As global climate change impacts our present day, understanding the intricate feedback mechanisms that regulate Earth's climate has become a focal point across geoscience disciplines. In the geological past, the Late Paleozoic Ice Age (LPIA) has a special relevance to current climate studies because it serves as our only deep-time analog for comprehending “climate change in an icehouse on a vegetated Earth” (Gastaldo et al., 1996; Montañez and Soreghan, 2006). This unique episode coincided with the highest rates of organic carbon burial in the past half billion years, resulting in low atmospheric pCO2 and high pO2 levels. These conditions likely sustained the longest-lived icehouse of the Phanerozoic Eon and may have catalyzed significant evolutionary innovations in terrestrial life, including such traits as insect flight and gigantism (Berner et al., 2007; Montañez, 2016).

Towards the end of the LPIA, during the C–P transition (∼ 304–290 Ma), a climatic shift to substantially more arid conditions resulted in the dramatic recession of the formerly vast paleotropical wetlands of low-latitude western and central Pangea (Sahney et al., 2010; Pardo et al., 2019). This loss of freshwater and classic “coal swamp” habitats, conceptualized as the Carboniferous rainforest collapse hypothesis, marked a profound shift in continental ecosystems. Indeed, many paleontological studies of Late Paleozoic central Pangea repeatedly demonstrate a stark contrast between Late Carboniferous paleoequatorial ecosystems, characterized by a blend of aquatic and terrestrial species and Cisuralian (Early Permian) environments dominated by more terrestrial faunal and floral assemblages (Figs. 1–2) (Vaughn, 1966, 1970; Olson and Vaughn, 1970; Frederiksen, 1972; Rowley et al., 1985; Gastaldo et al., 1996; Cleal and Thomas, 1999; Cleal and Thomas, 2005; Pardo et al., 2019). These changes also coincide with the emergence of the first large-bodied tetrapod herbivores, a pervasive feature of terrestrial ecosystems from the Permian through to the present day (Sues and Reisz, 1998; Reisz and Sues, 2000; Pearson et al., 2013; Reisz and Fröbisch, 2014; Brocklehurst et al., 2015).

Although at first glance this may suggest a mass extinction event, quantitative analysis of late Paleozoic equatorial fossil assemblages, as detailed by Pardo et al. (2019), supports the notion that this environmental and ecological shift was protracted and spatially diachronous, corroborating earlier hypotheses (Vaughn, 1966, 1970; Olson and Vaughn, 1970). This in turn led to the development of the Vaughn–Olson model (Pardo et al., 2019), which posits that the proliferation of terrestrial faunas during the C–P transition occurred earlier in western equatorial Pangea (Fig. 2) in the area roughly corresponding to the modern Colorado Plateau of southwestern North America.

Located within the hypothesized epicenter of dryland vertebrate expansion (Pardo et al., 2019), the Cutler Group in southeastern Utah, USA (Figs. 1–3), is ideal for studying this environmental and ecological transition. This unit's abundant fossil record (Fig. 1) and extensive outcrops of interbedded marine and continental “redbed” sediments preserve a diverse suite of depositional environments and associated biota spanning the C–P transition (Fig. 3), properties which make them a valuable resource for understanding the effects of this climatic transition on paleoequatorial ecosystems. Nevertheless, limited marine-biostratigraphic and geochronologic control (Fig. 3b) hinders a comprehensive paleoenvironmental reconstruction of these sediments and confident comparison to other contemporaneous fossil assemblages.

This collaborative research project, titled PERMIA (Paleozoic Equatorial Records of Melting Ice Ages), seeks to address these issues by studying the Elk Ridge no. 1 core (ER-1). This legacy core, collected in 1981 within what is now part of Bears Ears National Monument (Fig. 3a), recovered a significant portion of the lower Cutler Group and underlying units, making it an ideal archive for studying paleoenvironmental change during this critical period in terrestrial vertebrate evolution. Work is ongoing, yet much has been learned about the stratigraphic and geophysical properties through initial core processing. The purpose of this contribution is to introduce the project, share preliminary lithologic and geophysical results from the analysis of a legacy core (Elk Ridge no. 1), revise the position of lithostratigraphic boundaries within that core to align more accurately with the currently recognized regional stratigraphic framework, and discuss ongoing work with the core and complementary outcrop sites.

In early 2023, the top 450 m (corresponding to a depth of 127.77–581 m) of the ER-1 core was transported from the Austin Core Research Center to the Continental Scientific Drilling Facility at the University of Minnesota for preparation and analysis. Upon arrival, all core sections were photographed in their respective boxes and placed in transparent plastic core liners with end-caps indicating the stratigraphic top and bottom of each core section. The liners enabled detailed scanning and processing workflows aligned with best practices established in numerous recent scientific drilling projects (e.g., Clyde et al., 2013; Cohen et al., 2016; Olsen et al., 2018). Evidence of core degradation was minimal, limited to salt crystals from dried drilling fluid on the surface of some sandstone core segments. The geophysical properties of the whole cores were measured using a Geotek Standard Multisensor Core Logger and included magnetic susceptibility (loop), gamma density, p-wave velocity and amplitude, natural gamma radiation, and electrical resistivity.

Using a diamond-blade rock saw with continuous deionized water flush, the core was split lengthwise into two halves: an archive half which was placed into storage and a work half which was subjected to further analysis and subsampling. To eliminate saw marks and enhance the clarity of the split core face, the working halves were polished using either dry or wet rock polishers. This step was necessary to more clearly image sedimentary structures and microfossils that were obscured by the cutting process. The polished working halves were imaged at 20 pixels per millimeter resolution using the Geotek Core Imaging System. Additionally, high-resolution point magnetic susceptibility and color reflectance spectrophotometry of the core face were measured using a Geotek XYZ Multisensor Core Logger. Core depth scales were generated by applying a linear compression factor for core runs to fit the drilled interval when recovery wad > 100 % (equivalent to the CSF-B depth scale used in scientific ocean drilling, e.g., Expedition 337 Scientists, 2013).

Employing Corelyzer software, the PERMIA team directly compared the physical core sections with their high-resolution images and accompanying geophysical datasets (Ito et al., 2023). These visual representations enabled comprehensive lithologic descriptions of each core section, which were recorded using the PSICAT software program (Reed et al., 2007).

One of the initial goals of PERMIA was to refine the stratigraphy of ER-1 within an updated regional stratigraphic framework using currently accepted terms (Loope et al., 1990; Fig. 5). Examination of the polished split cores and high-resolution core images (e.g., Fig. 6) in the stratigraphic context using the Corelyzer visualization environment (Ito et al., 2023) allowed for more accurate determinations of lithology, stratigraphic contacts, sedimentary structures, and microfossils. By comparing this new dataset to recent stratigraphic work (Loope et al., 1990; Dubiel et al., 1996, 2009; Condon, 1997; Ritter et al., 2002; Mountney and Jagger, 2004; Jordan and Mountney, 2012), PERMIA updated the stratigraphy of the upper core to reflect more accurately the accepted lithostratigraphic nomenclature (Fig. 5) and associated boundaries (Fig. 7). Specifically, we place the Cedar Mesa Sandstone–lower Cutler beds contact at 318.9 m depth and the lower Cutler beds–Honaker Trail Formation (Cutler Group–Hermosa Group) boundary at 442.6 m depth. These placements are consistent with the lithostratigraphic boundaries as observed in nearby outcrops (e.g., the Arch Canyon and Valley of the Gods area) and descriptions in the published literature.

Our placement of the Cutler–Honaker Trail boundary is at a marked change to redder, predominantly non-calcareous siltstone with less abundant and much thinner marine carbonate layers and more common sandstone layers. This is consistent with the lithologic changes observed in outcrops along the southern margin of Cedar Mesa (e.g., Valley of the Gods and Goosenecks of the San Juan State Park areas), where a mixed siliciclastic–carbonate succession with abundant marine limestones (the Honaker Trail Formation) is overlain by siltstone and sandstone redbeds with rare, thin marine limestones (the local equivalent of the lower Cutler beds called the Halgaito Formation) (Baker, 1936; G. S. Soreghan et al., 2002; Ritter et al, 2002; Huttenlocker et al., 2021). The color change associated with this boundary is apparent in the core line scan image stack (Fig. 7), where our boundary placement also marks a change from drab brown and tan colors below to darker reddish-brown colors above. Also notable is that our placement of the Honaker Trail–Cutler boundary is just above three sandstone units associated with marine limestones (sandstones otherwise being rare in the Honaker Trail Formation), a lithologic association also observed in outcrops in the uppermost Honaker Trail Formation at its eponymous locality just south of Cedar Mesa (Ritter et al., 2002: Figs. 3, 5). Some previous stratigraphic studies emphasize that the contact in this area forms a change in the weathering profile from the canyon or cliff down-section to the slope and pillar-forming lower Cutler above (e.g., Baker, 1936; Gregory, 1938), but this cannot be evaluated in the core.

We place the lower Cutler beds–Cedar Mesa Sandstone boundary at the base of the first thick (> 20 m) cross-bedded sandstone, marking a rather abrupt change from strata dominated by dark reddish-brown siltstone with less common thin (typically < 5 m thick) sandstone packages, to thick (> 10 m), predominately cross-bedded light-colored sandstones separated by occasional thin reddish-brown siltstones (all < 5 m thick and often less than 1 m thick). This boundary placement is just above the highest unambiguous marine unit, a wavy-laminated dark reddish-brown sandy siltstone between 323 and 318.9 m depth containing abundant disarticulated echinoderm elements. Locating the lower Cutler–Cedar Mesa boundary above the last marine unit and at the base of the first thick sandstone sequence is also consistent with its placement in outcrops by recent stratigraphic studies (e.g., Stanesco and Campbell, 1989; Loope et al., 1990; Dubiel et al., 1996, 2009; Jordan and Mountney, 2012; Huttenlocker et al., 2021; Pettigrew et al., 2021). Our boundary placement is also associated with a noticeable decrease in magnetic susceptibility, gamma density, and natural gamma radiation values (Fig. 7), which other authors have observed at this contact from wells in the same area (Fig. 8 in Condon, 1997).

Overall, the core is dominated by a relatively small number of lithologies. Carbonate layers are typically either sandy to silty limestones containing abundant marine fossils (echinoderms, brachiopods, bivalves, fusulinids) or thin carbonate layers devoid of visible fossils and with mostly nodular textures. The fossiliferous marine carbonate facies are only observed in the Honaker Trail Formation and lower Cutler beds, whereas the non-fossiliferous carbonates are found throughout the core and appear to be mostly non-marine in origin (i.e., they are associated with pedogenically modified siltstones). Carbonate-rich siltstones are often associated with both of these limestone facies and are also present as thin layers associated with dark reddish-brown siltstones within the Cedar Mesa Sandstone. Brown to dark reddish-brown siltstones and sandy siltstones are either parallel-bedded and are sometimes laminated or massive. The latter are often both mottled and contain carbonate nodules. These siltstone facies comprise the majority of the Honaker Trail Formation and lower Cutler beds but also form thin (< 5 m thick) layers between the thick sandstones of the Cedar Mesa Sandstone. Predominantly orange, tan, and pinkish-grey fine- to medium-grained sandstones are typically trough cross-bedded or parallel-bedded, though a small number of units are apparently massive (acknowledging the restricted lateral view of a core). Sandstones are rare in the Honaker Trail Formation, except for an interval between 440 and 500 m depth, more common (but still subordinate to siltstone) in the lower Cutler beds, and dominate the Cedar Mesa Sandstone. In the latter, they are mostly trough cross-bedded, but parallel-bedded and massive units are present, particularly just above and below silty layers. The fact that we often observe gradational transitions between lithologies (i.e., sandy siltstones between sandstones and siltstones, calcareous siltstones between carbonates and siltstones) and a lack of observable erosive contacts supports our inference that the core lacks any significant discontinuities.

This variety of lithologies can be grouped into three informal lithofacies associations: (1) clastic to silty fossiliferous limestones; (2) massive to horizontally bedded calcareous and non-calcareous siltstone; and (3) massive, horizontally bedded, and cross-bedded fine-grained sandstone (Figs. 6–7). Siltstones in all three formations show extensive evidence of pedogenic modification (e.g., mottling, carbonate nodules) (Fig. 6). Though all three rock types are present within each formation, limestones become less abundant up-section, being most common and thickest in the Honaker Trail Formation, siltstones are most common in the Honaker Trail Formation and lower Cutler beds, whereas cross-bedded sandstones dominate the Cedar Mesa Sandstone.

Cyclic variation both within and among these three rock types is common throughout the core, regardless of stratigraphic unit. In the Honaker Trail Formation, this cyclicity is manifested predominantly as siltstone–limestone couplets, whereas in the overlying lower Cutler beds it consists of siltstone–sandstone couplets, with occasional limestone or calcareous siltstone layers in the middle of the siltstones. In the Cedar Mesa Sandstone, the main cyclicity is an alternation between trough cross-bedded sandstone, parallel-bedded or massive sandstone, and thin siltstone beds. These lithologic cycles match those observed by other authors in equivalent outcrops in Paradox Basin (e.g., Ritter et al., 2002; Mountney, 2006; Williams, 2009; Jordan and Mountney, 2012; Langford and Salsman, 2014; Olivier et al., 2023). Comparing these lithologic cycles to the geophysical data, siltstones generally have a much higher magnetic susceptibility than either sandstone or limestone layers. This results in a distinct cyclical pattern in the magnetic susceptibility data (Figs. 7–8), which aligns with corresponding cycles in the lithostratigraphy. On a larger scale, the magnetic data exhibit a negative secular trend as the lower Cutler beds transition into the more sandstone-dominated Cedar Mesa Sandstone (Fig. 7).

With initial core processing completed, the micropaleontological, geochemical, and paleomagnetic properties of ER-1 are currently being analyzed, the primary goal being the construction of a robust paleoenvironmental reconstruction anchored by reliable geochronologic controls. Though no known volcanic ash layers (and few juvenile detrital zircons) have yet to be identified in the lower Cutler Group, several approaches are available to date the core effectively. First, Nail et al. (1996) already recovered biostratigraphically useful fusulinids and conodonts from several levels within the core, and polished split core segments of ER-1 revealed highly fossiliferous carbonate layers in the Honaker Trail Formation (Fig. 6) and the lower Cutler Group (Fig. 7); together, this implies that the core can be precisely dated using biostratigraphic methods. Such an approach has been applied successfully to the underlying Carboniferous Hermosa Group (Ritter et al., 2002) as well as outcrop exposures of the lower Cutler beds in the region, where widespread presence of conodonts in marine carbonate layers (Figs. 3, 4) allowed for correlation with well-resolved marine biochronologies from elsewhere (Fig. 7) (Huttenlocker et al., 2021). Indeed, conodont biostratigraphy is the gold standard used to define the C–P boundary at both the Asselian GSSP (Usolka, Ural Mountains) (Chernykh and Ritter, 1997; Chernykh et al., 1997) and North American mid-continental sections (Ritter, 1995; Wardlaw, 2005; Boardman et al., 2009; Wardlaw and Nestell, 2014; Henderson, 2018). Formal identification of the C–P boundary within the Cutler Group remains critical for correlating Utah's rock record with the geologic timescale as well as other relevant C–P paleoenvironmental and paleontological archives across Pangea. Complementary to the biostratigraphic approach, the global marine 87Sr/86Sr record – a proxy for Phanerozoic seawater Sr (Veizer, 1989; McArthur et al., 2020) – shows a steep decrease from base-Ghzelian (latest Carboniferous) values of 0.7081–0.7087 to 0.7079–0.7081 by the end of the Asselian (earliest Permian) (Korte et al., 2006; Korte and Ullmann, 2018; Chen et al., 2018). Thus, a time series of 87Sr/86Sr isotope values from marine carbonates and the associated conodont bioapatite should accurately place the Cutler sequence along the C–P strontium reference curve. Therefore, we sampled all major marine limestone units in the studied portion of the core for conodonts, fusulinids, and 87Sr/86Sr isotopes.

Previous attempts at dating through paleomagnetic methods confirmed that deposition of the Cutler Group coincides with the Kiaman-reversed superchron (Gose and Helsley, 1972; Scott, 1975) (Fig. 1). This interval should also include the short-lived “Kartamyshian event”, a normal polarity magnetozone dated to ∼ 299 Ma (Fig. 1), and if found within this core and outcrop, it would serve as a useful maximum age constraint and datum for the top of the Carboniferous (Hounslow and Balabanov, 2016). Previous work (Gose and Helsley, 1972; Scott, 1975) indicates that the lower Cutler beds preserve a primary magnetic remanence, and we therefore should be able to identify the Kartamyshian event if it is present.

Furthermore, cyclic sedimentation patterns established in the Cutler Group (G. S. Soreghan et al., 2002; Jordan and Mountney, 2012; Golab, 2016; Golab et al., 2018; Tobey, 2020) strongly suggest that astronomically tuned geochemical and magnetic cyclostratigraphy can provide high-resolution age models for these deposits. Various studies have shown that long-term eccentricity cycles (∼ 405–413 kyr) have been stable throughout Earth's history (Olsen and Kent, 1999; Horton et al., 2012; Kent et al., 2017, 2018; Lantink et al., 2019; Olsen et al., 2019). Indeed, in the Canyonlands area, Jordan and Mountney (2012) identified lithologic cycles in the lower Cutler beds, which they hypothesized were reflective of long eccentricity cycles. The magnetic susceptibility record of ER-1 (Figs. 7–8), along with previous environmental–magnetic studies of the Halgaito Formation near Mexican Hat (G. S. Soreghan et al., 2002), demonstrate that cyclic facies changes correspond to variations in magnetic properties (Fig. 8). This provides strong evidence that it is possible to construct a magnetic cyclostratigraphic framework anchored by biostratigraphic, strontium isotope, and paleomagnetic age constraints.

This high-resolution geochronological dataset will be constructed in tandem with the measurement of various geophysical and geochemical paleoclimatic proxies. This will include the whole-core geophysical data presented in this paper (e.g., MS, NGR, color scans) (Fig. 7), high-resolution XRF core scans (in progress), and environmental magnetic (Verosub and Roberts, 1995; Evans and Heller, 2003; Liu et al., 2012) measurements of discrete specimens. Furthermore, similar datasets will be collected from outcrop sites (Fig. 3a), enabling the development of a robust and temporally constrained paleoenvironmental framework for this region during the C–P transition. Finally, this model will be correlated with a high-resolution dataset of georeferenced vertebrate sites and specimens (in progress) as a means of testing the Vaughn–Olson model (Pardo et al., 2019) and other important questions regarding the C–P transition.

Do independent paleoenvironmental proxies and magnetic mineral assemblages record a secular drying trend across the lower Cutler bed–Cedar Mesa Sandstone transition?

Our analysis of the ER-1 core will improve the geochronological and paleoenvironmental context for the important paleontological and geological archives of the C–P transition preserved within Bears Ears National Monument. Thus, these core data enrich and elevate the global importance of the at-risk scientific resources preserved within the monument (Gay et al., 2020). By clarifying the processes driving equatorial climate and biotic change during the Late Paleozoic Ice Age, the study of the ER-1 core strengthens our ability to use the LPIA and its decline as a natural deep time analog that can be compared with ongoing anthropogenically driven global warming and ecological change. Data and conclusions from our ER-1 studies will be integrated into all levels of education and public engagement, from informal settings such as public exhibits in local communities in southeastern Utah to K-12 public school and higher-education classrooms. We are working with education experts at the Natural History Museum of Utah to develop curriculum resources, and the core images and associated data will be investigated and analyzed by undergraduate students, as one of us (RBI) has done with similar data from the CPCP project (Olsen et al., 2018).

By continuing to analyze the ER-1 core, we expect to gain further insight into the precise timing of regional late Paleozoic drying and its relationship with the collapse of the LPIA, ca. 304–280 million years ago. Furthermore, by correlating this new dataset with planned paleontological field work, we intend to enhance our comprehension of the biotic impacts resulting from Earth's last pre-Quaternary icehouse–hothouse state change. As present-day anthropogenic climate change threatens to destabilize a multitude of environments, improvements in our understanding of how major climate processes have impacted biomes in the past are increasingly relevant. To this end, the outcome of this study will benefit climate modelers, policy makers, ecologists, and the many communities impacted by climate change.