Gerilyn S. Soreghan

Amplitudes of Late Pennsylvanian glacioeustasy

The late Paleozoic is well documented as a time of significant continental glaciation, but the extents of the glaciation and attendant glacioeustasy are not well constrained because precise amplitudes of eustasy are difficult to extract from the stratigraphic record. In this paper, we use preserved relief on ancient subaerial exposure surfaces of large algal bioherms to demonstrate directly that Late Pennsylvanian glacioeustasy reached minimum amplitudes of 80 m and probably exceeded 100 m. Upper Paleozoic algal bioherms accreted predominantly during sea-level falls, but also during sea-level rises and highstands, and were capable of remarkably rapid growth rates. Eustatic amplitudes in excess of 100 m approach amounts documented for the Pleistocene, and place constraints on models for Gondwanan ice volume, climate dynamics, and potential character and magnitude of glacioclimatic fluctuations.

Equatorial Aridity in Western Pangea: Lower Permian Loessite and Dolomitic Paleosols in Northeastern New Mexico, U.S.A.

ABSTRACT Lower Permian strata have been extensively cored in the subsurface of the Bravo Dome field, northeastern New Mexico. Analysis of core indicates that these strata consist of conglomeratic and sandy fluvial deposits and volumetrically significant eolian silt (loessite). Fluvial facies dominate the lower half of the study interval and include matrix-supported, massive conglomeratic debris-flow units and laminated arkosic sandstone, whereas loessite dominates the upper half of the study section and consists of massive, well-sorted quartzose siltstone that locally reaches thicknesses as much as 120 m in the greater study region. Paleosols are present throughout the study interval and consist of protosols and dolosols, commonly exhibiting vertic features. Dolomite that is interpreted to be of pedogenic origin is an unusual but volumetrically significant component in these paleosols. Paleogeographic reconstructions and paleomagnetic data indicate that these strata accumulated at equatorial (3-8°) latitudes, but depositional and pedogenic evidence both suggest seasonally wet to markedly arid conditions from early Wolfcampian to early Leonardian time. The loessite covers a substantial area (> 6000 km2), making this the largest pre-Cenozoic loess accumulation yet documented. This is significant, because loess generally suggests arid to semiarid conditions. Intercalated paleosols in the loessite section record repeated cessation of silt influx coupled with landscape stability, which we relate to high-frequency oscillation between dry and slightly wetter conditions, possibly attributable to glacial-interglacial climatic conditions that prevailed at low latitudes. At a lower frequency, the evolution from a predominance of fluvial to primarily eolian strata, in tandem with changes in pedogenic character, reflect a long-term aridification for the study interval. These data corroborate independent inferences of monsoon-induced equatorial aridity in western Pangea and help constrain the timing of the zonal-to-monsoonal transition to earliest Permian time.

Sedimentologic-magnetic record of western Pangean climate in upper Paleozoic loessite (lower Cutler beds, Utah)

Sedimentologic, pedologic, and magnetic data within the upper Paleozoic lower Cutler beds of the southwestern Paradox basin (Utah) record high- and low-frequency climate changes that operated at equatorial latitudes of western Pangea. The lower Cutler beds consist of ∼250 m of lithified eolian silt (loessite) and marine-reworked and fluvially reworked loessite, with abundant intercalated paleosols comprising Protosols, Argillisols, and Calcisols. The evolution from loessite and marine-reworked loessite with abundant Calcisols in the lower section to loessite and fluvially reworked loessite with abundant Argillisols in the upper section records a long-term transition from semiarid conditions in western equatorial Pangea in latest Pennsylvanian time to seasonally wet conditions in earliest Permian time. This shift could record intensification of the Pangean megamonsoon and associated seasonal incursions of moisture-laden westerlies. Paleosols record relatively high-frequency fluctuations between drier, dustier glacials and wetter interglacials of the late Paleozoic. Bulk magnetic-susceptibility values in paleosols exhibit variations that track paleosol type and are significantly elevated relative to parent loessite, attributable to the occurrence of both ultrafine-grained (superparamagnetic) and coarser-grained (remanence-carrying) magnetite. This signature reflects in situ pedogenic production of ferrimagnetic phases and a subordinate component of allochthonous, magnetic dust influx during pedogenesis, analogous to processes inferred for the magnetic signature in the Pliocene–Pleistocene loess-paleosol sequences of, e.g., the Chinese Loess Plateau. Integration of sedimentologic, geochemical, and magnetic data further suggests that enhancement of magnetic susceptibility in loessitic paleosols of this section relates primarily to climatic conditions and secondarily to durations of pedogenesis. Whereas peak susceptibility values in mature paleosols (Argillisols and Calcisols) do not vary significantly through the study section, peak values for Protosols track facies evidence for wetter conditions through time. Accordingly, relative changes in paleosol susceptibility values can provide paleoclimatic information, but should be integrated with other data to fully assess the origin of the signature. Overall, our data document the applicability of analytical approaches used on recent loess to very ancient loessite; this result is significant, because loess commonly records high- resolution evidence of terrestrial climate and climate change.

Anomalous cold in the Pangaean tropics

The late Paleozoic archives the greatest glaciation of the Phanerozoic. Whereas high-latitude Gondwanan strata preserve widespread evidence for continental ice, the Permo-Carboniferous tropics have long been considered analogous to today9s: warm and shielded from the high-latitude cold. Here, we report on glacial and periglacial indicators that record episodes of freezing continental temperatures in western equatorial Pangaea. An exhumed glacial valley and associated deposits record direct evidence for glaciation that extended to low paleoelevations in the ancestral Rocky Mountains. Furthermore, the Permo-Carboniferous archives the only known occurrence of widespread tropical loess in Earth9s history; the volume, chemistry, and provenance of this loess(ite) is most consistent with glacial derivation. Together with emerging indicators for cold elsewhere in low-latitude Pangaea, these results suggest that tropical climate was not buffered from the high latitudes and may record glacial-interglacial climate shifts of very large magnitude. Coupled climate–ice sheet model simulations demonstrate that low atmospheric CO 2 and solar luminosity alone cannot account for such cold, and that other factors must be considered in attempting to explain this “best-known” analogue to our present Earth.

Paleowinds inferred from detrital-zircon geochronology of upper Paleozoic loessite, western equatorial Pangea

U-Pb geochronology of detrital zircons from upper Paleozoic loessite (western United States) provides data bearing on atmospheric circulation within western equatorial Pangea. Zircon age spectra of four loessites from three localities representing middle Pennsylvanian (Desmoinesian) and Early Permian (Wolfcampian) time vary significantly, reflecting changing provenances attributable to temporal and spatial shifts in winds. Zircons from two Desmoinesian samples (from Arizona and Utah) show a dominant mode between 1800 and 1600 Ma, reflecting the Yavapai-Mazatzal terranes that cored the Ancestral Rockies uplifts and suggesting northeasterly winds. Both samples also contain a secondary cluster of Grenvillian grains (1300–1000 Ma), reflecting a south-southeasterly source. Ages for Wolfcampian samples (from New Mexico and Utah) differ from one another; the New Mexico loessite contains a large mode at 1700 Ma, missing in the Utah sample, reflecting their locations on opposing sides of the Ancestral Rocky Mountains, within a westerly wind regime. Inferred easterly winds for middle Pennsylvanian time match model predictions, and the presence of both northerly and southerly directions might reflect time-averaged fluctuation of the Inter-Tropical Convergence Zone. In contrast, monsoonal circulation and attendant westerly winds appear to have been well established by earliest Permian time.

Walther's law, climate change, and upper Paleozoic cyclostratigraphy in the ancestral Rocky Mountains

Geologists routinely apply Walther9s Law in interpreting paleoenvironments and paleogeographic scenarios from stratigraphic successions. The application of Walther9s Law, however, may be problematic in strata that have been influenced by significant and geologically rapid climate change. Such climatic fluctuation can force depositional environments to change fundamentally rather than migrate laterally, resulting in a stratigraphic succession wherein depositional facies vertically superposed rarely or never coexisted laterally. Upper Pennsylvanian carbonate-siliciclastic cycles deposited in basins of the southern Ancestral Rocky Mountains display stratigraphic relations that cannot be explained by autogenic and/or glacioeustatic mechanisms alone; rather, intracyclic (glacial-interglacial) climate change was a significant influence. Cycles of the northern Pedregosa basin, for example, comprise eolian-marine siltstone in sharp contact with subtidal-peritidal carbonate facies; absence of facies mixing suggests a climatic control on silt influx. Cycles of the western Orogrande basin comprise deltaic siliciclastic strata in sharp contact with subtidal-peritidal carbonate facies; absence of facies mixing, truncated siliciclastic progradation, and carbonate facies with emergence features similarly support a climatic influence on the siliciclastic component. In both systems, intracyclic (glacial-interglacial) climatic shift reconfigured depositional environments. In cases such as these, application of Walther9s Law could produce erroneous conclusions. Severe glacial-interglacial climate change helps explain the pervasive association of mixed carbonate-siliciclastic cyclicity that typifies upper Paleozoic strata in many parts of the world.

Pedogenically enhanced magnetic susceptibility variations preserved in Paleozoic loessite

Magnetic susceptibility variations observed in Quaternary loess sequences of China have been linked to pedogenesis and used to characterize terrestrial climatic variations through glacial-interglacial transitions. To date, however, magnetic studies of loess sequences have been applied exclusively to Quaternary deposits. Here we demonstrate preservation of magnetic susceptibility variations in an upper Paleozoic loessite sequence of western North America. Magnetic susceptibility through a continuously exposed test section of the loessite varies by more than an order of magnitude: low values occur in unaltered loessite horizons, and high values are found in sedimentologically confirmed or suspected paleosol horizons. Low-temperature remanence and hysteresis data indicate that the susceptibility variations between paleosol and loessite horizons relate to differences in concentration and grain size of the magnetic carrier. These results are consistent with in situ formation of ultra-fine-grained (superparamagnetic) material during pedogenesis. We suggest that these susceptibility variations relate to periodic pedogenesis linked to late Paleozoic climatic fluctuations in low latitudes of western Pangea.

Paleoclimatic inferences from paleopedology and magnetism of the Permian Maroon Formation loessite, Colorado, USA

The Maroon Formation in the eastern Eagle basin (Colorado) consists of >700 m of lithified loess with >200 paleosols interpreted as Protosols and Argillisols on the basis of field, petrographic and geochemical data. Additionally, magnetic susceptibility aids assessment of the intensity of pedogenesis. Bulk magnetic susceptibility (χ b ) through the section repeatedly fluctuates between low values (average 3.51 × 10 −8 ± 1.59 × 10 −8 m 3 /kg) in parent loessite and higher values (average 5.70 × 10 −8 ± 2.70 × 10 −8 m 3 /kg) in paleosols. Moreover, magnetic susceptibility positively correlates with abundance of clay-sized material as well as Al 2 O 3 and K 2 O and effectively distinguishes Protosols and Argillisols. Low- temperature demagnetization indicates the presence of ultra-fine-grained magnetite. The integration of geochemical, petrographic, and rock magnetic data suggest that changes in magnetic susceptibility reflect pedogenesis and relate primarily to climate- and time-dependent pedogenic production and concentration of ultra-fine- grained magnetite. The Maroon Formation loess and associated soils accumulated in an overall arid system, documented in part by formation of incipiently formed paleosols developed into Argillisols by eolian clay and carbonate additions rather than by in situ clay formation. However, the paleosols showing bulk magnetic susceptibility values of greater than 200 χ b document a high- frequency (10 4 −10 5 yr) fluctuation between arid times of loess accumulation and slightly wetter times of reduced silt influx and resultant pedogenesis. This fluctuation likely reflects glacial-interglacial climate shifts that operated in low-latitude Pangea during icehouse conditions. These results suggest that climate-related magnetic susceptibility variations within loess successions can be preserved and useful in very ancient (pre– Pliocene–Pleistocene) sequences.

Load-induced subsidence of the Ancestral Rocky Mountains recorded by preservation of Permian landscapes

The Ancestral Rocky Mountains (ARM) formed a system of highlands and adjacent basins that developed during Pennsylvanian–earliest Permian deformation of interior western North America. The cause of this intracratonic deformation remains debated, although many have linked it to far-field compression associated with the Carboniferous–Permian Ouachita-Marathon orogeny of southern North America. The ultimate disappearance of the ARM uplifts has long been attributed to erosional beveling presumed to have prevailed into the Triassic–Jurassic. New observations, however, indicate an abrupt and unusual termination for the largest of the ARM uplifts. Field evidence from paleohighlands in the central ARM of Oklahoma and Colorado indicates that Lower Permian strata onlap Pennsylvanian-aged faults and bury as much as 1000 m of relief atop the paleohighlands. In parts of Oklahoma and Colorado, late Cenozoic partial exhumation of these paleohighlands has revealed landscapes dating from Permian time. These relationships suggest cessation of uplift followed by active subsidence of a broad region that encompassed both basins and uplifted crustal blocks and that commenced in Early Permian time, directly following the Pennsylvanian tectonic apogee of the ARM. Independent from these geological observations, geophysical data reveal a regional-scale mafic load underpinning these paleohighlands, emplaced during Cambrian rifting associated with the southern Oklahoma aulacogen. Geophysical modeling of the effects of such a load in the presence of a horizontal stress field, such as that implied by ARM orogenesis, indicates that the amplitude of flexurally supported features is modulated nonlinearly. This leads to buckling and thrust formation with the application of sufficient compressive stress, and subsidence of topography formed by buckling upon relaxation of the high compressional stresses. We therefore infer that the core ARM highlands subsided owing to the presence of a high-density upper crustal root, and that this subsidence began in the Early Permian owing to relaxation of the in-plane compressional stresses that had accompanied the last phase of the Ouachita-Marathon orogeny of southern and southwestern Laurentia. Our results highlight the importance of tectonic inheritance in intraplate orogenesis and epeirogenesis, including its potential role in hastening the reduction of regional elevation, and enabling the ultimate preservation of paleolandscapes.

An Exhumed Late Paleozoic Canyon in the Rocky Mountains

Landscapes are thought to be youthful, particularly those of active orogenic belts. Unaweep Canyon in the Colorado Rocky Mountains, a large gorge drained by two opposite‐flowing creeks, is an exception. Its origin has long been enigmatic, but new data indicate that it is an exhumed late Paleozoic landform. Its survival within a region of profound late Paleozoic orogenesis demands a reassessment of tectonic models for the Ancestral Rocky Mountains, and its form and genesis have significant implications for understanding late Paleozoic equatorial climate. This discovery highlights the utility of paleogeomorphology as a tectonic and climatic indicator.