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At least 901 records · Page 50Linked to original sources
USGS1974

Molluscan phylogeny: The paleontological viewpoint

Stasek theorized that the extant mollusks are the progeny of three separate lineages that separated before the phylum was well established. He wrote that no known intermediate forms, fossil or living, bridge the "enormous gaps between any two of the three lineages," and therefore treated each as a separate subphylum. These subphyla are (i) the subphylum Aculifera Hatscheck 1891, containing only the class Aplacophora, derived from the most primitive ancestors of the Mollusca; (ii) the subphylum Placophora von Jhering 1876, containing only the class Polyplacophora, and emphasizing the pseudometamerism of its more advanced premollusk ancestor; and (iii) the subphylum Conchifera Gegenbaur 1878, containing the Monoplacophora and the other classes derived from it. We point out that the Polyplacophora may be derived from the Monoplacophora instead of a more primitive ancestral stock. We also suggest that the Conchifera can be separated into two major lineages, each worthy of the rank of subphylum. The fossil record indicates that the Monoplacophora gave rise to the Gastropoda, Cephalopoda, Rostroconchia, and possibly Polyplacophora, and that the Pelecypoda and Scaphopoda are derived from the Rostroconchia. These last three classes thus form a lineage that diverged from the Monoplacophora in the Early Cambrian. They emphasized a shell form that in all groups is primitively open at both ends, allowing the gut to remain relatively straight, with an anterior mouth and posterior anus. They became burrowing (infaunal) deposit or filter feeders. We coin the term Diasoma (through-body) for the subphylum containing these three classes (Rostroconchia, Pelecypoda, and Scaphopoda). The remaining three classes (Monoplacophora, Gastropoda, and Cephalopoda) emphasize a conical univalved shell, usually twisted into a spiral. The relatively small single aperture forces the anus to lie close to the mouth, and the gut is bent into a "U." Most are surface-dwelling (epifaunal) grazers or carnivores. We coin the name Cyrtosoma (hunchback-body) for the subphylum containing these three classes. Strictly speaking, the cyrtosomes are the ancestors of the diasomes but, in fact, both subphyla appeared and began to diversify within a few million years in the Early Cambrian.

Science↗
USGS1980

Seabed minerals and the law of the sea

If, as seems likely, a comprehensive law-of-the-sea treaty is successfully concluded, minerals of the continental margins would be almost entirely controlled by coastal nations, and the right to produce minerals of the deep ocean floor would be licensed by an International Seabed Authority. Mineral production from the continental margins is likely to increase and diversify with time. The only minerals from the deep ocean floor for which there are reasonably good prospects of production within the next decade are the metalliferous muds from some of the deeps in the Red Sea (which lie within what is likely to be the jurisdiction of Saudi Arabia and Sudan) and nodules in the northeastern equatorial Pacific that contain recoverable metals, namely nickel, copper, cobalt, and possibly manganese, molybdenum, and vanadium.

Science↗
USGS1984

Deposit from a giant wave on the island of Lanai, Hawaii

Limestone-bearing gravel, the newly named Hulopoe Gravel, blankets the coastal slopes on Lanai. The deposit, which reaches a maximum altitude of 326 meters, formerly was believed to have been deposited along several different ancient marine strandlines, but dated submerged coral reefs and tide-gauge measurements indicate that the southeastern Hawaiian Islands sink so fast that former worldwide high stands of the sea now lie beneath local sea level. Evidence indicates that the Hulopoe Gravel and similar deposits on nearby islands were deposited during the Pleistocene by a giant wave generated by a submarine landslide on a sea scarp south of Lanai.

Hawaii↗
USGS1989

Voyager 2 at Neptune: Imaging science results

Voyager 2 images of Neptune reveal a windy planet characterized by bright clouds of methane ice suspended in an exceptionally clear atmosphere above a lower deck of hydrogen sulfide or ammonia ices. Neptune's atmosphere is dominated by a large anticyclonic storm system that has been named the Great Dark Spot (GDS). About the same size as Earth in extent, the GDS bears both many similarities and some differences to the Great Red Spot of Jupiter. Neptune's zonal wind profile is remarkably similar to that of Uranus. Neptune has three major rings at radii of 42,000, 53,000, and 63,000 kilometers. The outer ring contains three higher density arc-like segments that were apparently responsible for most of the ground-based occultation events observed during the current decade. Like the rings of Uranus, the Neptune rings are composed of very dark material; unlike that of Uranus, the Neptune system is very dusty. Six new regular satellites were found, with dark surfaces and radii ranging from 200 to 25 kilometers. All lie inside the orbit of Triton and the inner four are located within the ring system. Triton is seen to be a differentiated body, with a radius of 1350 kilometers and a density of 2.1 grams per cubic centimeter; it exhibits clear evidence of early episodes of surface melting. A now rigid crust of what is probably water ice is overlain with a brilliant coating of nitrogen frost, slightly darkened and reddened with organic polymer material. Streaks of organic polymer suggest seasonal winds strong enough to move particles of micrometer size or larger, once they become airborne. At least two active plumes were seen, carrying dark material 8 kilometers above the surface before being transported downstream by high level winds. The plumes may be driven by solar heating and the subsequent violent vaporization of subsurface nitrogen.

Science↗
USGS2009

Ecophysiology of "halarsenatibacter silvermanii" strain SLAS-1 T , gen. nov., sp. nov., a facultative chemoautotrophic arsenate respirer from salt-saturated Searles Lake, California

Searles Lake occupies a closed basin harboring salt-saturated, alkaline brines that have exceptionally high concentrations of arsenic oxyanions. Strain SLAS-1 T was previously isolated from Searles Lake (R. S. Oremland, T. R. Kulp, J. Switzer Blum, S. E. Hoeft, S. Baesman, L. G. Miller, and J. F. Stolz, Science 308:1305-1308, 2005). We now describe this extremophile with regard to its substrate affinities, its unusual mode of motility, sequenced arrABD gene cluster, cell envelope lipids, and its phylogenetic alignment within the order Halanaero-bacteriales, assigning it the name "Halarsenatibacter silvermanii" strain SLAS-1 T . We also report on the substrate dynamics of an anaerobic enrichment culture obtained from Searles Lake that grows under conditions of salt saturation and whose members include a novel sulfate reducer of the order Desulfovibriales, the archaeon Halorhabdus utahensis, as well as a close homolog of strain SLAS-1 T . Copyright ?? 2009, American Society for Microbiology. All Rights Reserved.

Applied and Environmental Microbiology↗
USGS1993

Aerobic and anaerobic degradation of a range of alkyl sulfides by a denitrifying marine bacterium

A pure culture of a bacterium was obtained from a marine microbial mat by using an anoxic medium containing dimethyl sulfide (DMS) and nitrate. The isolate grew aerobically or anaerobically as a denitrifier on alkyl sulfides, including DMS, dimethyl disulfide, diethyl sulfide (DES), ethyl methyl sulfide, dipropyl sulfide, dibutyl sulfide, and dibutyl disulfide. Cells grown on an alkyl sulfide or disulfide also oxidized the corresponding thiols, namely, methanethiol, ethanethiol, propanethiol, or butanethiol. Alkyl sulfides were metabolized by induced or derepressed cells with oxygen, nitrate, or nitrite as electron acceptor. Cells grown on DMS immediately metabolized DMS, but there was a lag before DES was consumed; with DES-grown cells, DES was immediately used but DMS was used only after a lag. Chloramphenicol prevented the eventual use of DES by DMS-grown cells and DMS use by DES-grown cells, respectively, indicating separate enzymes for the metabolism of methyl and ethyl groups. Growth was rapid on formate, acetate, propionate, and butyrate but slow on methanol. The organism also grew chemolithotrophically on thiosulfate with a decrease in pH; growth required carbonate in the medium. Growth on sulfide was also carbonate dependent but slow. The isolate was identified as a Thiobacillus sp. and designated strain ASN-1. It may have utility for removing alkyl sulfides, and also nitrate, nitrite, and sulfide, from wastewaters.

Georgia↗
USGS2004

Vulnerability of drinking-water wells in La Crosse, Wisconsin, to enteric-virus contamination from surface water contributions

Human enteric viruses can contaminate municipal drinking-water wells, but few studies have examined the routes by which viruses enter these wells. In the present study, the objective was to monitor the municipal wells of La Crosse, Wisconsin, for enteric viruses and determine whether the amount of Mississippi River water infiltrating the wells was related to the frequency of virus detection. From March 2001 to February 2002, one river water site and four wells predicted by hydrogeological modeling to have variable degrees of surface water contributions were sampled monthly for enteric viruses, microbial indicators of sanitary quality, and oxygen and hydrogen isotopes. 18O/ 16O and 2H/1H ratios were used to determine the level of surface water contributions. All samples were collected prior to chlorination at the wellhead. By reverse transcription-PCR (RT-PCR), 24 of 48 municipal well water samples (50%) were positive for enteric viruses, including enteroviruses, rotavirus, hepatitis A virus (HAV), and noroviruses. Of 12 river water samples, 10 (83%) were virus positive by RT-PCR. Viable enteroviruses were not detected by cell culture in the well samples, although three well samples were positive for culturable HAV. Enteroviruses detected in the wells by RT-PCR were identified as several serotypes of echoviruses and group A and group B coxsackieviruses. None of the well water samples was positive for indicators of sanitary quality, namely male-specific and somatic coliphages, total coliform bacteria, Escherichia coli, and fecal enterococci. Contrary to expectations, viruses were found in all wells regardless of the level of surface water contributions. This result suggests that there were other unidentified sources, in addition to surface water, responsible for the contamination.

Applied and Environmental Microbiology↗
USGS1969

Yersinia philomiragia sp. n., a new member of the Pasteurella group of bacteria, naturally pathogenic for the muskrat (Ondatra zibethica)

A bacterium experimentally pathogenic for muskrats ( Ondatra zibethica ), white mice, mountain voles ( Microtus montanus ), and deer mice ( Peromyscus maniculatus ) was isolated from the tissues of a sick muskrat captured on the Bear River Migratory Bird Refuge (Brigham City, Utah) and from four surface water samples collected within 15 miles of that point. In culture, the cells are chiefly coccoid, but in the tissues of muskrats and voles they resemble the bizarre forms of Yersinia pestis , except for their smaller size. The characteristics of the organism are described and the name Yersinia philomiragia sp. n. is proposed.

Utah↗
USGS2011

In Vivo fitness associated with high virulence in a vertebrate virus is a complex trait regulated by host entry, replication, and shedding

The relationship between pathogen fitness and virulence is typically examined by quantifying only one or two pathogen fitness traits. More specifically, it is regularly assumed that within-host replication, as a precursor to transmission, is the driving force behind virulence. In reality, many traits contribute to pathogen fitness, and each trait could drive the evolution of virulence in different ways. Here, we independently quantified four viral infection cycle traits, namely, host entry, within-host replication, within-host coinfection fitness, and shedding, in vivo , in the vertebrate virus Infectious hematopoietic necrosis virus (IHNV). We examined how each of these stages of the viral infection cycle contributes to the fitness of IHNV genotypes that differ in virulence in rainbow trout. This enabled us to determine how infection cycle fitness traits are independently associated with virulence. We found that viral fitness was independently regulated by each of the traits examined, with the largest impact on fitness being provided by within-host replication. Furthermore, the more virulent of the two genotypes of IHNV we used had advantages in all of the traits quantified. Our results are thus congruent with the assumption that virulence and within-host replication are correlated but suggest that infection cycle fitness is complex and that replication is not the only trait associated with virulence.

Journal of Virology↗
USGS2015

Cyclic avian mass mortality in the northeastern United States is associated with a novel orthomyxovirus

Since 1998, cyclic mortality events in common eiders ( Somateria mollissima ), numbering in the hundreds to thousands of dead birds, have been documented along the coast of Cape Cod, Massachusetts, USA. Although longitudinal disease investigations have uncovered potential contributing factors responsible for these outbreaks, detecting a primary etiological agent has proven enigmatic. Here we identify a novel orthomyxovirus, tentatively named Wellfleet Bay virus (WFBV), as a potential causative agent of these outbreaks. Genomic analysis of WFBV revealed that it is most closely related to members of the Quaranjavirus genus within the family Orthomyxoviridae . Similar to other members of the genus, WFBV contains an alphabaculovirus gp64-like glycoprotein, which was demonstrated to have fusion activity, and also tentatively suggests that ticks (and/or insects) may vector the virus in nature. However, in addition to the six RNA segments encoding the prototypical structural proteins identified in other quaranjaviruses, a previously unknown RNA segment (segment 7) encoding a novel protein designated as VP7 was discovered in WFBV. Although WFBV shows low to moderate levels of sequence similarity to Quaranfil virus and Johnston Atoll virus , the original members of the Quaranjavirus genus, additional antigenic and genetic analyses demonstrated that it is closely related to the recently identified Cygnet River virus (CyRV) from South Australia, suggesting that WFBV and CyRV may be geographic variants of the same virus. Although the identification of WFBV in part may resolve the enigma of these mass mortality events, the details of the ecology and epidemiology of the virus remain to be determined. Importance The emergence or reemergence of viral pathogens resulting in large-scale outbreaks of disease in humans and/or animals is one of the most important challenges facing biomedicine. For example, understanding how orthomyxoviruses such as novel influenza A virus reassortants and/or mutants emerge to cause epidemic or pandemic disease is at the forefront of current global health concerns. Here we describe the emergence of a novel orthomyxovirus, Wellfleet Bay virus (WFBV), which has been associated with cyclic large-scale bird die-offs in the northeastern United States. This initial characterization study provides a foundation for further research into the evolution, epidemiology, and ecology of newly emerging orthomyxoviruses, such as WFBV, and their potential impacts on animal and/or human health.

Massachusets↗
USGS2004

Complete genome sequence of Fer-de-Lance Virus reveals a novel gene in reptilian Paramyxoviruses

The complete RNA genome sequence of the archetype reptilian paramyxovirus, Fer-de-Lance virus (FDLV), has been determined. The genome is 15,378 nucleotides in length and consists of seven nonoverlapping genes in the order 3??? N-U-P-M-F-HN-L 5???, coding for the nucleocapsid, unknown, phospho-, matrix, fusion, hemagglutinin-neuraminidase, and large polymerase proteins, respectively. The gene junctions contain highly conserved transcription start and stop signal sequences and tri-nucleotide intergenic regions similar to those of other Paramyxoviridae. The FDLV P gene expression strategy is like that of rubulaviruses, which express the accessory V protein from the primary transcript and edit a portion of the mRNA to encode P and I proteins. There is also an overlapping open reading frame potentially encoding a small basic protein in the P gene. The gene designated U (unknown), encodes a deduced protein of 19.4 kDa that has no counterpart in other paramyxoviruses and has no similarity with sequences in the National Center for Biotechnology Information database. Active transcription of the U gene in infected cells was demonstrated by Northern blot analysis, and bicistronic N-U mRNA was also evident. The genomes of two other snake paramyxovirus genotypes were also found to have U genes, with 11 to 16% nucleotide divergence from the FDLV U gene. Pairwise comparisons of amino acid identities and phylogenetic analyses of all deduced FDLV protein sequences with homologous sequences from other Paramyxoviridae indicate that FDLV represents a new genus within the subfamily Paramyxovirinae. We suggest the name Ferlavirus for the new genus, with FDLV as the type species.

Journal of Virology↗
USGS2015

Discovery of a novel hepatovirus (Phopivirus of seals) related to human hepatitis A virus

Describing the viral diversity of wildlife can provide interesting and useful insights into the natural history of established human pathogens. In this study, we describe a previously unknown picornavirus in harbor seals (tentatively named phopivirus) that is related to human hepatitis A virus (HAV). We show that phopivirus shares several genetic and phenotypic characteristics with HAV, including phylogenetic relatedness across the genome, a specific and seemingly quiescent tropism for hepatocytes, structural conservation in a key functional region of the type III internal ribosomal entry site (IRES), and a codon usage bias consistent with that of HAV.

mBio↗
USGS2020

Binning singletons: Mentoring through networking at ASM microbe 2019

The American Society for Microbiology (ASM) national conference, Microbe, is the flagship meeting for microbiologists across the globe. The presence of roughly 10,000 attendees provides enormous opportunities for networking and learning. However, such a large meeting can be intimidating to many, especially early career scientists, students, those attending alone, and those from historically underrepresented groups. While mentorship is widely valued by ASM and its members, finding concrete ways to develop new and diverse mentoring opportunities can be a challenge. We recognized the need for an initiative aimed at expanding peer-to-peer mentoring, facilitating networking, and providing support for Microbe attendees; therefore, we created the program Binning Singletons for ASM Microbe 2019. The program consisted of five steps named after tools or phenomena in the profession of microbiology: (i) Identify the Singletons (e.g., individuals attending alone), (ii) Bin the Singletons, (iii) Horizontal Transfer, (iv) Quorum Sensing, and (v) Exponential Growth. These steps resulted in the matching of participants unsure of how to get the most out of their conference experience (e.g., singletons) with mentors who assisted with meeting planning, networking, and/or impostor syndrome. Started on social media only a month before ASM Microbe 2019, the program successfully launched despite limited time and resources. Binning Singletons improved inclusivity and networking opportunities for participants at the conference. Here, we discuss what worked, and what can be improved, with an eye toward development of the Binning Singletons model for future conferences to provide opportunities to increase inclusivity, networking, and accessibility for singletons and build a stronger scientific community.

mSphere↗
USGS1999

Late Cenozoic stratigraphy and tephrochronology of the western Black Mountains piedmont, Death Valley, California: Implications for the tectonic development of Death Valley

Geologic mapping combined with the tephrochronology of spatially isolated sedimentary sections along the western Black Mountains piedmont adjacent the Death Valley fault zone (DVFZ) improves the late Cenozoic stratigraphy from relative age to correlated age. Pliocene tephra layers identified in Funeral Formation conglomerates at Artist Drive and Copper Canyon include a “Nomlaki-like” tephra bed (ca. 3.4 Ma), the tuffs of Mesquite Spring (3.1–3.3 Ma), and a tuff of the lower Glass Mountain family (1.86–1.92 Ma). We informally name the early(?) to middle Pleistocene Mormon Point formation1, which contains tephra layers correlated with the upper Glass Mountain/Bishop family of tephra layers (0.76–1.2 Ma), the Lava Creek B ash bed (ca. 0.66 Ma), and the Dibekulewe ash bed (ca. 0.51 Ma). Identification of these tephra layers indicates that the maximum age of the overlying and inset lacustrine gravel and alluvial fan deposits is 0.51 Ma. The correlated age stratigraphy indicates that the dextral-oblique DVFZ has stepped basinward at Mormon Point and Copper Canyon since the late Pliocene. In contrast, during that same time the DVFZ at Artist Drive has not stepped basinward, but developed into a graben. The age of faulting on the low-angle (~19°–40°) Mormon Point turtleback fault is bracketed between 0.76 and 0.18 Ma, and the overlying Mormon Point formation shows no evidence of tilting, indicating slip on the turtleback fault was at a low-angle. Early Quaternary slip on the low-angle turtleback fault conflicts with the present versions of the pure shear, rolling-hinge, and detachment/rift models for Death Valley extension. Early Quaternary slip is most compatible with turtleback faults as folded or warped detachment fault. We propose that the warping is thermally driven and related to the Black Mountains igneous complex.

California, Nevada↗
USGS2000

Overview: Ancient Lake Creede

Lake Creede was moderately saline closed-basin lake that developed in the 26.9 Ma Creede caldera in the San Juan Mountains in the southwest Colorado. The volcaniclastic sediments deposited within the late Oligocene lake were first described and named as the Creede Formation by Emmons and Larsen (1923). The lake and its sedimentary fill are of interest first as representatives of a caldera-hosted lake in a silicic volcanic terrane, and second because of the likely involvement of lake fluids or related pore waters in the deposition of the 25 Ma silver and base-metal ores of the Creede mining district north of the Creede caldera (Fig. 1), as proposed Bethke and Rye (1979). Much of the material presented in this volume is based on observation of core samples and on downhole geophysical measurements obtained as part of a U.S. Continental Scientific Drilling Program in the moat of the Creede caldera. These core and downhole studies are supplemented by outcrop studies, some initiated in support of the drilling program (Bethke and Lipman, 1987), and by conceptual studies of the evolution of the Creede caldera and its surrounding landscape. Not surprisingly, not all authors agree on all interpretation. Most disagreements are pointed out in this overview chapter, and may present opportunities for future study.

Colorado↗
USGS2002

Dinosaurs that did not die: Evidence for Paleocene dinosaurs in the Ojo Alamo Sandstone, San Juan Basin, New Mexico

Palynologic and paleomagnetic data confirm a Paleocene age for the Ojo Alamo Sandstone (and its contained dinosaurs) throughout the San Juan Basin of New Mexico. The recently reported discovery of 34 skeletal elements from a single hadrosaur in the Ojo Alamo provides unequivocal evidence that these bones were not reworked from underlying Cretaceous strata. Geochemical studies of samples from several single-dinosaur-bone specimens from the Paleocene Ojo Alamo Sandstone and the underlying Late Cretaceous (Campanian) Kirtland Formation show that mineralized bones from these two rock units contain distinctly different abundances of uranium and rare-earth elements and demonstrate that Cretaceous and Paleocene bones were mineralized at different times when mineralizing fluids had distinctly different chemical compositions. These findings indicate that the dinosaur bone from the Paleocene Ojo Alamo is indigenous and not reworked. These data show that a relatively diverse assemblage of dinosaurs survived the end-Cretaceous asteroid-impact extinction event of 65.5 Ma. The San Juan Basin’s Paleocene dinosaur fauna is herein named the Alamoan fauna. Magnetic-polarity chronology shows that these survivors lived for about one million years into the Paleocene and then became extinct around 64.5 Ma. We suggest that a plausible survival mechanism for this Lazarus fauna may have been the large numbers of buried dinosaur eggs, laid just before the asteroid impact occurred. These buried eggs would have provided a safe haven for developing dinosaur embryos for the first one to two years after the impact, thereby making it possible for them to survive the worst of the impact’s early devastation.

Colorado, New Mexico↗
USGS1987

Resurrection Peninsula and Knight Island ophiolites and recent faulting on Montague Island, southern Alaska

The Resurrection Peninsula forms the east side of Resurrection Bay (Fig. 1 ). The city of Seward is located at the head of the bay and can be reached from Anchorage by highway (127 mi;204 km). Relief ranges from 1,434 ft (437 m) at the southern end of the peninsula to more than 4,800 ft (1,463 m) 17 mi (28 km) to the north. All rock units composing the informally named Resurrection Peninsula ophiolite are visible and (or) accessible by boat.The eastern half of the peninsula is located within the Chugach National Forest; the western half is mainly state land, but there is some private land with recreational cabins. The Seward A6 and A7 and Blying Sound D6 and D7 maps at 1:63,360 scale (mile-to-the-inch) cover the entire Resurrection Peninsula. Knight Island is located 53 mi (85 km) east of Seward (Fig. 1 ). Numerous fiords indent the 31-mi-long (50 km) by 7.4-mi-wide (12 km) island and offer excellent bedrock exposures. The island is rugged and has a maximum elevation of 3,000 ft (914 m). It has numerous mineral prospects (Tysdal, 1978; Nelson and others, 1984; Jansons and others, 1984; Koski and others, 1985), and several abandoned canneries are located on the island. Knight Island lies entirely within the Chugach National Forest—state and private inholdings constitute less than five percent of its total land area. The Seward A2, A3, B2, B3, and C2, 1:63,360-scale U.S. Geological Survey topographic maps cover the entire island. Montague Island, 50 mi (80 km) long and up to 11 mi (18 km) wide, lies 10.6 mi (17 km) southeast of Knight Island. It belongs to an island group that forms the southern margin of Prince William Sound (Fig. 1 ). Montague Island is less rugged and less heavily vegetated than either the Resurrection Peninsula or Knight Island. Rock exposures are excellent along the beaches, and ground disruption due to recent fault movements is clearly visible. The Seward Al and A2 and Blying Sound Dl, D2, and D3 maps cover the areas of interest on Montague Island. In all areas, access is by float-equipped aircraft, helicopter, or boat. Wheel-equipped aircraft can land on the beaches or at several landing strips on Montague Island.

Alaska↗
USGS1947

Stratigraphy of the upper Cambrian, Llano Uplift, Texas

The two formations and eight members that constitute the Upper Cambrian in the Llano uplift of central Texas are described or redefined, and their lithic characters in 19 measured sections are graphically summarized. Standard reference to them is thus furnished. The Riley formation comprises the basal Paleozoic strata of the Llano uplift. Its initial sediments were deposited on a submerged pre-Cambrian terrane having a known topographic relief as great as 800 feet. Its thickness normally averages about 680 feet but ranges from probably less than 200 to about 800 feet. At most places it is subequally divisible between the Hickory sandstone member below and the Cap Mountain limestone member above, with the thin but widespread, glauconitic Lion Mountain sandstone member capping and completing the sequence. The Wilberns formation includes five named members between the Riley formation and rocks of the Lower Ordovician Ellenburger group. It normally averages about 580 feet thick and ranges from 540 to 610 feet thick, but in the southeastern corner of the Llano uplift truncation of the upper beds has reduced it to 360 feet. The thin but widespread, nonglauconitic Welge sandstone member introduces the sequence. Above it is the Morgan Creek limestone member, grading to the succeeding argillaceous beds of the Point Peak shale. At the top of the sequence are the San Saba limestone and Pedernales dolomite members. These two are essentially equivalent and gradational facies, with the Pedernales normally overlying the San Saba.

Texas↗