfaces, anomalously coarse sand layers, imbricated boulders, chaotic bedding, rip-up mud clasts, normal grading, inverse grading, landward-fining trend, horizontal planar laminae, cross-stratification, hummocky cross-stratification, massive sand rich in marine fossils, sand with high K, Mg and Na elemental concentrations and sand injections. These sedimentological features imply extreme variability in processes that include erosion, bed load (traction), lower flow regime currents, upper-flow regime currents, oscillatory flows, combined flows, bidirectional currents, mass emplacement, freezing en masse, settling from suspension and sand injection. The notion that a ‘tsunami’ event represents a single (unique) depositional process is a myth. Although many sedimentary features are considered to be reliable criteria for recognizing potential paleo-tsunami deposits, similar features are also common in cyclone-induced deposits. At present, paleo-tsunami deposits cannot be distinguished from paleo-cyclone deposits using sedimentological features alone, without historical information. The future success of distinguishing paleo-tsunami deposits depends on the development of criteria based on systematic synthesis of copious modern examples worldwide and on the precise application of basic principles of process sedimentology.

Stratigraphy, Sedimentology, and Microfossil Content of Hurricane Rita Storm Surge Deposits in Southwest Louisiana

Hurricane Rita generated a 4–5-m-high storm surge along the southwest Louisiana coast. The storm surge and accompanying waves transported sand and mud into woodland and freshwater marsh environments located immediately landward of sandy beaches. The resulting storm surge deposit is up to 0.5 m thick and extends at least 500 m inland. Analysis of the stratigraphy, sedimentology, and microfossil content of the deposit suggests two distinct phases of deposition: a thin layer of finer sand and mud and an overlying thicker layer of coarser sand. The thin layer of finer sand and mud is characterized by planar laminae and calcareous foraminifera, characteristic of shallow marine environments. The layer drapes preexisting surfaces, extends at least 500 m inland, and abruptly overlies woodland and marsh soils. These findings suggest deposition from suspension of offshore sand and mud in an early stage of storm surge inundation. This layer is overlain by up to 0.5 m of coarser sand with prominent foreset laminae. Foraminiferal tests are rare in this part of the deposit. The coarser sand layer has an abrupt termination 100–150 m inland. These findings suggest the sand is a traction load deposit, formed at a later stage of storm surge inundation. The storm surge deposit has a distinctive geologic signature in the study area, raising the possibility of extending paleotempestology studies to this and other similar nearshore environments. The more distal inland part of the deposit has characteristics commonly associated with tsunami-laid sand, suggesting that in studies of coastlines vulnerable to extreme storms and tsunamis, and where the sedimentary evidence is fragmentary, the geologic records of storms and tsunamis may not be easily differentiated.

Deposit Structure and Processes of Sand Deposition from Decelerating Sediment Suspensions

Turbidity currents are notoriously difficult to monitor directly, therefore interpretation of their deposits forms the basis for much of our understanding of these flows. The deceleration rate of a flow is a potentially important yet poorly understood control on depositional processes. A series of experiments were conducted in an annular flume, in which fast (up to 3.5 m/s) and highly turbulent flows of sand (up to 250 µm) and water were decelerated at different rates and processes of deposition and deposit character analyzed. Previously poorly documented depositional processes were observed in the experiments. This is because the flows were initially unusually fast and of prolonged duration, with sustained periods of sediment fallout as the flow slowed down. The conditions in these flows are thus likely to be closer to those at the base of a waning turbidity current than is achieved in other relatively slow experimental flows. The collapse of high-concentration, moving, thin (< 5 mm) near-bed layers (laminar sheared layers) were an important mechanism by which the bed aggraded beneath these unsteady flows. At bed aggradation rates in excess of 0.44 mm/s the sequential collapse of laminar sheared layers produced a structureless, poorly graded and poorly sorted deposit (Bouma Ta). When bed aggradation rates fell below 0.44 mm/s the collapsing laminar sheared layers were reworked by turbulence to form planar laminae (Bouma Tb). These laminae are formed in a very different manner than the planar laminae attributed to bedwaves in previous open-channel flow experiments. Collapse of laminar sheared layers is therefore an alternative process for generating the Bouma Tb division. Inverse grading developed at the base of the deposits of slowly decelerated flows. This inverse grading was probably a result of grain sorting in a high-concentration layer that persisted at the base of the flow for many minutes prior to the onset of deposition.

A Pliocene mega-tsunami deposit and associated features in the Ranquil Formation, southern Chile

An exceptionally large tsunami affected the coastline of southern Chile during the Pliocene. Its backflow eroded coarse beach and coastal dune sediments and redistributed them over the continental shelf and slope. Sandstone dykes and sills injected from the base of the resulting hyperconcentrated flow into underlying cohesive muds, assisted in plucking up large blocks of the latter and incorporating them into the flow. Locally, the rip-up intraclasts were fragmented further by smaller-scale injections to form a distinct breccia of angular to rounded mudstone clasts within a medium to coarse sandstone matrix. Sandstone sills in places mimic normal sedimentary beds, complete with structures resembling inverse gradation, planar laminae, as well as ripple and trough cross-lamination. These were probably formed by internal sediment flow and shear stress as the semi-liquefied sand was forcefully injected into cracks. In borehole cores, such sills can easily be misinterpreted as normal sedimentary beds, which can have important implications for hydrocarbon exploration.

Physical criteria for distinguishing sandy tsunami and storm deposits using modern examples

Modern subaerial sand beds deposited by major tsunamis and hurricanes were compared at trench, transect, and sub-regional spatial scales to evaluate which attributes are most useful for distinguishing the two types of deposits. Physical criteria that may be diagnostic include: sediment composition, textures and grading, types and organization of stratification, thickness, geometry, and landscape conformity. Published reports of Pacific Ocean tsunami impacts and our field observations suggest that sandy tsunami deposits are generally < 25 cm thick, extend hundreds of meters inland from the beach, and fill microtopography but generally conform to the antecedent landscape. They commonly are a single homogeneous bed that is normally graded overall, or that consists of only a few thin layers. Mud intraclasts and mud laminae within the deposit are strong evidence of tsunami deposition. Twig orientation or other indicators of return flow during bed aggradation are also diagnostic of tsunami deposits. Sandy storm deposits tend to be > 30 cm thick, generally extend < 300 m from the beach, and will not advance beyond the antecedent macrotopography they are able to fill. They typically are composed of numerous subhorizontal planar laminae organized into multiple laminasets that are normally or inversely graded, they do not contain internal mud laminae and rarely contain mud intraclasts. Application of these distinguishing characteristics depends on their preservation potential and any deposit modifications that accompany burial. The distinctions between tsunami and storm deposits are related to differences in the hydrodynamics and sediment-sorting processes during transport. Tsunami deposition results from a few high-velocity, long-period waves that entrain sediment from the shoreface, beach, and landward erosion zone. Tsunamis can have flow depths greater than 10 m, transport sediment primarily in suspension, and distribute the load over a broad region where sediment falls out of suspension when flow decelerates. In contrast, storm inundation generally is gradual and prolonged, consisting of many waves that erode beaches and dunes with no significant overland return flow until after the main flooding. Storm flow depths are commonly < 3 m, sediment is transported primarily as bed load by traction, and the load is deposited within a zone relatively close to the beach.

Stratigraphy, sedimentology and bulk organic geochemistry of black shales from the Proterozoic Vindhyan Supergroup (central India)

Four organic-rich shale units of the Proterozoic Vindhyan sedimentary succession have been scanned to reveal their origin and hydrocarbon potential. The wavy-crinkly nature of the carbonaceous laminae is suggestive of a microbial mat origin of the shales. These shales are thus different from Phanerozoic black shales which typically exhibit planar laminae. The hydrocarbon potential of the black shale units has been evaluated by Rock-Eval pyrolysis. Total organic carbon content of many of the shales exceeds 1%. The meanT max for the black shales translate to a vitrinite reflectance range of 2.05-2.40% Rm based on standard conversion techniques. These shales have reached the catagenetic stage near the beginning of anthracite formation.

Braided-River Deposits in A Muddy Depositional Setting: The Molina Member of the Wasatch Formation (Paleogene), West-Central Colorado, U.S.A.

The Molina Member is a Paleogene, sand-rich interval, up to 125 meters thick, found within the otherwise mud-dominated, nonmarine Wasatch Formation of west-central Colorado. Sandstone beds of the Molina Member form sheet-like units interbedded with subordinate volumes of mudstone. The sandstones contain a low-diversity assemblage of sedimentary structures that is commonly dominated by horizontal, planar laminae, and bedding is locally contorted by large-scale soft-sediment deformation. A braided fluvial model best explains the characteristics of these strata, yet the existing braided-river models do not fully account for the assemblage of sedimentary structures or the relatively high volumes (up to half of the section) of interbedded mudstones. Two modern braided rivers, Medano Creek (Colorado) and the Prairie Dog Town Fork of the Red River (Texas), have rapid, shallow flows that form horizontal, planar bedding. In both systems the presence of large volumes of easily mobilized sandy sediment, a flashy discharge regime, and a lack of reworking between flood stages lead to the development of high-energy, shallow-water bedforms and the preservation of the resulting high-energy sedimentary structures. Similar conditions and processes are suggested to have controlled deposition of the sand-dominated Molina Member, which is a depositional anomaly within the otherwise muddy Wasatch Formation. Short-term availability of readily eroded, second-cycle sandstones from a newly elevated source area to the southeast is the most likely reason for the abrupt change from a mud-dominated fluvial system to the sandy Molina system.

Late Archean Calcite-Microbe Interactions: Two Morphologically Distinct Microbial Communities That Affected Calcite Nucleation Differently

Microbialites in the 2521+ or -3 Ma Gamohaan and Frisco formations, South Africa, consist of three components: very thin, filmy laminae that are interpreted as remnants of microbial mats; irregular surfaces that acted as supports over which filmy laminae draped, which also are interpreted as microbial in origin; and cement-filled voids that formed contemporaneously with microbialite growth. The structure of the microbialites varies with the proportions of filmy laminae and supports, disruption of had filmy laminae, and abundance of voids. Seven end-member morphologies have been defined: planar laminae, contorted laminae, tented microbialites, cuspate microbialites, irregular columnar microbialites, plumose structures, and herringbone calcite beds. Similar structures have not been reported in younger rocks, but they may have partial analogs in Yellowstone hot springs, ice covered lakes in Antarctica, and Proterozoic conoform stromatolites. Herringbone calcite, a fibrous marine cement, precipitated contemporaneously with microbial growth. It preferentially precipitated on the supports over the laminated mat as demonstrated by: the concentration of herringbone calcite near supports; growth banding in herringbone calcite, which indicates that calcite nucleated on and grew away from supports but not laminated mat; and the abutment of herringbone calcite coatings against laminated mat attached to supports. These observations suggest that the laminated mat inhibited nucleation of calcite crystals. The differences in the morphology and relationships to herringbone calcite in the supports and laminated mat imply that they were composed of distinct microbial communities that interacted with their environment differently. Also, the complexity of the microbialites demonstrates that a remarkably diverse set of microbe-substrate interactions evolved by late Archean time.

Preservation of planar laminae due to migration of low‐relief bed waves over aggrading upper‐stage plane beds: comparison of experimental data with theory

ABSTRACTExperimental studies of the formation of planar laminae by migration of low‐relief bed waves over aggrading upper‐stage plane beds show that the average thickness of laminae at a point increases with both aggradation rate and the variance of the heights of bed waves passing that point. In general, the preserved laminae represent only a small proportion (generally less than 50%) of the height of the largest bed waves in the population. The theory developed by Paola &amp; Borgman (1991), relating the probability density function of stratal thickness to that of bed wave height for the case of no net aggradation, was adapted for aggrading conditions and shown to agree well with data. These results suggest that the theory can be used to estimate, from the distribution of thickness of planar laminae, either the distribution of bed wave heights, mean aggradation rate, mean bed wave length or mean bed wave celerity, provided the other parameters can be estimated.

Thickness of sets of cross strata and planar strata as a function of formative bed-wave geometry and migration, and aggradation rate

Cross strata and planar strata (formed by the migration of ripples and dunes or low-relief bed waves on upper-stage plane beds, respectively) are the most common internal structures in fluvial sands and sandstones. The thickness of cross sets or planar laminae at a given location is controlled by (1) aggradation rate relative to downstream migration rate of bed waves and (2) the succession of bed waves of varying height passing the location. A quantitative understanding of this relationship is now appearing, based on new theory and experimental studies with ripples, dunes, and upper-stage plane beds. In many cases, the effects of control 1 can be ignored. In such cases, only the largest bed waves in a population form cross sets or planar laminae; the ratio of mean cross-set thickness/mean height of formative bed waves is less than 0.4, and the ratio of mean cross-set thickness/mean height of all bed waves is less than 0.5. A simple quantitative model (a modified form of the Paola-Borgman model) is presented for predicting the height distributions of bed waves (ripples, dunes, low-relief bed waves) from the thickness distributions of cross sets or planar laminae. If the aggradation rate is appreciable, more of the smaller bed waves in a population can form cross sets or planar laminae, and the preservation ratios increase (to 1 in the case of supercritical climbing-ripple cross strata). In this case, it is necessary to determine the angle of climb and length of the bed waves in order to use the model to predict bed-wave height distributions.

Sedimentology of subaqueous volcaniclastic sediment gravity flows in the Neogene Santa Maria Basin, California

ABSTRACTSubaqueous tuff deposits within the lower Miocene Lospe Formation of the Santa Maria Basin, California, are up to 20 m thick and were deposited by high density turbidity flows after large volumes of ash were supplied to the basin and remobilized. Tuff units in the Lospe Formation include a lower lithofacies assemblage of planar bedded tuff that grades upward into massive tuff, which in turn is overlain by an upper lithofacies assemblage of alternating thin bedded, coarse grained tuff beds and tuffaceous mudstone. The planar bedded tuff ranges from 0.3 to 3 m thick and contains 1‐8 cm thick beds that exhibit inverse grading, and low angle and planar laminations. The overlying massive tuff ranges from 1 to 10 m thick and includes large intraclasts of pumiceous tuff and stringers of pumice grains aligned parallel to bedding. The upper lithofacies assemblage of thin bedded tuff ranges from 0.4 to 3 m thick; individual beds are 6‐30 cm thick and display planar laminae and dewatering structures. Pumice is generally concentrated in the upper halves of beds in the thin bedded tuff interval.The association of sedimentary structures combined with semi‐quantitative analysis for dispersive and hydraulic equivalence of bubble‐wall vitric shards and pumice grains reveals that particles in the planar bedded lithofacies are in dispersive, not settling, equivalence. This suggests deposition under dispersive pressures in a tractive flow. Grains in the overlying massive tuff are more closely in settling equivalence as opposed to dispersive equivalence, which suggests rapid deposition from a suspended sediment load. The set of lithofacies that comprises the lower lithofacies assemblage of each of the Lospe Formation tuff units is analogous to those of traction carpets and subsequent suspension sedimentation deposits often attributed to high density turbidity flows. Grain distributions in the upper thin bedded lithofacies do not reveal a clear relation for dispersive or settling equivalence. This information, together with the association of sedimentary features in the thin bedded lithofacies, including dewatering structures, suggests a combination of tractive and liquefied flows.Absence of evidence for elevated emplacement temperatures (e.g. eutaxitic texture or shattered crystàls) suggests emplacement of the Lospe Formation tuff deposits in a cold state closely following pyroclastic eruptions. The tuff deposits are not only a result of primary volcanic processes which supplied the detritus, but also of processes which involved remobilization of unconsolidated ash as subaqueous sediment gravity flows. These deposits provide an opportunity to study the sedimentation processes that may occur during subaqueous volcaniclastic flows and demonstrate similarities with existing models for sediment gravity flow processes.

Grain size segregation and stratigraphy in aeolian ripples modelled with a cellular automaton

AEOLIAN ripples are distinguished by coarse-grained crests, finegrained troughs, and thin veneers of coarse grains on the upwind (stoss) slopes. Migration of these sorted bedforms during periods of net deposition results in a characteristic inversely graded stratigraphy that distinguishes aeolian sandstones from those of fluvial origin1. Here we investigate the formation and evolution of aeolian ripples using a cellular automaton model of a sandbed which incorporates two grain sizes and is subject to the impacts of episodically hopping (saltating) grains. Using this model, we identify the physical processes responsible for both the spatial sorting and the stratigraphic signature of the ripples. High-energy saltating grains are found to eject small grains preferentially from the impact site, leading to a coarsening of the heavily bombarded stoss slopes. Coarse grains do not in general leap far enough to escape the shadow zones on the downwind (lee) slopes, and therefore tend to accumulate and cycle around the ripple crests. Small grains, on the other hand, are ejected at higher velocities, enabling them to hop further. Incorporating net deposition into the model results in a stratigraphy closely resembling thin planar laminae ('pinstriping') found in aeolian sandstones: the inverse grading of grain sizes is a direct consequence of their different hop lengths.

Fluvial and Deltaic Facies and Environments of the Late Permian Back-Reef Shelves of the Permian Basin of Texas and New Mexico

The Artesia Group is a sequence of carbonates, evaporites, and clastics that was deposited across the back-reef shelves of the Permian Basin during late Permian time. There has been some controversy over the depositional environments of the clastic members of the Artesia Group and the role of sea level fluctuations in their accumulation. However, the results of a regional core study of the Queen Formation of the Artesia Group indicate that they were largely deposited in desert fluvial and deltaic environments during low-stands of sea level. Three fluvial-deltaic facies are recognized within the clastic members of the Queen. The first consists of medium to very find sandstones and silty sandstones with cross-beds, ripple cross-laminae, and planar and wavy laminae. This facies forms wavy sheets that thicken and thin along linear trends, and was deposited in sandy braided streams. The second facies consists of very find to fine sandstones, silty sandstones, and siltstones with ripple cross-laminae, planar and wavy laminae, cross-beds, clay drapes and pedogenetic cutans, as well as siltstones and silty mudstones with haloturbation structures and evaporite nodules. This facies forms thick planar sheets, and was deposited in fluvial sandflats and adjacent fluvial-dominated continental sabkhas. The third facies consists ofmore » cyclic deposits of haloturbated silty mudstones that grade into siltstones and very fine sandstones with crossbeds, planar and wavy laminae, haloturbation structures and evaporite nodules. Each cycle forms a lobate body that is bounded by carbonates or evaporites and which was deposited in sheet deltas that formed along the landward margins of a back-reef lagoon.« less

The morphology and dynamics of low amplitude bedwaves upon upper stage plane beds and the preservation of planar laminae

ABSTRACTExperimental studies of subcritical, unidirectional flow over upper stage plane beds of medium grained sand reveal the ubiquitous presence of low amplitude bedwaves. Flow depth was 0·11 m, mean flow velocities were 0·86–1·0 m s−1, shear velocities were 0·058–0·71 m s−1 and dimensionless shear stresses were 0·56–0·87. Bedwaves are asymmetrical in profile and range from 0·75 to 11 mm in height (mainly 2–6 mm), from 0·7 to 1·3 m in wavelength and have mean celerities of 10 mm s−1. Flow records suggest that the bedwaves are associated with accelerating flow over the bedwave crests and flow which decelerates and diverges laterally over the troughs. High resolution bed profiling during aggradation of the bed combined with subsequent box coring illustrates that these bedwaves are responsible for the planar laminae characteristic of upper stage plane beds. Lamina preservation is dependent upon the mean aggradation rate and the sequence of bedwaves of different height crossing any point; individual laminae are more readily preserved at higher aggradation rates where the possibility of reworking by later bedwaves is reduced. Laminae are recognized by small changes in grain size and commonly a fining upwards at the top of laminae which is generated by fine grained material infiltrating a lower lamina in the leeside of a bedwave.

Depositional Environments and Reservoir Properties of the Upper Queen Formation, Concho Bluff and Concho Bluff North Fields, Midland Basin, Texas

The upper Queen Formation (Upper Permian) in Concho Bluff and Concho Bluff North fields (Crane, Upton, and Ector counties, Texas) consists of four thick (6-11 m) clastic members separated by evaporite members, and contains several major sandstone reservoirs. A study of cores and logs from the upper Queen was done to determine its depositional environments and the properties of its sandstone reservoirs. Four facies are present in the upper Queen. Facies 1 consists of planar- and wavy-laminated fine and very fine sandstones and silty sandstones deposited in fluvial sandflats and delta-plains. Facies 2 consists of wavy-laminated siltstones and haloturbated mudstones with anhydrite nodules deposited in subaqueous prodelta environments and subaerial saline mudflats. Facies 3 consists of well-sorted fine to very fine sandstones with horizontal and inclined planar laminae deposited in eolian sand sheets. Facies 4 consists of massive to laminated halite and anhydrite deposited in hypersaline playas. The vertical sequence of these facies indicates that the upper Queen was deposited during four cycles of fan-delta progradation into and retreat from coastal-plain playas during a sea level lowstand in the Midland basin. The sandstones of the fluvial sandflat and eolian sand-sheet facies constitute the reservoirs of the upper Queen in themore » fields. The average cumulative thickness of these facies is 8 m, and the fields average 16% porosity and 40-50 md of permeability. The remaining facies are all nonproductive with averagge porosities of less than 10% and average permeabilities of less than 1 md.« less