Earth and Planetary Science

A rock magnetic cyclo-stratigraphy was measured for the upper Barremian-lower Aptian Cupido Formation of northeast Mexico. Anhysteretic remanent magnetization (ARM; 97 μT DC field, 100 mT peak AC field) was used to measure magnetic mineral concentration variations for the uppermost 150 m of the Cupido Fm (the ‘Cupidito’). The Cupido Fm is characterized by meter-scale, peritidal and subtidal carbonate cycles representative of inner and middle platform settings. Isothermal remanent magnetization (IRM) acquisition experiments and SEM examination suggest that micron-sized detrital magnetite derived from eolian dust carries the ARM signal. At the sampled sections (Potrero Garcia and Potrero Chico, 30 km apart), ARM records a synchronous 30-35 m oscillation with maxima coinciding with 4th-order sequence boundaries; this oscillation is superimposed by prominent higher-frequency variability. Tuning the 30-35 m oscillation to a 405-kyr periodicity (long eccentricity) focuses the higher frequencies into the short eccentricity, obliquity, and precession bands; the precession-band signal modulates with a pronounced eccentricity signature. The ARM signal is tightly correlated between the two sections, but is decoupled from the 5th-order meter-scale cycles. ARM amplitudes diminish up-section as the facies record progressive deepening. We interpret that this deepening was associated with regional and/or global increases in temperature and humidity that would result in a decrease in the influx of eolian-derived magnetic grains to the study area. This Mexican ARM rock magnetic cyclo-stratigraphy can be correlated with Milankovitch cycles recorded in coeval Tethyan pelagic deposits at Gorgo a Cerbara and Cismon, Italy suggesting it is global in origin.

Rhythmically interbedded limestone and shale or limestone and chert ("rhythmites") are a common feature of many deep-water Phanerozoic carbonate marine deposits. Seventeen different Palaeozoic rhythmite successions from across North America (8 studied in detail, 9 studied in reconnaissance) are described and summarized. Individual rhythmite couplets (4-20 cm thick) are composed of fine-grained, laminated to massive detrital limestone alternating with shale (or marl), or laminated spiculitic chert. Stratigraphic and primary depositional features within rhythmites and associated facies indicates that the carbonate-rich and carbonate-poor interbedding is the result of repetitive changes in sediment input (primary) rather than due to diagenetic redistribution of calcareous material (secondary).

Basinal facies of the Middle Cambrian Marjum Formation (western Utah) and Middle Devonian Denay Limestone (central Nevada) are characterized by thin, rhythmically interbedded limestone and marl layers (deep-water rhythmites). Limestone layers (average thickness range of 4.3-5.5 cm) are composed of laminated to massive pelleted lime mudstone; marl layers (average thickness range of 0.7-1.7 cm) are characterized by laminated, argillaceous, dolomitic mudstone. The absence of current-or wave-reworked features, paucity of bioturbation and skeletal fossils, stratigraphic relationships with adjacent facies, and the dark color indicate that limestone and marl layers in both rhythmite successions were deposited in quiet, dysaerobic waters below storm-wave base. Both limestone and marl layers are composed of submillimeter-thick graded laminae which represent discrete depositional events from dilute density currents generated by high-frequency storms or distal turbidity currents. Fluctuations in primary pelagic productivity cannot account for variations in carbonate influx because calcareous microfossils did not evolve until the Mesozoic. Instead, the rhythmic interbedding is interpreted to reflect climatically controlled variations in fluvial and/or eolian influx, or changes in marine currents (storm or distal turbidity currents) which transported shallow platform-derived carbonate material into the deep-water region.

Rock magnetic variations record cyclicity within lithologically homogeneous basinal lime mudstones of the Lower Cretaceous San Angel Limestone, northeastern Mexico. Variations in ferromagnetic mineral concentrations, as measured by anhysteretic remanent magnetization (ARM), occur at frequencies consistent with Milankovitch orbital rhythms. Magnetic mineral compositions, grain-size distributions, and grain shapes from digested samples are congruent with far-traveled atmospheric dust. Prevailing winds and the proximity of the Cretaceous basin to an African eolian source support the encoding of orbitally modulated changes in wind intensity or source-area aridity. ARM measurements offer great potential to calibrate the pace of depositional processes in carbonates and to investigate high-frequency orbitally driven climate change in basinal strata throughout geologic time.

Study of cyclic carbonate platforms relies on field observations of repeating, often cryptic, shallowing upward facies. This can be difficult when cycles are thin or lack lithologic distinction. The cyclicity typically occurs at Milankovitch time scales, and needs to be analyzed for orbital forcing. An assessment of how the signal is encoded in the rocks is integral to assessing the validity of climate proxies. The challenge is how to render measured sections into objective time series using a facies-independent physical parameter.

Rocks of the Goshute-Toano Range, Elko County, Nevada, between White Horse Pass on the south and Silver Zone Pass on the north, record a complex Mesozoic and Tertiary history of metamorphism, contractional and extensional deformation, and igneous activity. Paleozoic strata comprise seven distinctive structural tracts, separated from each other by low-or high-angle faults. These tracts are grouped here into three structural-thermal levels separated by low-angle faults: (1) a lower level of Cambrian to Mississippian sedimentary rocks that were strongly folded and metamorphosed in Mesozoic time, (2) a middle level of Cambrian to Mississippian, nonmetamorphosed sedimentary strata that were heated moderately, weakly folded, thinned by bedding-parallel faults, and segmented by high-angle faults, and (3) an upper level of Pennsylvanian, Permian, and younger rocks that were heated only slightly and, over large areas, were broken up, tilted, and dispersed by normal faulting in Miocene time. Lower-level rocks were intruded in Middle Jurassic time by a granodiorite stock that transected metamorphic fabric. Middle-level strata were intruded in the Late Jurassic by granite bodies of extremely irregular shape and in Late Jurassic to middle Miocene time by stocks, dikes, and sills. Upper-level strata lack outcrops of intrusive rocks of any kind. Extensive Miocene rhyolite flows cover parts of middle-and upper-level strata. The Goshute-Toano Range is replete with welldeveloped examples of contractional and superposed extensional structures and offers abundant opportunities for potentially very productive detailed structural studies.