Wilberns Formation Research Articles

Stromatolite‐rimmed thrombolite columns and domes constructed by microstromatolites, calcimicrobes and sponges in late Cambrian biostromes, Texas, USA

ABSTRACTMicrobial carbonates formed stromatolitic, thrombolitic, dendrolitic and maceriate (mazelike) fabrics in shallow marine Cambrian–Early Ordovician carbonates encircling Laurentia. However, poor preservation often hinders recognition of their specific components. Well‐preserved examples of normal shallow marine limestones in the ca 490 Ma upper Cambrian Point Peak Member, Wilberns Formation, central Texas, include stromatolitic cones, steep‐sided laminated rimmed columns with grainy interiors, and laminated and maceriate domes. Together these form decimetre to metre‐thick biostromes. In these examples, a single component, microstromatolite, on its own or with minor calcimicrobes, creates macroscopic stromatolitic, dendrolitic, thrombolitic and maceriate fabrics. Microstromatolites constructed upward widening stromatolitic cones that developed into columns with laminated rims surrounding slightly depressed interiors. These columns accumulated allochthonous sediment by a ‘bucket effect’. Their interiors contain either clusters of dendrolitic microstromatolite or ragged columns of laminated stromatolite–sponge biolithite, and are often characterized by a ‘mottled’ fabric that superficially resembles thrombolite. This mottling was formed by localized dolomitization around millimetric burrows that otherwise do not appear to have significantly influenced the biolithite fabric. Calcimicrobes, including cyanobacteria (Razumovskia) and microproblematica (Renalcis and Tarthinia), impart a mesoscopic clotted appearance to maceriate fabric, and locally to column rims, both of which are dominated by microstromatolite. Similar component‐fabric relationships should be recognizable in rimmed columns and domes that were locally abundant elsewhere in Cambrian–Early Ordovician shallow carbonate seas.

Covering the Great Unconformity in southern Laurentia during Rodinia breakup: Detrital zircon studies of provenance evolution during Cambrian marine transgression (Llano Uplift, Texas)

The Llano Uplift in central Texas, USA, exposes the southernmost expanse of Laurentian crystalline basement in North America and the overlying lower Paleozoic strata deposited on the Great Unconformity. Systematic detrital zircon (DZ) U-Pb provenance analysis of the Hickory Sandstone—the basal unit of the Cambrian Riley Formation that onlaps the Mesoproterozoic core of the Llano Uplift—yielded locally variable DZ U-Pb signatures with ages ranging from 1800 Ma to 485 Ma (n >1700). The Hickory Sandstone zircons are dominated by 1550−1300 Ma (50%) and 1300−1000 Ma (46%) ages. These two dominant Mesoproterozoic DZ age components likely were sourced from local Grenville metamorphic and igneous basement of the Llano Province (1300−1000 Ma) and from the extra-regional Granite-Rhyolite Province basement to the north and northwest of the Grenville Front, which suggests both local sourcing and a regional drainage system supplying sediment to the southern Laurentian margin during the Cambrian. This interpretation is supported by a minor component of 1800−1600 Ma zircons, sourced from the Yavapai-Mazatzal Province, and zircons with Early Cambrian ages that were likely sourced from the Wichita Igneous Province, which is located to the northwest and to the north of the Llano Uplift, respectively. Documented NNW-trending topographic ridges in the Mesoproterozoic basement surface of the Llano Uplift could have funneled aeolian and fluvial sand supply to the southern Laurentian coast that was reworked in the marginal marine environment. Samples from the western Llano Uplift are dominated by regionally sourced, early Mesoproterozoic Granite-Rhyolite Province DZ ages, whereas samples from the eastern Llano Uplift exhibit a dominant locally sourced Grenville signature. Hickory Sandstone samples also contain a small number of Neoproterozoic (850−600 Ma) and Cambrian (541−487 Ma) zircons. Sources for Neoproterozoic zircons likely were located along the Laurentian continental margins and derived from extension-related magmatism associated with the breakup of Rodinia. Cambrian zircons are most common in Hickory Sandstone samples on the southern and western flanks of the Llano Uplift. Some are reasonably sourced from the Wichita Igneous Province to the north, but the younger Cambrian zircons suggest sources to the west. Upper Hickory strata are dominated (>60%) by 1550−1300 Ma grains with up to 10% >1600 Ma Paleoproterozoic grains. Upper Cambrian sandstones of the Wilberns Formation also contain a significant contribution of >1600 Ma grains, which suggests a possible enlargement of the fluvial headwaters over time extending into older Laurentian provinces to the north and northwest. Differences among DZ populations over relatively limited distances may have been related to NW-oriented ridge and corridor landforms that developed on the Great Unconformity surface, which were products of the Proterozoic tectonic and lithologic architecture affected by aeolian and fluvial processes at this unusual stage in Earth history. Paleocurrent data and atypical detrital grain characteristics for a Hickory Sandstone locality in the southeastern Llano Uplift and its DZ age similarities with Cambrian sandstones of the Argentine Precordillera suggest a common provenance prior to or during Rodinia breakup.

Controls on microbial and oolitic carbonate sedimentation and stratigraphic cyclicity within a mixed carbonate‐siliciclastic system: Upper Cambrian Wilberns Formation, Llano Uplift, Mason County, Texas, USA

AbstractThe upper Cambrian Wilberns Formation in central Texas records deposition on a low‐gradient shelf within a mixed carbonate–siliciclastic tidal‐flat system that changes offshore to subtidal shelf and open‐marine oolitic skeletal shoals with large microbial mounds. Siliciclastic sediment is interpreted to have been delivered to the tidal flat by aeolian processes because of the narrow range in grain size and paucity of clay. Tidal influence is dominant as evidenced by reversing currents and desiccation on the tidal flat, and megaripples with reversing current indicators in offshore shoals. Intraclastic conglomerates were deposited in broad channels on the tidal flats during storm surges. Microbialite deposition is interpreted to be controlled by accommodation favouring amalgamated thin biostromes developed in the tidal flat vs. larger mounds with greater synoptic relief in the offshore, and current energy resulting in preferential elongation of offshore mounds in a NE–SW orientation. Intertidal mounds and biostromes grew in the presence of significant siliciclastic flux and trapped it within their structure, whereas offshore large buildups incorporated little siliciclastic component. Oolite and skeletal grainstone formed in tide agitated shoals associated with large subtidal microbial mounds. Storms extensively recycled and redistributed skeletal and oolitic sands from the offshore shoals across the shelf as thin sand sheets. Spatial mixing of siliciclastic and carbonate sediment occurred across the tidal flat and shelf. Low‐frequency and intermediate‐frequency stratigraphic cycles were driven by shifts in the shoreline and changes in rate of siliciclastic flux in response to relative sea‐level fluctuation. Random facies stacking and the lack of metre‐scale cyclicity are interpreted to reflect stratigraphic incompleteness and an episodic signal introduced by storms.

New long-stemmed eocrinoid from the Furongian Point Peak Shale Member of the Wilberns Formation, central Texas

AbstractLlanocystis wilbernsensisn. gen. n. sp. (Eocrinoidea, Echinodermata) is described based on three specimens from the Furongian Point Peak Shale Member of the Wilberns Formation in central Texas. It displays a unique morphology including a very long stem constructed with holomeric columnals, few feeding appendages, and a polyplated theca. The specimens occur in an intraformational conglomerate deposited in a proximal carbonate platform environment and represent an example of the poorly documented “pelmatozoan” radiation that occurred in proximal facies by the end of the Cambrian.

Paragenesis of the Morgan Creek Limestone, Late Cambrian, central Texas: Constraints on the formation of glauconite

Abstract Deposition of the Morgan Creek Limestone, a member of the Cambrian Wilberns Formation, began a few meters above a disconformable surface and displays abundant indicators of accumulation in shallow marine to tidal flat environments. These indicators include: intercalation of a lithologic variety of thin beds (e.g. biosparites, biomicrites, oosparites, intrasparites), which display rapid vertical and lateral lithologic changes, numerous stromatolitic horizons, channels filled with graded oosparites and intrasparites that cut through micrite accumulations, and finely laminated (non-burrowed) siltstones. Glauconite is a ubiquitous minor constituent throughout in the form of pellets, replaced skeletal material and mica books, and, most informatively, as an authigenic precipitate in the form of fibroradiating rims on carbonate allochems and siliciclastic grains. Fibroradiating glauconite rims were disrupted and pushed away from the pelmatozoan nuclei on which they precipitated. Combined sedimentological and paragenetic constraints indicate that the glauconite was the earliest diagenetic event to affect these sediments and occurred essentially at the sediment–water interface within these relatively high-energy, shallow marine deposits. Precipitation of glauconite was closely followed by precipitation of carbonate cement as well as dissolution of aragonitic constituents. Later diagenetic processes included selective dolomitization and silicification. In modern seas glauconite is reported to form at and below mid-shelf water depths. Thus, the constraints pertaining to the origin of modern glauconite are not valid for Late Cambrian deposits and probably are also not applicable for Late Cretaceous through Early Paleogene glauconites.

NEOTYPE AND REDESCRIPTION OF THE UPPER CAMBRIAN ANTHASPIDELLID SPONGE, WILBERNICYATHUS DONEGANI WILSON, 1950

The Upper Cambrian sponge Wilbernicyathus donegani Wilson, 1950 was originally described as an archaeocyath. The original specimens were lost. Later, based on the original description and figures, this species was interpreted as an orchocladine lithistid, but family-level taxonomy has not been clear. A neotype is designated to help stabilize the taxonomy of this species. A redescription based on the neotype and other new material from the area of the type locality and elsewhere demonstrates that this species is an orchocladine lithistid sponge belonging to the family Anthaspidellidae. It has a regular skeleton consisting of dendroclones and trabs arranged in a ladderlike net, and radial canals are well organized. The species occurs within sponge-microbial and stromatolitic reefs in the Wilberns Formation of central Texas and possibly the Clinetop Bed of the Dotsero Formation of Colorado. Stratigraphically, it ranges from the Idahoia to the Saukia trilobite zones of the Upper Cambrian Ptychaspid Biomere. Ecologically, Wilbernicyathus Wilson, 1950 occupies a reefal niche and constitutes up to 30% of the reef volume. These Late Cambrian bioherms represent the initial Laurentian expansion in the sponge-microbial buildups that dominated reef environments worldwide during the Early Ordovician.

Paleomagnetic Dating of Depositional and Diagenetic Events in Upper Cambrian Rocks of Central Texas: ABSTRACT

Hematite, magnetite, and goethite are present in samples from the Riley and Wilberns Formations in central Texas. Samples containing abundant goethite possess characteristic paleomagnetic directions that are Tertiary to recent in age. The goethite component is easily removed by thermal demagnetization temperatures greater than 150/degrees/C. A minority of samples from the Wilberns Formation contains magnetite as the dominant magnetic component. These samples display shallow easterly characteristic directions that the authors interpret as a true detrital remanent magnetization. Most samples from both units possess hematite as the dominant carrier of magnetic remanence. Samples in which hematite occurs as ooids and as a replacement for carbonate allochems possess shallow easterly declinations similar to magnetite-dominant samples. This generation of hematite probably formed during or shortly after deposition. The paleopole for samples in which hematite occurs as a replacement for glauconite grains is similar to previously published Ordovician paleopoles. This indicated Ordovician age is corroborated by a previously published Late Ordovician radiometric age for glauconite from the Riley Formation. Samples in which hematite occurs as an infilling within fractures and pressure solution seams display characteristic paleomagnetic directions that indicate a late Paleozoic age. This age corresponds to tectonic activity along the Ouachita tectonic beltmore » to the east. Such an age may reflect hematization from meteoric fluids during structural uplift of the area, or it might reflect precipitation during mixing of tectonically driven basin-derived fluid and meteoric fluid.« less

Nature of Fort Chadbourne Fault System and its Relation to Petroleum Potential of Wilberns Formation (Upper Cambrian) of West-Central Texas: ABSTRACT

The Wilberns Formation (Upper Cambrian) in the subsurface of west-central Texas is composed predominantly of sandstone units. Subsurface mapping of the Wilberns shows the sandstones to be persistent throughout End_Page 517------------------------------ most of the study area. The Fort Chadbourne fault system is a linear zone of deformation trending from Sutton County northward into northeastern Nolan County. This structural zone contains uplifted and faulted Wilberns Formation. En echelon faults and folds as the predominant structures along the Fort Chadbourne fault system suggest wrench faulting. The orientation of these faults (north-south) and folds (northeast-southwest) suggests that the wrench system had left-lateral movement that was produced from compressive forces active during the Ouachita orogeny. Based on thin-section analyses of core samples, the general paragenetic sequence for the upper sandstone units of the Wilberns Formation consists of at least 4 diagenetic stages: (1) reduction of primary porosity and development of quartz overgrowths resulting from burial and compaction; (2) precipitation of dolomite cement into remaining pore space; (3) dissolution of calcareous material and development of secondary porosity in response to uplift and exposure; and (4) precipitation of hematite, pyrite, and clays as pore lining and pore fill after reburial (hydrocarbon migration probably occurred during this stage). This sequence of diagenetic events is responsible for the excellent reservoir quality of the sandstone units within the Wilberns Formation found along the Fort Chadbourne fault system. End_of_Article - Last_Page 518------------

Cambrian paleomagnetism of the Llano Uplift, Texas

Late Cambrian sandstones and limestones sampled from various members of the Riley and Wilberns formations of the Llano uplift show a progression of paleomagnetic pole positions as a function of age. The members, ages, and poles are the following for the Riley Formation: the Hickory Sandstone, Lower Dresbachian, 34°N, 145°E; the Cap Mountain Limestone, Dresbachian, 33°N, 140°E; the Lion Mountain Limestone, Upper Dresbachian, 24°N, 146°E. For, the Wilberns Formation they are the following: the Welge Sandstone/Morgan Creek Limestone, Lower and Middle Franconian, 24°N, 151°E; and the Point peak Shale, Upper Franconian, 6°N, 159°E. These poles are based on thermal, chemical, and alternating field demagnetizations and on vector analysis. Most directions are interpreted to be of reversed polarity, but the Cap Mountain Limestone also yielded normal polarity directions; all directions resided in hematite with blocking temperatures up to 680°C. Almost all Cambrian poles appear to fall in a broad streak between the equator at about 155°E (e.g., the poles from the Tapeats Sandstone and from the Late Cambrian Point Peak Shale of the Wilberns Formation) and about 60°N, 90°E, where Cambrian poles have been obtained by Elston and Bressler (1977) and French et al. (1977). Although, at the present time, partial to complete remagnetization or nondipole behavior of the geomagnetic field are adequate ad hoc hypotheses to explain some of the data, it is suggested that the simplest and most unifying hypothesis to explain all data involves a Cambrian loop of apparent polar wander with respect to North America. This loop occurs before the middle Late Cambrian, with the poles from the Llano uplift falling on the return track.

Paleomagnetism of the Wilberns Formation (Texas) and the Late Cambrian paleomagnetic field for North America

The natural remanent magnetizations of Late Cambrian siltstones of the Wilberns Formation have been analyzed with alternating field, thermal, and chemical demagnetization techniques. From mainly thermal demagnetization analyses a mean direction of magnetization has been calculated: declination D of 101.5°, inclination I of +13.5°, k = 171, and α95 = 4.6° for seven sites (55 samples), yielding a virtual pole position at 6.4°N, 158.8°E, dp = 2.4°, and dm = 4.7°. This polar error (dp, dm) represents the oval of 95% confidence about the pole position. Comparisons with previously published Late Cambrian data reveal good agreement between the thermally isolated directions of magnetization. The difference in the published results obtained through alternating field demagnetization can be explained by a late Paleozoic remagnetization or by a previously unexpected Cambrian shift or loop of the apparent polar wander path.

Paleoenvironment, Algal Structures, and Fossil Algae in the Upper Cambrian of Central Texas

For many years, algal limestone bodies have been known in the Point Peak and San Saba Members of the Wilberns Formation (Late Cambrian (Dresbachian) to Early Ordovician) in the Llano Uplift region of central Texas, but they have never been studied in detail. The relationships between environment, fossil algae, and algal structures are not well understood, and the descriptive terminology is becoming more complicated. The writer has examined algal macrostructures along with their corresponding microfabrics and fossil algae from a paleoecologic viewpoint. Several previously unknown microfabrics have been found that reflect the growth patterns of various algal taxa in response to different hydrologic conditions. For example, fossil algae with wispy or tufted growth forms are found in nonlaminated algal macrostructures (limited hydrologic stress) while laminated macrostructures such as stromatolites exhibit tightly laminated microfabrics (considerable hydrologic stress). The various environmentally produced structures are found commonly in sequences that depict the long-term paleoecologic history of the algal reefs from their inception (commonly in the sublittoral) to their culmination (commonly in the intertidal). Four different genera of fossil algae, Girvanella, Epiphyton, Renalcis and Nuia , were found in this study.

Trilobites of the Upper Cambrian Ptychaspid Biomere Wilberns Formation, Central Texas

Trilobites collected during the past 20 years from the Morgan Creek, Point Peak, and San Saba Members of the Wilberns Formation comprise 89 species assigned to 45 genera belonging to zones of the upper Franconian and Trempealeauan Stages of the Upper Cambrian Croixan Series. New zonal names are proposed in the interest of a regionally applicable nomenclature. Although none of the zonal nomenclature is identical to that of the 1944 Cambrian Correlation Chart of Howell et al., the four zones recognized in central Texas are equivalent to the eight highest zones on the Chart. Stratigraphically lowest is the Franconian Taenicephalus Zone, with a locally recognized Parabolinoides Subzone at its base; this zone is equivalent to the Conaspis Zone of the Correlation Chart. The Franconian Idahoia Zone, with a locally recognized Idahoia lirae Subzone at its base, is equivalent to the Ptychaspis Subzone of the Ptychaspis-Prosaukia Zone of the Correlation Chart. The sparsely fossiliferous Ellipsocephaloides Zone corresponds to the Prosaukia Subzone of the Ptychaspis-Prosaukia Zone on the Chart. Almost two-thirds of the trilobite species described occur in the Trempealeauan Saukia Zone, which corresponds to the five highest zones of the Correlation Chart; local subzones, in ascending order, are the Saukiella pyrene Subzone, the Saukiella junta Subzone, the Saukiella serotina Subzone, and the Corbinia apopsis Subzone.The succession of ptychoparioid trilobite faunas within these zones characterizes the Ptychaspid Biomere. The base of the biomere is at the base of the Taenicephalus Zone; the top coincides with the lowest occurrence of an Ordovician trilobite fauna. Trilobite families that characterize the Ptychaspid Biomere are the Ptychaspididae and the Parabolinoididae.Systematic descriptions include two new subfamilies, Drumaspidinae and Ptychaspidinae, and eight new species, Conaspis leptoholcis, Idiomesus infimus, Euptychaspis frontalis, Keithiella scapane, Saukiella serotina, Prosaukia remora, Calvinella prethoparia, and Westonaspis? texana.

Paleoenvironment, Algal Structures, and Fossil Algae in Upper Cambrian of Central Texas: ABSTRACT

The presence of algal limestone bodies in the Point Peak and San Saba Members of the Wilberns Formation (late Dresbachian to Early Ordovician) in the Llano uplift region of central Texas has been known for many years, but they have not been studied in detail. The relations between environment, fossil algae, and algal structures are not well understood, and the descriptive terminology is becoming more complicated. The writer examined algal macrostructures together with their corresponding microfabrics and fossil algal flora from a paleo-oceanographic viewpoint. Several previously unknown microfabrics were found that reflect the growth patterns of various algal taxa in response to different hydrologic conditions. For example, fossil algae with wispy or tufted growth forms are found in nonlaminated algal macrostructures (limited hydrologic stress), whereas laminated macrostructures such as stromatolites exhibit tightly laminated microfabrics (considerable hydrologic stress). The various environmentally produced structures are found commonly in sequences that depict the long-term oceanographic history of the algal limestone buildups (reefs) from their inception (commonly in the sublittoral) to the r culmination (commonly in the intertidal). Four genera of fossil algae found in this study were Girvanella, Epiphyton, Renalcis, and Nuia. End_of_Article - Last_Page 703------------

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.