Kevin R. Evans

Evidence of major Middle Cambrian deformation in the Ross orogen, Antarctica

Long-lived polyphase tectonism in the Cambrian history of the Ross orogen is demonstrated by recent stratigraphic studies in both the central and Weddell Sea segments of the Transantarctic Mountains. We postulate that strong early Middle Cambrian deformation preceded a more widely recognized Late Cambrian-Early Ordovician period of contraction and magmatism. These Middle Cambrian movements were locally the principal phase of orogenic deformation. They profoundly altered sedimentation patterns along the Antarctic margin, so that in the Weddell Sea region, when carbonate platform sedimentation resumed, the platform margin was reestablished far oceanward of its Early Cambrian location. Intra-Cambrian tectonism along the paleo-Pacific margin of Gondwana was not confined to its Antarctic segment; comparable activity recently has been described from Australia and is known from South America. These movements may be related to amalgamation of East and West Gondwana.

Small shelly fossils from Antarctica: an Early Cambrian faunal connection with Australia

Kennardiids, members of a family of organisms that bore phosphatic sclerites, are present in Antarctica; previously, they had been found only in Australia. This new occurrence reinforces the concept of a faunal province shared between the continents during Early Cambrian time. Although the two known genera of kennardiids, Dailyatia Bischoff and Kennardia Laurie occur in Antarctica and Australia, no species are common to both continents. Dailyatia is interpreted as having had tightly fitting sclerites that armored a bilaterally symmetrical, bipolar body; two alternative reconstructions are presented. Taxa include two new species, Dailyatia braddocki and Dailyatia odyssei , and two unnamed species of Kennardia.

The Great American Carbonate Bank in the Miogeocline of Western Central Utah: Tectonic Influences on Sedimentation.

Cambrian and Ordovician strata in Millard and Juab counties, western central Utah, are a thick (17,500 ft [5334 m]) succession that was deposited on a tropical miogeoclinal platform that experienced rapid thermal subsidence after a Neoproterozoic sea-floor spreading ridge formed along the western margin of Laurentia. In this area, which includes the Cricket Mountains, Drum Mountains, Fish Springs Range, House Range, Confusion Range, and Wah Wah Mountains, the Cambrian to Middle Ordovician Sauk megasequence is approximately 15,875 ft (4839 m) thick, and the Upper Ordovician part of the Tippecanoe megasequence is approximately 1525 ft thick (465 m). Basal deposits of the Sauk megasequence are the transgressive Lower Cambrian Prospect Mountain Quartzite, and the top of the Sauk megasequence is the upper Whiterockian Watson Ranch Quartzite. Strata between these sandstones are mostly limestone with several shaly intervals. The Sauk megasequence is divided into four parts, Sauk I to IV, in this area, and these parts have been divided into smaller sequences. The Ordovician part of the Tippecanoe megasequence is mostly dolomite and quartzite. Major influences on the depositional history of these strata include rapid generation of accommodation space caused by thermal subsidence following continental rifting, in-situ generation of tropical carbonates that generally kept pace with accommodation, eustatic fluctuations, influx of siliciclastics during sea level lowstands, and vertical tectonic adjustments of regional tectonic elements inherited from Neoproterozoic rifting: the Wah Wah arch, House Range embayment, Tooele arch, and Ibex Basin. The resulting strata comprise one of the best known Middle Cambrian–Middle Ordovician stratigraphic successions in North America and include the reference sections of the Upper Cambrian Millardan Series and the Cambrian–Ordovician Ibexian Series. Stratigraphers established a Global boundary Stratotype Section and Point (GSSP) for the base of the Middle Cambrian Drumian Stage in the Drum Mountains and proposed another GSSP for the base of the uppermost Cambrian stage in the Wah Wah Mountains. Middle Cambrian–Middle Ordovician strata are very fossiliferous, and some intervals have incredibly abundant fossils, such as the numerous complete specimens of the Middle Cambrian trilobite Elrathia kingii in the central House Range. Trilobites, conodonts, brachiopods, and other fossil groups have been used for biozonation and correlation, and these strata comprise a North American standard for uppermost Cambrian–Middle Ordovician trilobite and conodont zonations. Upper Ordovician dolomites and quartzites are less fossiliferous. These Cambrian and Ordovician strata are the lower half of a Lower Cambrian–Lower Triassic succession that is approximately 34,000 ft (10,300 m) thick and was thrust onto the Jurassic Navajo Sandstone in the southern Wah Wah Mountains during the Sevier orogeny. These strata are exposed in block-faulted mountain ranges resulting from basin and range extension during the late Tertiary.

A new post-Early Cambrian archaeocyath from Antarctica

A new species of archaeocyath sponge, Dictyocyathus neptunensis , is described from the lower third of the Nelson Limestone of the Neptune Range, Antarctica. Based on the presence of the trilobites Nelsonia schesis and Amphoton oatesi , these strata are interpreted to be mid- to late middle Cambrian in age. D. neptunensis shows a branching, low integration modular form and a very simple skeletal type. Such an organization is predicted from known morphological trends within the early Cambrian Archaeocyatha. Specimens are not in life position, and probably form part of a storm deposit. The absence of Dictyocyathus from latest early Cambrian (Toyonian) strata worldwide suggests a convergent nature for this highly simplified skeletal type in archaeocyaths.

Alumni of Geology B.S. Program Express Strong Support for Field Geology and Related Field and Laboratory Experiences

In a recent survey, alumni of a Bachelor of Science program in geology expressed strong support for a required summer Field Geology course and for related field and laboratory learning experiences in the undergraduate geology curriculum. On a 4-point scale (4=essential; 1=unimportant and unnecessary), geology alumni rated “summer Field Geology course” 3.77, higher than any of the ancillary requirements (mathematics, chemistry, physics, biology, computer skills) or independent study. When asked about possible options for cutting the costs of the B.S. program in geology, only 1.8% of the alumni favored reducing the ratio of laboratory experiences to lecture contact hours and only 2.3% favored reducing the number of field trips and other out-of-classroom experiences.

Sea-Level Changes and Stratigraphy of the Nelson Limestone (Middle Cambrian), Neptune Range, Antarctica

ABSTRACT The first detailed history of relative sea-level changes known from lower Paleozoic strata in Antarctica is documented in this study of the Nelson Limestone. Basal beds of the formation constitute a nonmarine terrigenous valley-fill succession. Thickness of these strata varies from 0.1 m to 72 m, suggesting that significant topographic relief existed below the sub-Nelson unconformity. Locally, base level must have risen at least 72 m because, following fluvial infilling of the valley, a succession of carbonates over 400 m thick was deposited as seas inundated the Antarctic margin of Gondwana during late Middle Cambrian time. On a global scale, the broad relative sea-level rise recorded in the Nelson Limestone is synchronous with previously recognized eustatic events during Lejopyge laevigata time. In the Nelson Limestone, smaller fluctuations of relative sea level are indicated by three sequences, each about 100 m thick. The lower sequence is bounded below by the sub-Nelson unconformity and includes the valley-fill succession, transgressive sandstone, and highstand carbonate deposits. The top of a subaerially exposed interval of carbonates in one section is interpreted as a sequence boundary with correlative conformity in the deeper-water areas. The subaerially exposed interval is roughly correlative with a succession of five or more slump deposits 1-2 m thick in another section. Subaerial exposure at nearly the same stratigraphic level as the slumps marked a fundamental shift in depositional style. Following marine flooding, an offshore carbonate shoal with an associated restr cted lagoon was established during deposition of the middle sequence. The upper sequence records marine flooding above the middle sequence and subsequent reestablishment of an offshore carbonate shoal prior to deformation and volcanism in the urea related to the Ross Orogeny.

The proposed GSSP for the base of Cambrian Stage 10 at the First Appearance Datum of the conodont Eoconodontus notchpeakensis (Miller, 1969) in the House Range, Utah, USA

The Stage 10 Working Group of the International Subcommission on Cambrian Stratigraphy is tasked with recommending a stratotype section and horizon for the base of Stage 10, the uppermost stage of the Cambrian System. We identify three sections in the House Range in western Utah, USA, for consideration as locations for defining and characterizing the base of the proposed stage. We also propose a boundary horizon at the base of the Eoconodontus conodont Zone combined with a distinctive negative carbon-isotope excursion named the HEllnmaria–Red Tops Boundary (HERB) event. These and other biological and nonbiological tools can be used for correlating the proposed stage.

Inexpensive Jacob's Staff with Laser Sight

This paper provides information on the construction and use of an inexpensive fiberglass Jacobs staff and laser sighting device. Laser technology can facilitate high-precision sighting over long and short distances when measuring stratigraphic sections.

Well Study—Sauk III-IV Interval in the American Quasar 16-21A Horse Heaven-state Well, Confusion Range, West-central Utah

More than 15,000 ft (4572 m) of Sauk megasequence strata are exposed in mountain ranges across the eastern Great Basin of western Utah (Figure 1; see Miller et al., 2012). The upper parts, Sauk II to Sauk IV, are particularly well exposed in the western desert of Utah, in and around the Ibex area (Figure 2). Division of the Sauk megasequence into four parts follows Miller et al. (2012). This area was also the location of several wildcat wells that were drilled in the late 1970s and early 1980s (Figure 1). From 1991 to 2003, Evans et al. have collected gamma-ray profiles from measured stratigraphic sections in the Confusion, Fish Springs, and House Ranges, as well as the Wah Wah Mountains and sections in eastern Nevada. Profiles from the Ibex area can be correlated confidently with some subsurface gamma-ray logs. As a consequence, key stratigraphic markers exposed in measured sections can be identified in well logs, and conversely, well logs can be used to help delineate the distribution of facies and large-scale structures. Figure 1. Locations of wells penetrating Cambrian and Ordovician strata and locations of measured stratigraphic sections with gamma-ray profiles in west-central Utah. ARCO = Atlantic Richfield Company. 10 mi (16.1 km). Figure 2. Lithostratigraphic units associated with the Sauk III-IV interval in the mountain ranges of west-central Utah. The gamma-ray profiles from measured stratigraphic sections are indicated at the right side.