Caliche Horizons Research Articles

Geophysical Reconnaissance for Siting Dryland Critical-Zone Monitoring Experiments in Southern New Mexico, USA

A dryland critical-zone observatory is planned on a piedmont setting of the Jornada Experimental Range northeast of Las Cruces, New Mexico, near a ∼10-yr eddy flux covariance tower and vegetation monitoring experiment and a 2-yr old water-uptake rainfall infiltration experiment. We carried out several geophysical surveys to help select sites that minimize geologic complexity for follow up hydrologic and biogeochemical studies that will be conducted by other researchers. First, we conducted a review of regional topography, gravity, and magnetics prior to a site visit and then collected reconnaissance magnetic and electromagnetic data to aid in planning more detailed geophysical site characterization surveys. Our initial topographic analysis using 1/3 arc-second digital elevation models (DEMs) showed the proposed area had an out-of-equilibrium curvature pointing to active erosion and possible faulting. Short-wavelength step-like topographic anomalies in the DEMs were confirmed in LiDAR elevations, and are consistent with erosionally resistant soil horizons in the old alluvial fan deposits. Comparison of 2-D density and susceptibility models based on nearby (3-8 km) hydrostratigraphic studies established that the observed regional gravity and magnetic anomalies were larger than could be modeled with the 2-D structural constraints, and established the station spacing our reconnaissance surveys would require to sample shallow soil variations. Our first site visit confirmed the general fault locations and we identified three outcropping caliche horizons distinct to alluvial channel, proximal splay and distal splay deposits in a several hundred-meter traverse that are consistent with the short-wavelength topographic features. In order to plan additional seismic, radar, gravity, and electrical surveys within a region of such high potential variability, we collected magnetic field and magnetic susceptibility measurements along two profiles at 10-50 m spacing. We found anomalies consistent with two projected faults, as well as other bedrock structures, a result significantly more complex than prior regional hydrostratigraphic mapping had suggested. We also conducted a more limited 0.5 km long ground conductivity survey with 5 m spacing that traversed the rainfall infiltration experiment site and found anomalies that aligned with one of the projected faults. The results showed deep (>6 m) 50 mS/m (milliSiemens/meter) values, indicating moister soils, on the footwall side, dropping to 20 mS/m after crossing the fault, consistent with previous observations that normal faults in the Rio Grande Valley asymmetrically influence fluid flow.

Brackish water algal reefs – facies analysis as a tool to identify palaeoenvironmental variations in Miocene deposits (Mainz‐Weisenau, Germany)

Brackish‐water carbonates are far less studied than their marine or limnic counterparts. However, their association with few, specialized species enables the documentation of fine‐scale changes in the depositional environment. The Cenozoic Mainz Basin (Germany) was only sporadically connected to the North Sea and the Paratethys, exposing several transitions from marine to fresh water influence. Focusing on one outcrop of the Rüssingen Formation of Mainz‐Weisenau (Aquitanian, Miocene), we present a detailed analysis of the faunal and sedimentological responses to changing salinities and water depth, including algal reef growth and facies development. The deposits include allochthonous limestones surrounding an autochthonous reef complex and several smaller reef patches. The allochthonous facies is dominated by the gastropod Hydrobia inflata, and the reef facies is mainly made up by the green alga Cladophorites sp. The algal thalli are overgrown by cryptocrystalline, organic precipitations, and laminated, chemical precipitations. Locally, quiver‐shaped structures of Trichoptera sp. protective cases occur. The depositional setting was a shallow, low energy, and brackish environment supersaturated by carbonate. We could not confirm a general trend of reducing salinities as reported for the Rüssingen Formation. Our results question previously reported episodic desiccation events, because apparent caliche horizons actually represent thin beds of increased Cladophorites growth. Set‐up, distribution of the reef facies, and reef debris indicate short‐time variations of temperature, salinity and water depth. We conclude that these variations are based on the geographic position at the edge of an algal reef barrier, separating the Mainz Basin from the Rhine Rift Valley.

Assessing shallow rock conditions over a brine cavern using seismic methods

The 16 July 2008 collapse of a brine well southeast of Artesia, New Mexico, has led to concern about the stability of a brine well facility located in Carlsbad, New Mexico. Characterization, monitoring, and mitigation studies were initiated in 2009, and emergency response procedures are in place should it collapse into a large sinkhole that could threaten nearby structures, two highways and a major irrigation canal. Results of precision survey indicate that differential subsidence is occurring across the facility and is greatest in the vicinity of the brine well. It is anticipated that ground movement-induced fracturing or crushing of competent, shallow caliche horizons could be a precursor to collapse of the material over the brine well cavern into a large sinkhole. A 24-channel seismograph was used to obtain compression wave (p-wave) seismic refraction and refraction microtremor surface wave (s-wave) data at selected locations in the vicinity of the brine well to characterize the condition of the shallow caliche horizons. When intact and unfractured, these caliche horizons are anticipated to be high strength with high seismic p-wave velocities of at least 3,000 m/s. Fracturing or crushing of these materials dramatically reduces the material mass strength and seismic velocities. Shallow seismic profiles totaling about 730 m were completed around the facility. High seismic velocities were interpreted in the caliche adjacent to the highways abutting the facility, while low seismic velocities were interpreted adjacent to a major irrigation canal where measured subsidence was greatest.

What are the Spratly Islands?

Seismic records, combined with dredged samples and a core, indicate that the Spratly Islands of the Dangerous Ground Province are constructed of presently active carbonate build-ups, known to extend back continuously at least to the Pleistocene and presumed to have initiated in the Miocene, most likely upon the crests of sea-floor cuestas that trend north-east–south-west parallel to the sea-floor spreading magnetic anomalies of the contiguous abyssal plain of the southern part of the South China Sea. The cuestas range from spectacular to subdued, constructed of Triassic and Cretaceous strata and no older rocks have been identified from dredges. The cuesta axes plunge towards the south-west away from the islands, suggesting that the reefs began colonising their more elevated parts, but the timing is uncertain. The highest seismically recorded cuesta crest is in 440 m of water and the islands and reefs are generally closely surrounded by water deeper than 1500 m. Since the so-called Mid-Miocene Unconformity (MMU), the region has been undergoing post-rift thermal subsidence. However, the nearby seismic lines show no evidence of drowned carbonate reefs. It is suggested that the coral-algal reefs colonised the crests of the most elevated cuestas that have maintained stability as shown by the 165 m core of one reef indicating periodic exposure with caliche horizons. Deepening water has protected the build-ups from extinction by post-rift draping strata in contrast to the Central Luconia Province, and the build-ups have been able to keep up with regional thermal subsidence. The dredged Mesozoic strata indicate that the Dangerous Ground is not exotic and should be interpreted as an integral part of the pre-rift Sundaland continent that included South China, Vietnam, Peninsular Malaysia, western Sarawak and possibly part of Sabah. Igneous and metamorphic samples have been dredged. Although individual spot K/Ar dates cannot be accepted at face value, such rocks can also be interpreted as an integral part of Sundaland. Post-MMU dredged samples are predominantly deep-water calcareous mudstones typified by the draping strata of the Ocean Drilling Program (ODP) Site 1143 cored from Recent to Late Miocene.

Biogenic caliches in Texas: The role of organisms and effect of climate

Biogenic constituents are ubiquitous and abundant in the caliches of Texas. Investigation of 51 caliche profiles on various host strata (alluvium, limestone, igneous rocks, etc.) across approximately 900 km of Texas from subhumid east to arid west has shown that 43 of these profiles exhibit prominent biogenic constituents. These profiles exhibit significant differences in thickness (varying from centimeters to meters) and maturity (varying from I to VI). All of the different caliche facies are composed of low-Mg calcite. Biogenic features generally occur in the upper part of the profiles, including the uppermost portion of massive caliche horizons, platy horizons, laminar crusts, and pisoids. The main biogenic caliche facies include rhizoliths (calcified root structures), stromatolite-like laminar crusts, and coated grains. Compared to the abiogenic massive micritic to microsparitic calcite groundmass, biogenic constituents are morphologically distinct. These biogenic constituents are composed of several microscopic mineral components, including calcified filaments, needle fiber calcite (e.g., single crystalline needles and needle pairs, triangular crystals, and polycrystalline chains of rhombohedrons), spherulites, micro-rods, and nano-spheres. A large number of calcified root cellular structures and micro-organisms, e.g., fungal filaments, actinomycetes, and rod-like bacteria, are also present. Plant roots as well as soil biota produce distinctive structures and also enhance lithification by inducing calcite precipitation in the caliches, i.e., biologically controlled or influenced processes. Host strata did not significantly influence the abundance nor type of biogenic features in the caliches. In contrast, climate had an evident effect on the development of biogenic constituents in these caliches in terms of the amount as well as type. The thickness of laminar crusts and grain coatings and the abundance of biotic constituents within those facies decrease as the climate shifts from subhumid and subarid, in southeast and central areas, to subarid and arid, in west and northwest Texas. In addition, root structures and micro-rods diminish significantly from the subhumid east to the arid west.

Paleoenvironment and taphonomy of the dinosaur-bearing Ischigualasto Formation (Upper Triassic, Argentina)

The oldest-known dinosaurs (Herrerasaurus, Pisanosaurus) occur within the Ischigualasto Formation. Recent work in the formation has brought to light significant new material, including the complete skeleton of a new primitive dinosaur. We sketch below the paleoenvironment and faunal succession during the range of these early dinosaurs, and review some of the taphonomic factors that shaped their fossil record.The Ischigualasto Formation (Carnian?) is included within the Agua de la Peña Group, a series of continental Triassic deposits exposed in the Ischigualasto-Ville Union Basin of northwest Argentina. Ischigualasto sediments rest unconformably upon the carbonaceous fluvial/lacustrine Los Rastros Formation; this contact is characterized locally by marked angular discordance. The upper contact is gradational into red-beds of the Los Colorados Formation. Medium- to coarse-grained conglomeratic sandstones, siltstones, and silty mudstones dominate the section. Sand bodies are characterized by medium- to large-scale trough cross-stratification and broad lenticular/narrow sheet geometries, and are interpreted as deposits of shallow, low-sinuosity streams. Siltstones and mudstones show pervasive evidence of soil development, including root traces, nodular caliche horizons, and pedogenic slickensides. Deposits attributable to lacustrine/paludal sedimentation are scarce, and freshwater vertebrates and invertebrates are extremely rare. These data suggest an upland depositional setting on a low-relief alluvial plain with seasonal climate.The Ischigualasto vertebrate fauna includes archosaurs, rhynchosaurs, traversodontid and carnivorous cynodonts, and temnospondyl amphibians. Rhynchosaurs dominate (relative specimen abundance) in the lower half of the section, but are absent from the upper half. Traversodontid cynodonts occur throughout the formation, but are much more abundant up-section. Archosaurs, carnivorous cynodonts, and particularly temnospondyls are rare throughout, with dinosaurs limited to the lower half. No major stratigraphic or sedimentologic changes occur up-section, and there is no evidence for significant shifts in physical or chemical taphonomic processes. Thus, trends in relative taxon abundance likely record a true biotic signal (e.g., local extinction, immigration) rather than a taphonomically-driven preservational bias.Fossils are preserved as isolated carcasses or disarticulated elements, most often in fine-grained overbank facies. Bone beds and microsites are conspicuously absent. Temnospondyl remains were found within a local carbonaceous lens developed upon a sand body, suggesting autochthonous burial in an abandoned-channel setting. Isolated skulls, particularly those of the traversodontid Exaeretodon, are extremely common. Fifteen isolated crania of this cynodont were mapped in a single stratum with limited areal exposure. Abundant preservation of isolated therapsid crania has also been reported in the Beaufort Series (Permo-Triassic) of the Karoo Basin, South Africa (Smith, 1980). Post-disarticulation hydrodynamic sorting (enhanced by scavenging?) of an areally dispersed mass-mortality assemblage may explain this unusual occurrence.

Taphonomy of Three Dinosaur Bone Beds in the Upper Cretaceous Two Medicine Formation of Northwestern Montana: Evidence for Drought-Related Mortality

Two bone beds, Canyon Bone Bed and Dino Ridge Quarry, have yielded the near-exclusive remains of a new species of Styracosaurus (Family Ceratopsidae); the third bone bed, Westside Quarry, is dominated by a new species of Prosaurolophus (Family Hadrosauridae). Evidence supporting a drought hypothesis includes: 1) a seasonal, semiarid paleoclimate, 2) associated caliche horizons, 3) aqueous depositional settings, 4) apparent age distributions characteristic of modern drought mortality (CBB and DRQ), and 5) the intraformational recurrence of low-diversity bone beds. Several alternative scenarios were considered, but drought proved most reasonable in light of the enhanced probability of preserving drought assemblages, and the species-selective and recurrent nature of modern drought mortality

Upper Pennsylvanian Paleosol in Stranger Shale and Underlying Iatan Limestone, Southwestern Iowa

ABSTRACT Cores of the Iatan Limestone and overlying Stranger Shale in southwestern Iowa display characteristics of paleosol development other than the weathered-clay or caliche horizons or underclay type of paleosol profiles from the Upper Pennsylvanian in the Midcontinent described by other authors. Paleosol features in the Iatan-Stranger sequence include clay-filled fractures and solution channels (cryptokarst) in the top of the Iatan Limestone which display some degree of secondary clay orientation along the edges (neostrians). The clay-filled fractures widen upward and grade into the gray to red mottled, non-calcareous, blocky mudstone of the Stranger Shale, which is characterized by an absence of fauna and presence of slickensides, clay coatings on grains (argillans), quartz silt-filled f actures (skeletans), and possible former rooted zones. The mudstone matrix locally displays the sepic plasmic fabrics that are characteristic of soil development. Pedogenesis in this unit resulted from subaerial exposure during retreat of the sea following the transgressive-regressive marine cycle that deposited the Iatan Limestone. Solution-channelling of the top of the Iatan and leaching of clays within its fractures indicates initial exposure to a humid climate with strong meteoric flux. The Stranger paleosol, however, displays features of Entisol, Inceptisol, or Vertisol development in terrestrial deposits under a somewhat drier climate following the partial solution of the upper Iatan. These features include a thick (1 to 2.4 m) paleosol profile with weak morphologic expression which contains crystalline illitic clays and oxidized iron minerals. Lack of fluvial-alluvial features in the thick, monotonous quartz-silty mudstone sequence of th lower Stranger Shale suggests that fine aeolian detrital influx (desert loess) may have contributed much of the material to this unit. The lack of caliche or weathered clays in the Stranger mudstone indicates the importance of using multiple lines of evidence for recognizing paleosols.

Missourian (early late Pennsylvanian) climate in Midcontinent North America

The abrupt decrease in mineable coals from Desmoinesian to Missourian rocks in Midcontinent North America has been related by several lines of evidence to the probability that Missourian climate became at least seasonally drier than Desmoinesian climate. This represents a transition from the equatorial Desmoinesian rainforest climate to the arid Permian climate, as North America moved northward from the equator. This change is reflected in the progression of evaporites from western Colorado in the Desmoinesian to Kansas in the Permian. Direct climatic evidence from soils in two Missourian shales deposited at low stands of sea-level includes caliche horizons, incompletely leached mixed-layer clays (in contrast to intensely leached Desmoinesian kaolinite), and soil profiles resembling Vertisols, which develop under semi-arid conditions. Less direct climatic evidence includes the greater proportion of marine limestone deposited at intermediate sea-level stands in the Missourian than in the Desmoinesian part of the sequence, in conjunction with the greater abundance and thickness of oolite and shoreline facies in Missourian limestones than in Desmoinesian counterparts. This probably reflects increasing dryness of the climate, which would have led to decreased detrital influx and increased salinity as rainfall and runoff diminished. Indirect climatic evidence in offshore black phosphatic shales deposited at highest sea-level stands involves possible seasonality in organic and phosphatic laminations, related to periodicity of upwelling in the tropical trade-wind belt under the strong monsoonal influence of Pangaea. Midcontinent Missourian climates probably ranged from tropical monsoon to tropical savanna or hot steppe, all having wet-dry seasonality. The coincident extinctions of conodont, brachiopod, and other marine genera as well as the loss of swamp lycopods at the end of the Desmoinesian suggest the possibility that a greater than usual drop in sea level affected both realms at this time, while the climate was becoming drier.

Environmental Significance of Pisoliths, Mississippian Madison Formation of the Williston Basin, Bottineau, Renville, and McHenry Counties, North Dakota: ABSTRACT

Pisoliths from the Mississippian Madison Formation of North Dakota originated in an agitated, shallow-water marine environment. Evidence for this interpretation includes: (1) association of pisoliths with ooliths and marine fossils; (2) well-rounded clastic pisolith morphology; (3) vertical facies succession in core from open-marine facies through the pisolith-oolith facies band into lagoonal and intertidal facies featuring fenestral fabrics and possible tepee structures; (4) upward-shallowing cycles with reverse grading, submarine hardgrounds, and desiccation features. Madison pisolites (pisolith-rich carbonate) differ markedly from those set forth by Dunham as indicators of vadose, soil conditions: (1) they lack fitted polygonal structure; (2) they have large amounts of primary pore space or cemented pore space; (3) they contain no peds (soil components); (4) no evidence of rooting is visible; (5) no classic caliche horizons are present; (6) only minor occurrence of vadose cements postdate pisolith initiation, development, and deposition; (7) no evidence for replacement of parent rock was observed. Similar evidence was used by Esteban, and Esteban and Pray, to show that the type Capitan Formation pisoliths were not formed by soil-forming processes. Pisoliths and pisolites should be viewed in relation to other carbonate allochems, sedimentary structures, and diagenetic fabrics to determine their origin and environmental significance. End_of_Article - Last_Page 507------------

Evolution of a Fluvial Clastic Wedge, Abo Formation (Wolfcampian), Sacramento Mountains, New Mexico: ABSTRACT

Outcrops of the Abo Formation (Wolfcampian) in the northern and central Sacramento Mountains of south-central New Mexico record the evolution of fluvial systems within a major clastic wedge. This wedge was derived from the Pedernal uplift and prograded westward into the Orogrande basin. Regionally, the Abo Formation thickens from 60 m (197 ft) in the central Sacramentos, to over 545 m (1,788 ft) in the northern portion of the range, a distance of approximately 29 km (18 mi). Superposed on the regional pattern are local thickness variations of up to 91 m (300 ft). These occur within distances as small as 0.4 km (0.25 mi), and represent local tectonic and paleotopographic controls on sedimentation. Clast-supported cobble conglomerates, associated with laminated mudstones and discontinuous coarse arkosic sandstones, are present near the base of the Abo Formation. The conglomerate lenses range from 0.5 to 20 m (1.5 to 66 ft) thick, and from 15 m (49 ft) to several kilometers wide. They exhibit poor stratification, occasional clast imbrication, and near horizontal to concave-upward erosional bounding surfaces. Locally, nodular CaCO3 occurs in beds or scattered throughout the associated mudstones. The central portion of the sequence is characterized by abundant conglomeratic medium to very coarse-grained arkosic sandstone lenses interbedded with laminated mudstones. These lenses range from 0.25 to 10 m (0.8 to 33 ft) in thickness, and 10 to 110 m (33 to 361 ft) in width. Commonly, several lenses will coalesce, forming laterally extensive belts. Both erosional and depositional (lateral accretion) bounding surfaces are well developed within individual lenses. Low-angle, large-scale trough cross-bedding is the dominant stratification type within these units, however, small to large-scale tabular foresets occur, as do planar, ripple, and small-scale trough stratification. Occasionally, individual units fine upward in grain size and/or scale of structures. This sandstone-rich horizo varies laterally in thickness, and grades upward into a mudstone-rich horizon at the top of the Abo Formation. The upper interval of the Abo sequence contains infrequent fine to coarse-grained arkose lenses enclosed in laterally extensive red mudstones. Paleosols and caliche horizons are present, but not common, in the mudstones. The basal conglomeratic horizon of the clastic wedge represents braided stream deposition at the distal end of arid alluvial fans associated with the tectonically active Pedernal uplift. Caliche and paleosols developed in the overbank mud drapes and levees of these deposits. The conglomerates grade into arkosic sandstones, reflecting the intense weathering and erosion of the granitic core of the uplift. The multilateral sandstone lenses characteristic of the mid portion of the Abo Formation were deposited in low-sinuosity, coarse-grained (bed-load) meander belts; however, higher sinuosity, mixed-load channel and crevasse splay deposits also occur locally. Finally, as the Pedernal source area was buried by its own debris, thick, broad flood-plain deposition predominated. Sandstones ass ciated with the red mudstones were deposited in shallow ephemeral channels and crevasse splays. End_of_Article - Last_Page 550------------

Regional implications of the geology of the Ogallala Group (upper Tertiary) of southwestern Morrill County, Nebraska, and adjacent areas

A part of a filled Ogallala Group (upper Tertiary) paleovalley system exhumed by recent stream erosion in southwestern Morrill County, Nebraska, and adjacent areas contains steep gradient gullies filled with locally derived sediments. Younger channels filled with granitic gravels derived by stream erosion of the Rocky Mountains to the west and southwest cut across and through these older filled gullies, indicating that there were at least two periods of downcutting followed by periods of filling during the development of this system. Individual clasts from younger Ogallala gravels are often as large as or larger than clasts from Quaternary gravels transported comparable distances by streams. This fact supports the idea that streams of Ogallala age were often at least as competent as their Quaternary counterparts. In a tributary to Greenwood Creek, six major volcanic ash lentils occur in superposition. These ash deposits range in geometry from tabular to channel-shaped and were deposited on land surfaces or in ponds and gullies at different times during the deposition of the Ogallala Group. Some of the lentils grade laterally into diatomites. Caliches are common throughout the Ogallala Group. Major caliches are developed in older rocks directly beneath the Ogalalla and in sediments beneath the eroded upper surface of the group. Caliche horizons occur at many horizons within the group and have “tepee,” honeycomb, and other typical structures. The preceding observations are in part directly opposed to observations made by earlier workers for the Ogallala in general or specifically for the study area. The Ogallala Group should be re-examined to see whether the earlier models or those offered here better explain its geologic history.

Fluvial sedimentation in the lower carboniferous of Clew Bay, County Mayo, Ireland

A succession of about 300 m of fluvial sediments from the Lower Carboniferous of northwest Ireland is described and interpreted. A lower, mainly red, formation contains fluvial channel deposits dominated by flat laminated sandstone. These are separated by interbedded sandstones and mudrocks with local caliche horizons and abundant mudcracks interpreted as levee and flood basin deposits. An upper, mainly non-red, formation contains fluvial channel deposits with common trough cross-stratification and epsilon cross-stratification also separated by interbedded sandstones and mudrocks. Evidence of desiccation is less common in the uppermost beds which pass transitionally upwards into marine sediments. The change in fluvial channel style is interpreted as due to increasing sinuosity and permanence of flow which may have been partly temporally and partly spatially controlled. The predominance of coarse sediments is thought to be largely controlled by limited subsidence. The Lower Carboniferous transgression was the major overall control of alluviation.

Mid-eocene climatic change, Southwestern California and Northwestern Baja California

A widespread paleosol of Paleocene and Early Eocene age occurs in southwestern California and northwestern Baja California. The dominant quartz-kaolinite mineralogy and cation-depleted chemistry of the buried soil indicate a humid, tropical paleoclimate similar to the modern equatorial belt. Although the Paleocene—Eocene paleomagnetic latitudes are similar to the modern latitudes (36–37°N to 32–33°N, respectively), rainfall was about 125–190 cm per year and average annual temperature was about 20–25°C in marked contrast to the present annual rainfall of 25 cm and average annual temperature of 16°C. A variety of indicators in the Late Eocene sedimentary succession suggests a change to a semi-arid paleoclimate. The nonmarine portions of the Late Eocene sedimentary record are dominated by a cobble conglomerate lithosome deposited in fluvial, alluvial fan and fan delta systems. Intertongued with the conglomerate is a sandstone lithosome deposited in flood-plain and nearshore marine environments. The conglomerate clasts were transported to the depositional site via a long-distance (200–300 km), moderate gradient, braided river system mostly by flash floods. Characteristic post-depositional, in situ fracturing of conglomerate clasts probably occurred due to salt crystallization. Within the flood plain sandstones, and to a lesser degree the conglomerates, are multiple well-developed caliche horizons of probable pedogenic origin. Clay minerals from the Late Eocene rocks are dominantly vermiculite and smectite with lesser chlorite and illite; this is in marked contrast to the kaolinite that comprises the underlying Early Paleogene lateritic paleosol. The character of the Late Eocene sedimentary succession indicates a semi-arid climate. Rainfall probably did not exceed 63 cm per year; it probably was seasonal and by occasional flash floods. This paleoclimate contrasts markedly with the earlier humid tropical paleoclimate and must indicate a widespread climatic change in late Middle Eocene time.

Eocene Conglomerate Sedimentology and Basin Analysis, San Diego and the Southern California Borderland

ABSTRACT Eocene conglomeratic strata in the San Diego area were deposited as a narrow, west-trending progradational system that changed facies from fluvial channel, to alluvial fan, to coastal plain-fan delta, to paralic, to shelf and subsea channels. Continuing this system westward are the large Eocene subsea fan deposits that include inner, middle, and outer subfan and basin plain facies of the southern California borderland. In all environments, 81 to 96 percent of the conglomerate clasts are distinctive, well-rounded, siliceous metavolcanic stones (Poway clasts) that reach 60 cm in size and average 6 cm; 2 to 13 percent are quartzite; and 0 to 12 percent are locally derived (Peninsular Ranges) crystalline basement (granitic and volcanic) rocks. The composition, shape, and average size of t e metavolcanic and quartzite clasts are constant in all environments, while the less resistant granitic clasts show the effects of progressive abrasion by a decrease in size and abundance with increased transport distances. Three major sedimentologic trends characterize the Eocene conglomerate. (1) Conglomerate uniformly decreases, and finer grained rocks increase, in abundance and thickness from east to west. (2) The nonmarine and shallow marine conglomerate facies each contain disorganized and organized beds with no grading. Organized conglomerates are framework supported, and have imbricated clasts that dip up-current. The long axes of the clasts are oriented perpendicular or parallel to the paleoflow or are distributed randomly. Some nonmarine sandstone beds contain caliche horizons. (3) The subsea fan conglomerate is both disorganized and organized, and the beds are graded, or inversely graded. The clasts are matrix-supported, imbricated, and dip up-current. Long axes of clasts are oriented both par llel to and perpendicular to paleocurrent trends, and angular mudstone rip-up clasts and reworked marine fossils are present.

Caliche in Southwestern Idaho

AbstractDeposition of caliche in the soils of southwestern Idaho probably results from soil‐forming processes. Caliche occurring on the lower, intermediate, and upper terraces of the Boise and Snake Rivers is described. Close correlation of the present ground surfaces with the top surfaces of the caliche layers is found. The pedogenic development of the soils is related to the ages of the terraces on which the soils occur, and also to the thickness and induration of the caliche layers. The thickness of the Cca horizon decreases or disappears entirely as the age of the soil and terrace increases. Calcium carbonate and silicates increase in the caliche horizons as the soils and terraces increase in age.