Rex E. Crick

Magnetosusceptibility event and cyclostratigraphy method applied to marine rocks: detrital input versus carbonate productivity

Magnetic susceptibility data from marine rocks can be used for global correlation due to synchronous variations in global erosion. We show here that the magnetic susceptibility signature, found in two forms, resides mainly in paramagnetic and other detrital constituents in most marine rocks. The first form is a short-term, low-magnitude, high-frequency cyclic climate signature that is often useful for regional correlation. The second form is a longer term, higher magnitude, low-frequency signature resulting from transgressive and regressive events that can be used for global correlation. Fluctuations in detrital input, due to eustatic-based erosion, are the primary cause of events. These fluctuations are driven by large-scale processes such as global orogenic cycles. However, variations in carbonate productivity cannot be ruled out when explaining the low-magnitude climate-driven cyclicity also observed in magnetic susceptibility data sets.

The Magneto-Susceptibility Event and Cyclostratigraphy (MSEC) Method Used in Geological Correlation of Devonian Rocks from Anti-Atlas Morocco

Magneto-Susceptibility Event and Cyclostratigraphy (MSEC) is used here as a means of establishing sensitive chronostratigraphic markers and chronohorizons useful for high-resolution correlation. MSEC is a composite of the magnetic susceptibility (MS) record of marine strata and the coeval biostratigraphic record and, similar to chronozones, zones based on MSEC data have boundaries that are isochronous. MS, a measure of the concentration of magnetic grains in sediments, proxies for the ratio of lithogenic to biogenic components. Controls on the detrital input of lithogenic material include eustasy and climate, however induced, and sea floor/basin subsidence. The development of an initial MSEC composite reference curve for the Middle-Upper Devonian of the Anti-Atlas region of southern Morocco is presented and discussed in the context of global sea level transgressive and regressive cycles. MSEC trends, composed of increasing MS magnitudes, correlate well with episodes of regression, whereas trends of decreasing MS magnitudes correlate with episodes of transgression.

Magnetostratigraphy susceptibility of the Frasnian/Famennian boundary

Magnetosusceptibility event and cyclostratigraphy (MSEC) is used to establish a non-polarity-based magnetostratigraphy susceptibility (MSS) between the stratotype region for the Frasnian/Famennian (F/F) boundary sequence in the Montagne Noire of southern France, the eastern Rheinisches Schiefergebirge of Germany, Belgium, the eastern Anti-Atlas of Morocco, and the Arbuckle Mountains of southern Oklahoma (USA). Despite differences of depositional environment, the MSS is remarkably consistent and can be described in the context of a hierarchy of magnetozones that allow the extension of correlation away from the Montagne Noire reference section. The nature of the controls on the influx of iron into the marine system produces a natural hierarchy of at least seven orders or magnetozones designated MSZ1, MSZ2, MSZ3, MSZ4, MSZ5, MSZ6, and MSZ7. These are characterized in terms of the magnitude of their duration. The MSS reference section of choice for the F/F boundary is the well-known Trench C at La Serre (LSC) in the northeastern Montagne Noire of southern France. The F/F boundary is located at the lower boundary of magnetozone La Serre Iα3b at La Serre as well as in all other sections studied. MSZ4 and MSZ5 magnetozones are used to establish intra- and inter-regional correlation between the reference sequence and the other regions. The base of the Upper Kellwasser event is as variable within the MSS zonation as it is within the biostratigraphic zonation, as is the position of the Lower Kellwasser event.

Global correlation using magnetic susceptibility data from Lower Devonian rocks

Magnetic susceptibility data from marine rocks can be used for global correlation because of synchronous variations in global erosion. We show here correlations between magnetic susceptibility data from two Lower Devonian, biostratigraphically well defined sections, one in northwestern Spain, and one from the Anti-Atlas region of Morocco. Using the Moroccan outcrop section, we correlate to cuttings from a Bolivian well with minimal biostratigraphic control. Using the magnetic susceptibility data, we identify the Lower Devonian, Pragian-Emsian, and lower-upper Emsian stage boundaries in the Bolivian well samples.

Sr, Mg, Ca and Mn chemistry of skeletal components of a Pennsylvanian and recent nautiloid

Abstract The atomic ratios of Sr/Ca, Mg/Ca, and Mn/Ca were determined for mineralogically and chemically unaltered shells and cameral deposits of three species of the orthoconic nautiloid Mitorthoceras Gordon (Pennsylvanian, Desmoinesian—Westphalian C) and for shells of four species of Nautilus Linnaeus. These data are used to investigate and document three aspects of nautiloid physiochemistry: (1) differences in the Sr/Ca ratios of shell and cameral deposit aragonite of Mitorthoceras species indicate that the fluids from which shells and cameral deposits were precipitated were of different compositions; and (2) although Sr/Ca ratios of shells are the same for species of Mitorthoceras, significant differences in the Mg/Ca ratios, and to some extent the Mn/Ca ratios, of shells of these species conform to the specific taxonomy of the genus and reflect physiochemical differences between these species. Among species of Nautilus it is the Mg/Ca and Sr/Ca ratios of shells that conform to the accepted specific taxonomy of the genus while Mn/Ca ratios are invariant among species. Third, differences in the Sr, Mg, and Mn content of shells of Mitorthoceras and Nautilus may reflect evolution of the physiochemical system of nautiloid cephalopods or differences in seawater chemistry of their respective premortem habitats.

The practicality of vertical cephalopod shells as paleobathymetric markers

The depth to which an intact coiled cephalopod shell will sink with its plane of symmetry vertical is related to shell geometry. Equations that relate shell geometry to physical constants of pressure, sea-water density, and gravity yield the maximum depth of stable verticality of a shell. Water depths calculated from the geometries of vertical shells of Cymatoceras hilli (Shattuck), Drakeoceras drakei Young, and Mortoniceras wintoni (Adkins) from the Fort Worth Formation (Upper Cretaceous, Albian) of north-central Texas are used to reconstruct a portion of the paleobathymetry of the East Texas Embayment during late Fort Worth time. The maximum water depth to which shells with these geometries would have sunk with the plane of symmetry vertical range from 1.6 m for the least stable shell geometry ( D. drakei ) to 2.6 m for the most stable shell geometry ( C. hilli ). Water depths were deeper than 2.6 m at localities where no vertical shells were observed and shells with the plane of symmetry horizontal showed no evidence of being reworked from the vertical orientation.

Biomineralization and Systematic Implications

Nautilus is the only cephalopod with an external, camerated shell composed of calcium carbonate (CaCO3). Interest in the chemistry of the calcified structures of Nautilus has developed slowly during the past three decades, but sufficiently to provide insight into the chemical relationship of calcified structures, cephalopod biomineralization, and the physiology of Nautilus. Understanding the skeletal chemistry of Nautilus provides a means of studying the cephalopod physiochemical system, which facilitates the exploration of four factors: (1) the differences in the genetic makeup of the physiochemical system responsible for CaCO3 production among species and populations, which forms the basis for biochemical taxonomy and its phylogenetic implications; (2) the ontogenetic variation in the chemistry of shell aragonite and its possible relationship to the life cycle of Nautilus; (3) the concentrations of trace elements in skeletal carbonate unrelated to CaCO3 production, which serve as indicators of nutrient concentrations in seawater; and (4) the implications of all aspects of Nautilus chemistry for the study of similar problems in extinct cephalopods, the skeletons of which have survived in a chemically unaltered state.

Ordovician endocerid genus Anthoceras: its occurrence and morphology

The description of the type species, Anthoceras decorum Teichert & Glenister 1954, is emended, based on collections from Mt Arrowsmith, New South Wales. The generic description is emended to include changes made to the type species and attributes of Siberian and Chinese species. Range of the genus is Arenigian.

Skeletal chemistry of Nautilus and its taxonomic significance

Abstract The strontium (Sr) and magnesium (Mg) chemistry of the shell wall and septum as well as the spherulitic-prismatic and nacreous layers of these structures was determined for Nautilus species: N. belauensis, N. macromphalus, N. pompilius and N. scrobiculatus . Each species of Nautilus exhibits greater variability and higher concentrations of Mg in juvenile portions of the shell than in more mature portions of the shell. This decrease in the variability and amount of Mg in the aragonite lattice suggests a physiochemical system which becomes more efficient with time relative to carbonate production. Statistically significant differences in the Sr and Mg content of spherulitic-prismatic and nacreous layers of the shell and septum indicate that these layers were formed from extracellular fluids of different compositions. Concentrations of Sr and Mg in aragonite of the shell wall are characteristic for each species and sufficiently invariant within species to allow species of Nautilus to be distinguished statistically on the basis of either the Sr or Mg content of the shell wall or the Mg content of septa.

Late Ordovician (Caradoc-Ashgill) ellesmerocerid Bactroceras latisiphonatum of Irian Jaya and Australia

An Ordovician nautiloid ellesmerocerid originally described as Irianoceras antiquum (Plectronoceratidae) Kobayashi 1971 from Irian Jaya and assigned a probable age of post Tremadoc is shown to be a member of the Llanvirn ellesmerocerid genus Bactroceras Holm 1878 (Baltoceratidae). Irianoceras antiquum is placed in synonymy with Bactroceras latisiphonatum Glenister 1952 described from New South Wales, Australia. The new material provides a better analysis of the variability of the genotype, extends its geographic range, and documents the presence or former presence of upper Caradoc to lower Ashgill strata in Irian Jaya.