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At least 469 records · Page 26Linked to original sources
USGS2002

Outcrop-scale acoustic facies analysis and latest Quaternary development of Hueneme and Dume submarine fans, offshore California

The uppermost Quaternary deposits of the Hueneme and Dume submarine fans in the Santa Monica Basin have been investigated using a closed-spaced grid of boomer seismic-reflection profiles, which give vertical resolution of a few tens of centimetres with acoustic penetration to 50 m. Acoustic facies integrated with geometry define six architectural elements, some with discrete subelements that are of a scale that can be recognized in outcrops of ancient turbidite systems. In the Santa Monica Basin, the relationship of these elements to fan morphology, stratigraphy and sediment source is precisely known. The width of upper Hueneme fan valley has been reduced from 5 km since the last glacial maximum to 1 km at present by construction of laterally confined sandy levees within the main valley. The middle fan comprises three main subelements: thick sand deposits at the termination of the fan valley, low-gradient sandy lobes typically 5 km long and < 10 m thick, and scoured lobes formed of alternating sand and mud beds with many erosional depressions. The site of thickest lobe sediment accumulation shifts through time, with each sand bed deposited in a previous bathymetric low (i.e. compensation cycles). The lower fan and basin plain consists of sheet-like alternations of sand and mud with shallow channels and lenses. Variations in the rate of late Quaternary sea level rise initiated changes in sediment facies distribution. At lowstand, and during the approximately 11 ka stillstand in sea level, the Hueneme Fan was fed largely by hyperpycnal flow from the Santa Clara River delta, depositing high sediment waves on the right hand levee and thick sandy lobes on the middle fan. At highstand of sea level, most turbidity currents were generated by failure of silty prodelta muds. In contrast, the smaller Dume Fan was apparently always fed from littoral drift of sand through a single-canyon point source.

Sedimentology↗
USGS2001

Petrographic and geochemical evidence for the formation of primary, bacterially induced lacustrine dolomite: La Roda 'white earth' (Pliocene, Central Spain)

Upper Pliocene dolomites ('white earth') from La Roda, Spain, offer a good opportunity to evaluate the process of dolomite formation in lakes. The relatively young nature of the deposits could allow a link between dolomites precipitated in modern lake systems and those present in older lacustrine formations. The La Roda Mg-carbonates (dolomite unit) occur as a 3??5- to 4-m- thick package of poorly indurated, white, massive dolomite beds with interbedded thin deposits of porous carbonate displaying root and desiccation traces as well as local lenticular gypsum moulds. The massive dolomite beds consist mainly of loosely packed 1- to 2-??m-sized aggregates of dolomite crystals exhibiting poorly developed faces, which usually results in a subrounded morphology of the crystals. Minute rhombs of dolomite are sparse within the aggregates. Both knobbly textures and clumps of spherical bodies covering the crystal surfaces indicate that bacteria were involved in the formation of the dolomites. In addition, aggregates of euhedral dolomite crystals are usually present in some more clayey (sepiolite) interbeds. The thin porous carbonate (mostly dolomite) beds exhibit both euhedral and subrounded, bacterially induced dolomite crystals. The carbonate is mainly Ca-dolomite (51-54 mol% CaCO3), showing a low degree of ordering (degree of ordering ranges from 0??27 to 0??48). Calcite is present as a subordinate mineral in some samples. Sr, Mn and Fe contents show very low correlation coefficients with Mg/Ca ratios, whereas SiO2 and K contents are highly correlated. ??18O- and ??13C-values in dolomites range from -3??07??? to 5??40??? PDB (mean = 0??06, ?? = 1??75) and from -6??34??? to -0??39??? PDB (mean = -3??55, ?? = 1??33) respectively. Samples containing significant amounts of both dolomite and calcite do not in general show significant enrichment or depletion in 18O and 13C between the two minerals. The correlation coefficient between ??18O and ??13C for dolomite is extremely low and negative (r = -0??05), whereas it is higher and positive (r = 0??47) for calcite. The lacustrine dolomite deposit from La Roda is interpreted mainly as a result of primary precipitation of dolomite in a shallow, hydrologically closed perennial lake. The lake was supplied by highly saturated HCO3-/CO32- groundwater that leached dolomitic Mesozoic formations. Precipitation of dolomite from alkaline lake waters took place under a semi-arid to arid climate. However, according to our isotopic data, strong evaporative conditions were not required for the formation of the La Roda dolomite. A significant contribution by bacteria to the formation of the dolomites is assumed in view of both petrographic and geochemical evidence.

Sedimentology↗
USGS2003

Direct numerical simulation of bedload transport using a local, dynamic boundary condition

Temporally and spatially averaged models of bedload transport are inadequate to describe the highly variable nature of particle motion at low transport stages. The primary sources of this variability are the resisting forces to downstream motion resulting from the geometrical relation (pocket friction angle) of a bed grain to the grains that it rests upon, variability of the near-bed turbulent velocity field and the local modification of this velocity field by upstream, protruding grains. A model of bedload transport is presented that captures these sources of variability by directly integrating the equations of motion of each particle of a simulated mixed grain-size sediment bed. Experimental data from the velocity field downstream and below the tops of upstream, protruding grains are presented. From these data, an empirical relation for the velocity modification resulting from upstream grains is provided to the bedload model. The temporal variability of near-bed turbulence is provided by a measured near-bed time series of velocity over a gravel bed. The distribution of pocket friction angles results as a consequence of directly calculating the initiation and cessation of motion of each particle as a result of the combination of fluid forcing and interaction with other particles. Calculations of bedload flux in a uniform boundary and simulated pocket friction angles agree favourably with previous studies.

Sedimentology↗
USGS2006

Sediments from bay St. George, Newfoundland

Sediments cored to a depth of about 1 m in Bay St . George , Newfoundland , were examined for grain‐size distribution and minerals. The sediments are light brown silty clays, the principal minerals of which are chlorite and muscovite mica. The scarce sand consists of fresh detrital grains of blue‐green amphibole, biotite, epidote, zoisite, magnetite, garnet, hypersthene, apatite, chlorite, and scarce zircon. Quartz, plagioclase feldspar, and muscovite are abundant. These minerals are all present in the rocks of the adjacent land areas and have been deposited in the sediments with little alteration or change due to weathering. The clay minerals of the sediments are predominantly muscovite and chlorite with a slight admixture of vermiculite and montmorillonite. There is very little mixed‐layering of these minerals. Montmorillonite may be due to diagenetic changes after deposition of these clays in the marine environment of the bay .

Newfoundland↗
USGS2006

Factors influencing the pore volume of fine-grained sediments under low-to-moderate overburden loads

An anomalous increase of pore volume with increasing depth in the range 0—1,900 ft. occurs in fine‐grained sediments along the east side of the San Joaquin Valley of Cali‐ fornia. Several possible causes for the anomaly were inferred from a literature search and from study of the core samples. Statistical analyses of the core sample data suggest the principle causes to be variations in particle size, the diatom‐skeleton content, and the type of exchangeable cation adsorbed by the clay‐mineral constituents of the sediments .

California↗
USGS2006

Pebble segregation and bed lenticularity in wave-worked versus alluvial gravel

Two aspects of bedding may permit visual discrimination between wave-worked and alluvial gravel. Pebbles in gravel worked by waves tend to be better segregated into discrete beds than those in alluvial deposits, and bedding in wave-worked gravel tends to be more laterally regular, or less lenticular, than that in stream gravel. Pebble segregation and lenticularity were quantitatively analysed in twenty-five different marine and fluvial deposits of Quaternary or Tertiary age in southwestern Oregon in order to evaluate their use as environmental indicators. The results indicate that these two factors combined provide a visual criterion for discriminating between many gravels of wave-worked and alluvial origin.

Oregon↗
USGS2006

Distributary channels, sand lobes, and mesotopography of Navy Submarine Fan, California Borderland, with applications to ancient fan sediments

The deep-tow instrument package of Scripps Institution of Oceanography provides a unique opportunity to delineate small-scale features of a size comparable to those features usually described from ancient deep-sea fan deposits. On Navy Fan, the deep-tow side-scanning sonar readily detected steep channel walls and steps and terraces within channels. The most striking features observed in side-scan are large crescentic depressions commonly occurring in groups. These appear to be large scours or flutes carved by turbidity currents. Four distinct acoustic facies were mapped on the basis of qualitative assessment of reflectivity of 4 kHz reflection profiles. There is a distinct increase in depth of acoustic penetration, number of sub-bottom reflectors, and reflector continuity from the upper fan-valley to the lower fan. These changes are accompanied by a decrease in surface relief. Navy Fan is made up of three active sectors. The active upper fan is dominated by a single channel with prominent levees that decrease in height downstream. The active mid-fan region or suprafan is where sand is deposited. Well defined distributary channels with steps, terraces, and other mesotopography terminate in depositional lobes. Interchannel areas are rough, containing giant scours as well as other relief. The active lower fan accumulates mud and silt and is without resolvable surface morphology. The morphological features seen on Navy Fan other than levees, interchannel areas, and lobes are principally erosional. The distributary channels are up to 0.5 km wide and 5–15 m deep. Such features, because of their large size and low relief, are rarely completely exposed or easily detectable in ancient rock sequences. Some flute-shaped scours are larger than channels in cross section but many are 5-30 m across and 1-2 m deep. If observed in ancient rocks transverse to palaeo-current direction, they would perhaps be indistinguishable from channels. Surface sediment distribution combined with fan morphology can be used to relate modern sediments to facies models for ancient fan sediments. Gravel and sand occur in the upper valley, massive sand beds in the mid-fan distributary channels, classical complete Bouma sequences on depositional lobes, incomplete Bouma sequences (lacking division a) on the lower mid-fan, and Bouma sequence with lenticular shape or other limited extent on mid-fan interchannel areas and on levees.

California↗
USGS2006

Bedform climbing in theory and nature

Where bedforms migrate during deposition, they move upward (climb) with respect to the generalized sediment surface. Sediment deposited on each lee slope and not eroded during the passage of a following trough is left behind as a cross-stratified bed. Because sediment is thus transferred from bedforms to underlying strata, bedforms must decrease in cross-sectional area or in number, or both, unless sediment lost from bedforms during deposition is replaced with sediment transported from outside the depositional area. Where sediment is transported solely by downcurrent migration of two-dimensional bedforms, the mean thickness of cross-stratified beds is equal to the decrease in bedform cross-sectional area divided by the migration distance over which that size decrease occurs; where bedforms migrate more than one spacing while depositing cross-strata, bed thickness is only a fraction of bedform height. Equations that describe this depositional process explain the downcurrent decrease in size of tidal sand waves in St Andrew Bay, Florida, and the downwind decrease in size of transverse aeolian dunes on the Oregon coast. Using the same concepts, dunes that deposited the Navajo, De Chelly, and Entrada Sandstones are calculated to have had mean heights between several tens and several hundreds of metres.

Sedimentology↗
USGS2006

Bedform distribution and inferred sand transport on Georges Bank, United States Atlantic continental shelf.

Four bedform provinces have been identified using sidescan-sonar and echo-sounding techniques: large sand waves superimposed on sand ridges, small sand waves, megaripples, and featureless seafloor. Sand-wave asymmetry and surface-sediment texture have been used to infer bedload transport paths and although the asymmetry of megaripples could not be determined, the occurrence of megaripples between the small sand-wave province and areas of featureless seafloor suggests a decreasing effectiveness of sand transport away from the bank crest. This sand dispersal pattern is further supported by the surface sediments which become progressively finer to the N and SW away from the crest of Georges Bank.-from Author

Massachusetts↗
USGS2006

Element mobility during alteration of silicic ash to kaolinite - A study of tonstein

A laterally persistent kaolinitic mudstone parting (tonstein) occurring in a Wyoming coal bed of Eocene age was sampled to estimate the compositional contrast with a probable silicic ash precursor, and to determine the compositional influence of leached ash on immediately adjacent coal. With the exception of Al, and possibly Ti, the tonstein is highly to moderately leached of major elements, relative to a range of compositions estimated for silicic ash of the region. In agreement with the behaviour of geochemically similar major elements, alkali trace elements (Rb, Cs) are highly leached, transition-series metals moderately leached, and Ga is residual. Additional immobile trace elements are Zr and Hf but some other trace elements that are considered relatively immobile during low-temperature alteration (Th, Ta, Nb, REE, Y) were apparently leached by the low-Eh, low-pH, organic-rich pore fluids of the coal-forming swamp. The migrational range of many of the leached elements is highly limited by the intimate association of tonstein with enclosing organic matter. Mixtures of coal + minor tonstein that occur within 20 mm of contacts are consistently enriched in some elements relative to either tonstein or nearby coal (U, Cu), or relative to calculated mixtures of tonstein and coal in their measured proportions (Th, Y, REE, Pb, Ba, V, Ti). Direct observations by fission-track radiography and electron microprobe indicate a preference of U and Fe for the organic component of mixtures. Metal fixation is probably by adsorption on insoluble organic matter (humic acids), or by precipitation as minor sulphides in these low-sulphur coals.

Wyoming↗
USGS2006

A kinematic model for the structure of lee-side (dune-like) deposits

A kinematic model for the structure of the lee-side deposit of a dune-like bedform, Gilbert-type delta, or similar step is developed, based on the assumptions that initial deposition is entirely by grainfall, that the rate of deposition decreases as a power function of distance downflow from the brink of the slipface, and that the resulting steepening of the slipface is periodically interrupted by avalanching. The parameters used in the model are: (1) the deposition rate at a given distance from the brink, (2) the exponent in the equation relating the deposition rate to distance from the brink, (3) the bedform migration rate, (4) the bedform height, (5) the avalanche speed, (6) the angle of initial yield, and (7) the residual angle after avalanching. From these parameters can be calculated structural characteristics such as the proportions of bottomset and foreset deposits, the proportions of avalanche and grainfall deposits in the foreset deposit, and the spacing of avalanche-grainfall couplets. The model correctly predicts the trends of changing avalanche activity and changing structural character with changes in flow character, grain size, and bedform height in both air and water. Moreover, the model correctly predicts certain consistent structural differences between aeolian and subaqueous lee-side deposits. Quantitative evaluation of the model requires more accurate data on the values of the input parameters than are presently available.

Sedimentology↗
USGS2006

Cycle of earthquake-induced aggradation and related tidal channel shifting, upper Turnagain Arm, Alaska, USA.

Aggradation of intertidal sediment in upper Turnagain Arm, a macrotidal estuary located in southern Alaska near Anchorage, began because of subsidence caused by sediment consolidation and tectonic lowering of the land during the earthquake of March 27, 1964. At the head of the Arm, decrease in percent sand of surface sediments with time and return of rodents, pulmonate gastropods, trees, bushes, and grasses to the intertidal region, indicate that the rate of sediment deposition has slowed and the Upper Turnagain Arm area has returned to a state of preearthquake equilibrium only 17 years after the event. Comparison of channel locations before and after the earthquake indicates the possibility of regional postseismic tilting of the estuary.

Alaska↗
USGS2006

Theoretical and measured aeolian sand transport on a barrier island, Louisiana, USA

Over the past 100 years, the Isles Dernieres, a low lying barrier island chain along the coast of central Louisiana, U sa , has undergone more than 1 km of northward beach face retreat with the loss of 70% of its surface area. The erosion results from a long term relative sea level rise coupled with day to day wind and wave action that ultimately favours erosion over deposition. At a site in the central Isles Dernieres, 8 days of wind and beach profile measurements during the passage of one winter cold front documented aeolian erosion and deposition patterns under both onshore and offshore winds. For offshore winds, the theoretical erosion rate, based on wind shear velocity, closely matched the measured erosion rate; for onshore winds, the theoretical rate matched the measured rate only after being corrected by a factor that accounted for beach face morphology. In late February 1989, a strong cold front moved into coastal Louisiana. That cold front stalled over the Gulf of Mexico, resulting in 4 days of strong northerly winds at a study site on the Isles Dernieres. During those 4 days, the wind moved sand from the backshore to the upper beach face. When the cold front finally moved out of the area, the wind shifted to the south and decreased in strength. The onshore wind then restored some of the upper beach face sand to the backshore while increased wave activity moved the rest into the nearshore. The theoretical estimate of 1·28 m 3 m −1 for the rate of sand transport by the northerly wind compares well with the measured backshore erosion rate of 1·26 m 3 m −1 , which was determined by comparing beach profiles from the start and end of the period of northerly winds. The theoretical estimate of 0·04 m 3 m −1 for the rate of sand transport by the southerly wind, however, is notably less than the measured rate of 0·45 m 3 m −1 . The large discrepancy between the two rates can be explained by a difference in the shear velocity of the wind between the beach face, where the erosion occurred, and the backshore, where the wind stress was measured. Using an empirical relationship for the wind shear drag coefficient as a function of coastal environment, the theoretical estimate for the rate of sand transport by the southerly wind becomes 0·44 m 3 m −1 .

Louisiana↗
USGS2006

Groundwater dolocretes from the Upper Triassic of the Paris Basin, France: A case study of an arid, continental diagenetic facies

Thick dolomite-cemented horizons (dolocretes) occur within a fluvial sandstone-mudstone sequence of Late Triassic age in the western part of the Paris Basin, France. Two types of dolomites can be distinguished: (a) nodular dolomitic beds less than a few metres thick, which formed within mottled overbank siltstones and mudstones; and (b) massive dolomite up to 16 m thick, which occurs in coarse grained channel sandstones and conglomerates. The majority of the dolomite consists of a finely crystalline groundmass of dolomicrospar and, less commonly, dolomicrite. Glaebules, irregular spar-filled cracks, spheroidal dolomite, silicification and vuggy porosity are locally abundant in the massive dolomite. In contrast, biologically induced micromorphological features such as rhizocretions and alveolar-septal fabrics were observed in the thin, nodular dolomite beds. The dolomite is near stoichiometric, well ordered and non-ferroan. 18O values range from −7·7 to −0·4%o PDB and 18O values range from −5·1 to + 1·8%0 PDB and no obvious difference in the stable isotopic composition between both types of dolomites was observed. Sr isotope ratios range from 0·7101 to 0·7126 and are invariably higher than the contemporary Triassic sea water. A vadose—pedogenic origin for the thin dolocrete layers is indicated by the occurrence of rhizocretions and other biological structures. Several features, however, argue against a pedogenic origin for the massive carbonates, most notably the absence of biologically induced structures, the occurrence in coarse grained channel (and not overbank) deposits, and the great thickness. These units are thus interpreted as groundwater in origin. Phreatic calcretes of Quaternary age, widespread in inland Australia, are regarded as a modern analogue for the Triassic Paris Basin dolocretes. Petrographic observations argue in favour of primary (proto)dolomite precipitation, although early diagenetic replacement of calcite by (proto)dolomite cannot be ruled out. Strontium and carbon isotope data of early diagenetic dolocrete cements and oxygen isotope data of early diagenetic silica indicate an entirely non-marine, continental origin for the groundwaters. The poorly ordered and non-stoichiometric protodolomite probably underwent stabilization upon further burial resulting in a near-stoichiometric, well ordered dolomite that clearly lacks evidence for pervasive recrystallization.

Paris↗
USGS2006

Controls on the accumulation of coal and on the development of anastomosed fluvial systems in the Cretaceous Dakota Formation of southern Utah

Alluvial strata of the Cretaceous Dakota Formation of southern Utah are part of a transgressive systems tract associated with a foreland basin developed adjacent to the Sevier orogenic belt. These strata contain valley fill deposits, anastomosed channel systems and widespread coals. The coals constitute a relatively minor part of the Dakota Formation in terms of sediment volume, but may represent a substantial amount of the time represented by the formation. The coals are separated by clastic units up to 20 m thick. The stratigraphically lowest clastic unit of the Dakota Formation lies above an unconformity cut into Jurassic rocks. Incised valleys associated with the unconformity are up to 12 m deep. Two discrete episodes of valley fill sedimentation are recognized, including a lower sandstone unit with conglomerate layers, and an upper, discontinuous, coal-bearing unit. After the valleys filled, the area became one of low relief where extensive mires formed. Peat accumulation was interrupted at least three times by deposition of clastic sediment derived from the west. The clastic units consist of sandstone, mudstone or heterolithic ribbon bodies, stacked tabular sandstones, and laminated mudstones, and contain minor coal beds less than 0·35 m thick. Ribbon bodies are 1–9 m thick and 15–160 m wide, have pronounced basal scours, and are filled with both lateral and vertical accretion deposits. An anastomosed channel complex is suggested by the large number of coeval channels of varying dimensions, the variation in the structure and grain size of channel fills, and the presence of abundant tabular sandstones interpreted as crevasse splays. Although some sandstone bodies have well developed lateral accretion surfaces, the overall ribbon geometry indicates that mature meandering streams were not well developed. This is in contrast to modern anastomosed systems, which are commonly thought to be a transitional morphology caused by avulsion of a meander belt to a new position on its floodplain. Rather than being a transitional channel pattern related to river avulsion, the anastomosed channels of the Dakota Formation may have formed part of a large inland delta that episodically invaded widespread mires. The mires developed during periods when clastic influx was reduced either by high rates of subsidence close to the thrust belt or by deflection of rivers by emergent thrusts.

Utah↗
USGS2006

Dolomitization of Quaternary reef limestones, Aitutaki, Cook Islands

Six holes were drilled to depths of 30–69 m in the shallow lagoon of Aitutaki in the southern Cook Islands. One hole encountered pervasively dolomitized reef limestones at 36 m subbottom depth, which extended to the base of the drilled section at 69·3 m. This hole was drilled near the inner edge of the present barrier reef flat on the flank of a seismically defined subsurface ridge. Both the morphology and biofacies indicate that this ridge may represent an outer reef crest. Mineralogy, porosity and cementation change in concert downhole through three zones. Zone 1, 0–9 m, is composed of primary skeletal aragonite and calcite with minor void-filling aragonite and magnesian calcite cement of marine phreatic origin. Zone 2, 9–36 m, is composed of replacement calcite and calcite cement infilling intergranular, intragranular, mouldic and vuggy porosity. Stable isotopes (mean δ 18 O=—5·4‰ PDB for carbonate; δD =—50‰ SMOW for fluid inclusions) support the petrographic evidence indicating that sparry calcite cements formed in predominantly freshwater. Carbon isotope values of —4·0 to —11·0‰ for calcite indicate that organic matter and seawater were the sources of carbon. Zone 3, 36–69·3 m, is composed of replacement dolostone, consisting of protodolomite with, on average, 7 mol% excess CaCO 3 and broad and weak ordering X-ray reflections at 2·41 and 2·54 A. The fine-scale replacement of skeletal grains and freshwater void-filling cements by dolomite did not significantly reduce porosity. Stable isotopes (mean δ 18 O=+2·6‰ 0 PDB for dolomite; maximum δD =—27‰ SMOW for fluid inclusions) and chemical composition indicate that the dolomite probably formed from seawater, although formation in the lower part of a mixed freshwater-seawater zone, with up to 40% freshwater contribution, cannot be completely ruled out. The carbon (δ 13 C=2·7‰) and magnesium were derived from seawater. Low-temperature hydrothermal iron hydroxides and associated transition metals occur in void space in several narrow stratigraphic intervals in the limestone section that was replaced by dolomite. The entire section of dolomite is also enriched in these transition metals. The metals dispersed throughout the dolostone section were introduced at the time of dolomitization by a different and later episode of hydrothermal circulation than the one(s) that produced the localized deposits near the base of the section. The primary reef framework is considered to have been deposited during several highstands of sea level. Following partial to local recrystallization of the limestone, a single episode of dolomitization occurred. Both tidal and thermal pumping drove large quantities of seawater through the porous rocks and perhaps maintained a wide mixing zone. However, the isotopic, geochemical and petrographic data do not clearly indicate the extent of seawater mixing.

Sedimentology↗
USGS2006

Tuffaceous ephemeral lake deposits on an alluvial plain, Middle Tertiary of central California

The Oligocene and Miocene Valley Springs Formation is one element in the middle to late Cenozoic alluvial fill of the Central Valley of California and represents a large fluvial depositional system that extended westward from sediment-filled palaeovalleys in the high Sierra Nevada to a piedmont alluvial plain under the present Central Valley. The Valley Springs Formation consists largely of tuffaceous mudrock, tuffaceous sandstone, polymict conglomerate and rhyodacitic tuff. The most prominent lithofacies in the formation is yellowish grey or greyish yellow, tuffaceous mudstone and claystone characterized by crude, commonly wavy layering or bedding, an irregular fracture, and common clay-lined partings, fissures, and small branching tubules. Petrographic and X-ray diffraction analyses of the composition, texture, fabric, mineralogy and organic content of an 8 m thick section of the tuffaceous mudrock lithofacies have provided evidence for an origin quite different from the earlier interpretations of this lithofacies as altered tuffs. Numerous rounded mudstone or claystone clasts in a detrital mud matrix, together with abundant freshwater microfossils (megaspores, chrysophyte cysts, diatoms and sponge spicules), indicates that the lithofacies is a mostly epiclastic sedimentary deposit. Root traces, fissuring, orientated clay skins lining pores and secondary surfaces, and mineralogical data are evidence of periodic exposure and desiccation. The tuffaceous mudrock lithofacies of the Valley Springs Formation, interpreted in the context of the middle to lower regions of a piedmont alluvial system, most probably represents a complex of ephemeral lake and marsh environments on a low gradient alluvial plain. The inferred abundance of shallow lakes, ponds and marshes implies a climate that was wetter than the semi-arid climate of the region today.

California↗