Outcrop spectral gamma-ray signals of distal arid continental basins: Facies identification and controls

Spectral gamma ray (SGR) studies can help link outcrop exposures to downhole data and provide useful case studies to reduce uncertainty in interpreting geophysical logs where core and other geological data is unavailable. However, this technique is underused in continental outcrop studies due to difficulties in determining facies comprising similarly sourced sediment, despite the fact that these deposits form prolific reservoirs for hydrocarbon and geothermal exploration, as well as carbon capture and hydrogen and storage. This work demonstrates the usefulness of spectral gamma ray measurements as an outcrop logging technique in determining quantifiable facies from the dryland continental sediments of the Cedar Mesa Sandstone Formation, Utah, USA. Complex sedimentary interactions between the aeolian, fluvial and lacustrine depositional environments are present, resulting in shared similar sedimentary characteristics within the preserved strata, such as miner-alogy, grain size, sorting and provenance signals. Consequently, the geophysical signatures observed within the spectral gamma ray logs, have been suppressed, making it difficult to distinguish sedimentary facies from solely the spectral gamma ray responses. However, by combining K:Th cross-plot analysis and gamma ray log motif interpretation, this work demonstrates that individual facies within arid continental settings can be distinguished based on SGR outcrop logging, and unique spectral trends and values can be quantified and used to correlate facies across the depositional basin.


Introduction
A wide range of geophysical techniques and methods are used to document and characterise subsurface and outcrop data.One of the most commonly conducted geophysical logging procedures is gamma ray (GR) logging (Rider and Kennedy, 2011 and references within).This technique measures the total radiogenic emissions from bedrock and is used primarily to determine shale versus sand content of the strata, and for correlation of the strata between wells (Slatt et al., 1992).By contrast, spectral gamma ray (SGR) logging, whilst conducted less frequently, is a powerful method to distinguish between rock types because it measures the discrete spectral bands for potassium (K), thorium (Th), and uranium (U) that contribute to the total radiogenic emissions of the rock.By analysing the concentrations and ratios of these individual spectral signatures, detailed petrophysical and geochemical relationships can be extracted (Rider, 1996).Both GR and SGR logging techniques are valuable for characterising stratigraphy (Doveton, 1994;Bristow and Williamson, 1998), estimating mineralogy, grain size, porosity and organic richness (Lüning et al., 2003), as well as interpreting climatic regimes (Yang and Baumfalk, 1997;Sierro et al., 2000), depositional environments (e.g.Myers and Bristow, 1989;Phujareanchaiwon et al., 2021), stratigraphic surfaces and sequence boundaries (Davies and Elliott, 1996;Ehrenberg and Svana, 2001;Lüning and Kolonic, 2003).
Comparatively fewer studies have used GR and SGR logging within continental depositional settings.and those that have utilised the techniques have largely focused on identifying and classifying fluvial/deltaic settings (Myers and Bristow, 1989;Svendsen and Hartley, 2001;Evans et al., 2007;Howell et al., 2022)., Examples of application within arid continental depositional settings are limited (e.g.North and Boering, 1999;Zuchuat et al., 2019;Gross et al., 2023) and interpretation of the data typically proves problematical (North and Boering, 1999), as it is difficult to distinguish individual facies from the geophysical log trends due to signal overlap in the natural radioactivity of common continental facies (Rider, 1990).
The work presented herein evaluates the usefulness of spectral gamma ray measurements as an outcrop logging technique for determining quantifiable facies from continental erg-margin sediments.Consequently, the specific objectives of this study are: (i) to compare sedimentary facies to spectral gamma ray responses; (ii) to utilise potassium-thorium cross plots to identify facies from spectral gamma response alone; (iii) to identify unique log motifs from the distinct facies; and finally, (iv) to build idealised spectral gamma ray responses which can be used to correlate facies across the basin, and guide subsurface interpretation where core or outcrop control is absent.

Geological setting
The Cedar Mesa Sandstone of early Permian (Cisuralian) age is a predominantly clastic formation deposited within the Paradox Basina Carboniferous aged flexural foreland basin, defined by the depositional extent of evaporite deposits of the early Carboniferous Paradox Formation (Mallory, 1960;Condon, 1997;Barbeau, 2003).The basin was formed by loading in response to uplift of the Ancestral Rocky Mountains (ARM) resulting in multiple highlands which surrounded the basin.The sedimentary fill of the Paradox Basin comprises a 4 km thick package of late Pennsylvanian to mid-Permian aged deposits sourced from the Uncompahgre uplift present to the north-east of the basin (Loope, 1984).
The Permian fill of the Paradox Basin comprises the mixed alluvial, fluvial, aeolian, lacustrine and shallow marine sediments of the Cutler Group.Within the proximal region, towards the north-east of the basin, the sediments of the Cutler Group are undivided.However, towards the medial to distal part of the basin, several stratigraphic formations are identifiable (Fig. 2).The oldest unit of the Cutler Group -the lower Cutler beds, is an informal, but widely accepted lithostratigraphical subdivision that comprises aeolian, fluvial and shallow marine sediments (Jordan and Mountney, 2010) related to repeated cyclic transgressions (Jordan and Mountney, 2012).These deposits are conformably overlain by the predominantly aeolian sediments of the Cedar Mesa Sandstone Formation (Mountney and Jagger, 2004).The Cedar Mesa Sandstone Formation is conformably overlain by the terminal fluvial fan sediments of the Organ Rock Formation (Cain and Mountney, 2009) or the laterally equivalent aeolian sediments of the De Chelly Sandstone (Blakey, 1996;Dubiel et al., 1996;Condon, 1997;Stanesco et al., 2000), which in turn are overlain by the aeolian coastal dunes of the White Rim Sandstone Formation (Loope, 1984).
The individual lithostratigraphic units of the Cutler Group have frequently been examined in isolation, with varying interpretations of climate among authors.Some propose a tropical climate during glacial intervals characterised by higher humidity, indicating increased precipitation and reduced seasonality (Cecil et al., 2003(Cecil et al., , 2014;;Eros et al., 2012), while others argue for an arid climate with lower humidity and more seasonal precipitation (Rankey, 1997;Olszewski and Patzkowsky, 2003;Soreghan et al., 2008;Jordan and Mountney, 2012;Wakefield and Mountney, 2013) compared to interglacial intervals.Recent work (Olivier et al., 2023) has interpreted the Cutler Group within a sequence stratigraphic context related to sea-level and sediment supply variations within glacial-interglacial phases in equatorial Pangea.
The aeolian sediments of the Cedar Mesa Sandstone represent the deposits of a north-eastsouth-west trending coastal erg system that developed along the shoreline, and was supplied with sediment sourced from the local marine shelf (Loope, 1984;Blakey, 1988;Blakey et al., 1988).The erg extended 100 km south, with sediments of the erg centre preserved 130 km west of Blanding (Fig. 1), in south-eastern Utah (Langford and Chan, 1993;Mountney, 2006).Major flooding surfaces that are preserved in the aeolian strata to the south-east of Canyonlands National Park (Fig. 1) subdivide the erg sediments as the dune-fields became progressively smaller and more isolated away from the erg centre (Langford and Chan, 1989).Major dune field sediments of the erg are separated by the sediments of wet interdunes, which grade into sabkha-like evaporitic deposits towards the south-east (Fig. 1) (Blakey, 1988;Blakey et al., 1988;Peterson, 1988;Condon, 1997;Huntoon et al., 2000;Mountney and Jagger, 2004;Langford and Massad, 2014;Pettigrew et al., 2020Pettigrew et al., , 2021)).The sabkha-like evaporitic deposits of the Cedar Mesa Sandstone Formation are exposed around the town of Bluff in south-eastern Utah (Fig. 1) and contain sedimentary features including nodular evaporites and enterolithic growth structures interbedded with aeolian sandstones and fine-grained clastic sediments.Sulphur isotope studies suggest these strata were deposited following marine incursions (Stanesco and Campbell, 1989).However, reinterpretation of these deposits suggests deposition occurred in inland sabkha environments associated with the periodic expansion and contraction of playa lakes and coeval aeolian ergs completely isolated from potential marine input (Langford and Massad, 2014;Pettigrew et al., 2020Pettigrew et al., , 2021)).This interpretation is based on abundant evidence of freshwater vegetation and mud cracks (Langford and Massad, 2014), distinct carbonate microfacies of continental origin (Pettigrew et al., 2020), and sedimentological and isotopic analysis, which shows that the marine sulphur isotope signature recorded within the sulphate was probably derived from the recycling of older marine evaporites of the late Carboniferous Paradox Formation (Pettigrew et al., 2021).

Methods
Five spectral gamma ray logs and five sedimentary logs were collected in tandem to relate the sedimentary deposits directly to their gamma ray response.Sedimentary logging and spectral gamma ray logging were conducted across the aeolian to continental lacustrine transition, where all facies of the spatial transition from the marginal aeolian erg and dominant saline lake setting are present.The logs record full successions of the Cedar Mesa Sandstone Formation, from the marker bed of the last marine limestone of the underlying Lower Cutler Beds to the first terminal fluvial fan deposits of the overlying Organ Rock Formation (Pettigrew et al. (2021) for detailed sedimentological analysis).Logging locations were chosen based on well exposed sections within incised canyons of the modern land surface (Fig. 1B).
Total and spectral gamma ray measurements were collected using a handheld BGO-Super-Spec 230 gamma ray spectrometer (Radiation Solutions, Inc., Canada) (Fig. 3F).This tool was chosen because it uses a 103 cm 3 Bismuth-Germanate (BGO-Bi 4 Ge 3 O 12 ) crystal, which provides high resolution spectral readings without the need for a caesium-137 source to fix background values (e.g.Zanzonico, 2012).Additionally, this tool provides a convenient portable set up with negligible difference in log quality to that obtained from larger, vehicle-mounted systems (Cripps and McCann, 2000).
American Petroleum Institute (API) values were calculated using the following formula (Ellis and Singer, 2008): The total (API) and spectral gamma-ray values were plotted against the sedimentary logs to compare the gamma ray patterns with the sedimentary facies (Fig. 4).Cross-plots of spectral gamma-ray components (K:Th) were then used to examine the relationships and abundances in each sedimentary facies.
Statistical analysis was conducted in R. One way ANOVA and Kruskal-Wallis H omnibus testing identified that there were statistically significant differences in recordings between the different facies for each element studied (API, K, Th, U) with at least one of each median facies reading being statistically different from another identified facies.Further pairwise comparisons tests using either Wilcoxon rank sum or Tukey HSD tests were conducted to show which specific groups were statistically different from each other and is presented in supplemental information.

Sedimentology
From the five sedimentary logs, twelve unique lithofacies have been identified based upon differences in grain size, lithology, and sedimentary structures, and are described in detail in Pettigrew et al. (2021).The lithofacies form nine facies associations (aeolian dunes, sandsheets, interdunes, ponded waters, stratified waters, unconfined flows, bedded gypsum/anhydrite, displacive gypsum/anhydrite, and brecciated gypsum/anhydrite) for which detailed descriptions are given in Pettigrew et al. (2021) and key characteristics are summarised in Table 1.For ease Fig. 1. (A) Reconstructed paleogeography of the Cedar Mesa Sandstone Formation during the early Permian Period (after Blakey et al., 1988).Location of the dune field is marked in dark yellow, with the location of evaporitic sediments shown in grey against an inferred land surface light yellow).Present day state boundaries are superimposed, and the study area is highlighted.(B) Study area and log localities pictured with roads and state boundaries.Palaeogeographical location of the sabkha facies (grey) is also shown (after Blakey et al., 1988).
of comparison with the spectral gamma ray data, combinations of the nine facies associations have been broadly classified into one of three end-member depositional environments: erg margin, fluvial/lacustrine, or saline lake, as described below.

Erg margin
The erg margin depositional environment contains three facies associations: predominantly, aeolian dune associations, sandsheet associations and minor interdune associations.
Aeolian dune associations form laterally extensive tabular bodies with planar basal bounding surfaces comprising planar and trough crossbedded sandstones (Sxb, Stxb).Lateral extents are over tens of metres, although stratigraphic thicknesses are limited (ca 5 m).Cross-bedded sandstones (Sxb) with planar foresets depicting reverse grading (1-5 cm in thickness) are the predominant lithofacies, with sporadic occurrences of trough cross-bedded sandstones (Stxb) with tangential reverse graded foresets of similar thickness (1-5 cm).The cross-bedded sandstones are arranged into 1-5 m thick sets and cosets no greater than 10 m in thickness.Planar cross-bedded and trough cross-bedded sets typically interfinger with inversely graded, wind-rippled sandstone (Sxr) and/or structureless sandstone (Sm) along the toesets of the foresets.
Sandsheet associations primarily form thin, sheet-like, laterally extensive, tabular bodies with planar basal and top surfaces, and predominantly occur between sets of aeolian dunes.Lateral extents of the sandsheet associations are greater than tens of metres, whereas stratigraphic thicknesses are limited (1-2 m).The association comprises structureless sandstones (Sm) and wind-ripple sandstones (Sxr) lithofacies.
Sporadic interdune associations comprise lensoidal bodies composed of structureless sandstones (Sm), fine-grained carbonates (Lm) and structureless pedogenic (Sfr) lithofacies and occur between aeolian dune associations.Interdune associations pinch out over distances of less than 1 km and have limited stratigraphic thicknesses (no greater than 2 m).Limestones, where present, are clastic-rich, have lensoidal geometries, and occur between fining upwards pedogenic (Sfr) or structureless sandstone (Sm) beds with rhizoliths and desiccation cracks.
4.1.1.1.Interpretation.The combination and arrangement of the associations described above represent the deposits of a aeolian erg margin (Mountney and Jagger, 2004).Aeolian dune associations primarily consist of planar-cross bedded sandstone (Sxb), with limited occurrences of trough-cross bedded sandstone (Stxb), but despite the dominance of dune associations within the environment, the thickness of the cross-bedded dune sets is limited, when compared to central-erg locations.An abundance of interdune and sandsheet associations comprising structureless sandstone (Sm), wind-ripple sandstone (Sxr), fine-grained carbonates (Lm) and structureless pedogenic (Sfr) lithofacies suggest a depositional area that is characterised by limited sediment supply and/or periods where water was sufficient for modification and stabilisation of the aeolian deposits.This results in small and isolated, straight crested dunes migrating across a sandflat, subject to frequent periods of stabilisation and/or deflation, typical of a depositional setting situated away from the central erg.

Fluvial/lacustrine
The fluvial/lacustrine depositional environment contains three facies associations: unconfined flow associations, stratified water associations and ponded water associations.
Stratified water associations are dominantly composed of dark grey to purple-coloured siltstones to very fine-grained parallel-laminated sandstones (Ssl) and oscillatory-rippled sandstones (Swr) which are characterised by tabular bodies with planar basal surfaces, and are intercalated sporadically with fine-grained carbonates (Lm) and horizontally-laminated pedogenic lithofacies (Sfo).Lateral extents are commonly over tens of metres, whereas stratigraphic thicknesses range greatly (less than 1 m to greater than 10 m), with the thickest deposits present in the southern portion of the study area.Lithofacies Ssl is typically mottled, has a sporadically high organic content and typically grades laterally into pedogenic lithofacies (Sfo).
Ponded water associations are dominantly composed of tabular bodies comprising oscillatory-rippled sandstones (Swr), fine-grained carbonates (Lm) with sporadic gypsum nodules and minor parallellaminated siltstones (Ssl).Lateral extents are greater than 10 m, and stratigraphic thicknesses range from 0.2 m to 10 m.The association typically contains bioturbated sediment, gypsum nodules and rhizoliths.The rhizoliths are horizontal to sub-vertical and the bioturbated sediment consists of thin (less than 10 cm) branched vertical and horizontal tube-like burrows.
Sporadic unconfined flow associations form thin, sheet-like, laterally extensive bodies with relatively flat to slightly concave-upward, erosional, basal surfaces.Lateral extents are highly variable and range from 1 m to 10s of metres, which often branch into thinner sheet-like units, and have vertical extents are no greater than 1.5 m.Crossbedded sandstones (Sfxb) are the predominant lithofacies in the association, and are typically arranged into multiple fining upwards sets (up to 1 m thick) with sporadic basal mud clasts, and are overlain by sporadic planar (Sfpl) and ripple-laminated siltstones to very fine-grained sandstones (Sfrl).

Interpretation.
The combination and arrangement of the associations are interpreted as the deposits of dryland lakes with periodic fluvial input.Deposits of laterally extensive stratified and ponded water associations dominated by parallel-laminated siltstone to fine-grained sandstones (Ssl) lithofacies indicate a low energy subaqueous setting where sediment settled mostly from suspension with depths occasionally sufficient to cause thermal stratification (Fielding, 1984;Tanner and Lucas, 2007;Boehrer and Schultze, 2008).Intermittent intercalation with ponded water associations primarily composed of wave-ripple (Swr) lithofacies shows a shallowing of the water level, potentially caused by climate fluctuations, and an increasing influence from wind shear (Martel and Gibling, 1991).The occurrence of rhizoliths and horizontally laminated pedogenic lithofacies (Sfo) suggests stabilisation around the margins of a long-standing body of water (Eberth and Miall, 1991;Platt and Wright, 1991;Owen et al., 2008).

Saline lake
The saline lake depositional environment contains three facies associations: bedded gypsum/anhydrite associations, displacive gypsum/ anhydrite associations and brecciated gypsum/anhydrite associations.
Bedded gypsum/anhydrite, displacive gypsum/anhydrite and brecciated gypsum/anhydrite facies associations dominate this depositional environment and comprise laterally extensive (1 m-10s of metres), 0.5-5.0m thick beds that and are predominantly composed of crystalline gypsum (G) with alabastrine and porphyroblastic textures.Small gypsum nodules (up to 20 cm in diameter) and laminated-bands of enterolithic convoluted folds, polygonal hummocks and chicken-wire structures are also abundant features within these deposits.The gypsum/anhydrite facies associations are commonly interbedded with laterally extensive (up to 10s of metres) dry pedogenic (Sfr) lithofacies which range in thickness from 0.5 to 3.0 m and comprise very fine to fine-grained sandstone with abundant gypsum nodules (up to 50 cm) and veins.Sporadic, thin (less than 20 cm thick) beds of pastel blue, parallel-laminated gypsum-bound sandstone (Gspl), and fine-grained carbonates (Lm), both of which have limited lateral extents (typically less than 1 m), are also observed between thick gypsum (G) deposits.4.1.3.1.Interpretation.The combination and arrangement of the associations described above are interpreted as the deposits of a continental saline lake environment comprising ephemeral saline lakes, saline pans and mudflats which formed during arid periods via the evaporation and desiccation of previously long-lived lakes.
As climate shifted from more humid to more arid, lacustrine settings begin the process of desiccation.Layers of bedded gypsum formed as the lake water evaporated, leaving behind smaller ephemeral saline lakes (e. g.Kendall, 1978;Last, 1984;Smoot and Lowenstein, 1991) (see Pettigrew et al., 2020 for detailed sedimentological study).These lakes experienced occasional influxes of sediment-laden (Gspl) floodwater during climatic fluctuations.Complete desiccation of the saline lake produced a saline salt pan composed of displacive and brecciated gypsum associations.
Saline mudflats formed around the edges of these ephemeral saline lakes.Comprising displacive gypsum associations, the saline mudflats formed due to variations in groundwater levels and circulation, which promoted subsurface phreatic evaporite growth as random crystals or nodules, or as concentrated thick layers, displacing surrounding sediment to form chickenwire and enterolithic structures (Smoot and Lowenstein, 1991;Boggs and Boggs, 2009;Warren, 2016).Subsequent stabilisation and pedogenesis of these saline mudflats formed thick and

Gamma ray results
Each of the identified lithofacies have gamma ray signatures with varying total gamma radiation, and potassium, uranium, or thorium radionuclide content.Statistical analysis was conducted to determine what, if any, statistical differences are present between the lithofacies identified from the gamma ray data.A holistic approach which describes each facies by using their individual counts (Table 2, Fig. 5), K:Th cross plots (Fig. 6) and log motifs (Table 3) -a visual qualitative analysis of total counts to determine recurring patterns and trends (cf.Martinius et al.,2002) -has been used to analyse the data and determine what effect sedimentation and geology plays on recorded signals.
The gamma ray response of each facies can also be grouped into three end member depositional environments.Though not mutually exclusive, the gamma ray signatures of the dominant depositional environment for each facies, along with the effects of the sedimentological and formational processes have on the gamma ray signatures are described below.

Erg margin
Four facies: planar cross-bedded sandstone (Sxb), trough crossbedded sandstone (Stxb), structureless sandstone (Sm), and windrippled sandstone (Sxr), have been identified from the gamma ray signals within the erg margin depositional environment, all characterised by relatively low API values.Statistically significant differences in API, K, Th and U are present between each facies except for between facies Sxb and Stxb where there is only a statistical difference in uranium.
Facies Sxb and Stxb have the lowest gamma ray responses of sediments within the study area, with averages of 41.26 API and 39.76 API and ranges of 13.60-81.20API and 23.60-56.80API, respectively.Whereas facies Sm and Sxr have relatively higher gamma ray responses.

Table 1
Summary of lithofacies and facies associations from Pettigrew et al. (2021) and dominant mineralogy (Pettigrew, 2019), grouped into erg margin, fluvial/lacustrine and saline lake depositional environments for comparison with the gamma ray results presented in this paper.See section 4.11-4.13for full description of lithofacies abbreviations.2, Fig. 5).K:Th cross plots show the erg-margin setting has the lowest average values (1.20% K, 3.97 ppm Th) of the three depositional environments.The erg margin deposits exhibit a strong exponential correlation of average values and cluster between 1.00 and 10.00 K (%) and 0.40-2.10Th (ppm) (Fig. 6C and D).Facies Sxb and Stxb share similar cluster distributions towards the lower left-hand side of the cross plot and have average values of 1.09 K (%), 3.27 Th (ppm) and 1.05 K (%), 2.88 Th (ppm) respectively.Facies Sxr has the highest values of the environment (1.41% K, 5.65 ppm Th) and display a visual cluster distribution towards the middle to upper right-hand side of the cross plot, whereas facies Sm appears visually as an intermediate with average values of 1.24 K (%), 4.06 Th (ppm) and a cluster distribution in the middle of the cross plot (Fig. 6C and D).
Overall, the log motifs of the erg-margin depositional environment have low gamma ray responses and display serrate funnel or cylinder patterns, with occasional erratic higher spikes (Table 3).However, facies Sxb and Stxb both display funnel and cylindrical log motifs commonly expressed as aggrading cylinder patterns with rarer examples of serrate funnel patterns (Table 3).Sporadic signal spikes within the cylinder or funnel patterns of facies Sxb and Stxb are also observed (Figs. 4 and 7).The log motifs for facies Sxr and Sm are very similar and are often represented as a single high value peak, however, where the thickness of the facies is greater than 20 cm, facies Sxr has a log motif with a serrate bell pattern, and facies Sm has a log motif with an aggrading cylinder or funnel pattern.Facies Sxb and Stxb, which represent planar and trough cross-bedded sandstones deposited by wind-blown aeolian dunes, have the lowest gamma ray responses due to the highly abrasive aeolian dune formation and migratory processes of reptation and saltation.These processes result in compositionally mature, quartz-rich sandstone.Facies Sxb and Stxb stack into sets and co-sets of cross-bedded sandstone, representing the migration and climb of the dune trains resulting in the aggrading cyclindrical to funnel shaped log motifs (Table 3).Funnel shaped motifs, may indicate increased compositional maturity up-section.Differentiation between the cross-bedding styles is impossible to detect purely from gamma ray responses as reflected in the statistical analysis.Nevertheless, on the micro-scale the subtle internal heterogeneities of the dune deposits are visible on the log response.
Bounding surfaces such as set, coeset, or bed bounding surfaces frequently have higher gamma ray responses, resulting in a signal spike (Fig. 7).The degree of foreset spacing also effects the log motif.Wider spaced, asymptotic foresets seemingly show log motifs with less variability in the gamma ray response, resulting in a smoother pattern when compared to closer spaced foreset bounding surfaces, which display motifs with more serrate aggrading signals, and often have a higher overall gamma ray response.
The gamma ray signals are therefore likely controlled by the compositional maturity of the dunes and whether the internal crossstrata are grainfall, grainflow, or wind-ripple dominated.Cross-strata with wider spaced foresets are likely to be dominated by grainflow strata and represent middle to top of the duneform.Whereas cross-strata with closer spaced foresets, couplets of grainflow/grainfall, and some wind-ripple strata represent the toeset of duneforms.

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Spikes in the gamma ray response are commonly observed at and/or above set, coset and interdune bounding surfaces.This is due to the higher abundance of wind-ripple and grainfall laminae, which commonly have higher levels of cementation (Carr-Crabaugh and Dunn, 1996), being present in the toesets of subsequent overlying dunes.Alternatively, within wetter aeolian systems signal spikes at interdune bounding surfaces may be as a result of a hiatus in time allowing for mica-rich minerals to concentrate along these surfaces, thus increasing the gamma ray response.These surfaces may also act as fluid pathways and either allow or baffle flow.
Facies Sxr, which represents ballistic wind ripples, has the highest gamma ray response of the environment.This facies is abundant along the toesets of dunes and within sandsheet associations.Petrographic studies (North and Boering, 1999) have shown that significant quantities of feldspar (orthoclase and plagioclase), biotite mica, and metamorphic lithic fragments are present within the deposits.This is reflected in the high gamma ray response, cross plots and log motifs, which is present as either a single high peak or a bell pattern.
Facies Sm represents suspension settlement from air of fine to medium-grained sandstone formed in sandsheet and interdune areas between successive dune trains.Interdunes are composed of similar quartz-rich sandstone to dunes, however, are readily influenced by water, either by rise in groundwater level or the influx of fluvial deposits through interconnected interdune corridors (Mountney and Thompson, 2002).This influence of water may result in the higher gamma ray responses observed within facies Sm, as lithic and potassium bearing minerals are introduced via fluvial processes, whereas areas devoid of water/fluvial influence show lower gamma ray responses due to the dominance of mature aeolian derived sediment.

Fluvial/lacustrine
Four facies: oscillatory-rippled sandstone (Swr), parallel-laminated siltstone/sandstone (Ssl), horizontally laminated pedogenic siltstone/ sandstone (Sfo), and cross-bedded sandstone (Sfxb), have been identified from the gamma ray signals within the fluvial/lacustrine depositional environment, and are characterised by moderate often overlapping gamma ray responses.This is also reflected in the statistical analysis which shows that over 50% of the facies lack significant differences when compared against each other (Supplementary information).
Facies Swr has the lowest gamma ray response of the depositional environment (65.35 API) but also displays the broadest range of responses (24.40-176.40API).Facies Ssl and Sfxb have similar average gamma ray responses of 68.64 API and 67.75 API respectively, however, facies Ssl has a narrower range of responses (35.60-106.00API) when compared to facies Sfxb (24.00-157.6API).Facies Sfo has the highest average gamma ray response of the depositional environment (78.84 API) and a range of 37.60-156.0API.The spectral gamma ray data also depict similar trends to those observed in the gamma ray responses.The broad range of gamma ray responses observed in facies Swr, is also reflected in the spectral gamma ray response due to the presence of multiple outliers within the uranium values (Table 2, Fig. 5).
K:Th cross plots show the fluvial/lacustrine setting has the highest average values (1.35% K, 5.22 ppm Th) of the three depositional environments (Fig. 6B).All of the facies (Ssl, Swr, Sfxb, Sfo) within the fluvial/lacustrine deposits show a central overlapping cluster within the middle to left hand side of graph with average values of 1.30-1.50K (%) and 4.00 to 6.00 Th (ppm) and an overall cluster between 0.50 and 2.30 K (%) and 1.60 to 8.30 Th (ppm) (Fig. 6E and F).
Overall, fluvial/lacustrine motifs have low to medium gamma ray responses in aggrading serrate funnel patterns with minor cylinder patterns (Table 3).However, when analysing the individual facies within the depositional environment, facies Swr and Ssl both display cylindrical and funnel log motifs commonly expressed as aggrading serrate cylinder patterns with rarer examples of serrate funnel patterns (Table 3).Frequent erratic signal spikes are observed within facies Swr (Figs. 4 and 7).Log motifs for facies Sfxb are often represented as a single low gamma ray response trough, however, where the thickness of the facies is greater than 20 cm, facies Sfxb has a serrate cylinder or funnel motif.The log motifs for facies Sfo are frequently represented as a single very high gamma ray response peak, however, where the thickness of the facies is greater than 20 cm, facies Sxr has a serrate bell or cylinder pattern.

Interpretation.
Fluvial/lacustrine deposits have the highest average gamma ray responses of the three depositional environments and display significant overlap in values and clusters.This reflects the similar formational processes and sourcing of the sediments within this environment.The lack of statistical differences between over 50% of the identified facies, is likely due to the frequent sediment recycling observed within the depositional systems.The high gamma ray responses relate to higher organic and clay content due to anoxic conditions (Lüning and Kolonic, 2003) and high concentrations of montmorillonite and illite (Brooks and Ferrell Jr, 1970) commonly associated with saline desert lakes.
Facies Swr and Ssl represent ripple-cross-laminated to planarlaminated siltstone to fine-grained sandstone deposits formed from suspension fallout in a low energy subaqueous settings such as a lake or pond.The aggrading nature of the log motifs for these facies is due to suspension settlement of the deposits.Random erratic high gamma ray response spikes within Swr may relate to the organic content within rhizoliths associated with the shallow water facies, commonly found   around the perimeter of larger long-lived lakes.Whereas, the overall higher gamma ray response from facies Ssl may be a result of the facies been formed within deeper water with more anoxic conditions.Facies Sfxb, which represents planar cross-bedded fluvial-derived sandstones, have on average low gamma ray responses with occasional seemingly anomalously high gamma ray peaks.SGR responses for this fluvial facies fall within range of SGR responses observed within dune facies (Sxb, Sxtb) (Table 2) suggesting the fluvial sediment is partly sourced from reworked aeolian dunes, or there has been extensive subaqueous transport leading to compositional maturity.Given the highly ephemeral nature of desert fluvial systems (Mabutt, 1977;Tunbridge, 1984;Billi et al., 2018;Horn et al., 2018;Priddy and Clarke, 2020), the former is more likely.The high gamma ray responses observed within the facies may relate to the presence of mud rip-up clasts, which have high potassium and thorium content, and/or from extraformational clasts containing heavy mineral elements.
Palaeosols are common features within fluvial/lacustrine settings and relate to the chemical, physical and/or biological modification and stabilisation of the sediment, commonly associated with more humid conditions around bodies of water.Facies Sfo generally has a lower range of gamma ray responses and a smoother log motif than the more evaporitic Sfr palaesol.Separation of these two interpreted pedogenic facies based on either cluster plots or log motifs is difficult, as both have similar signals and responses, which may reflect the strong modification of the facies, with reworking of the previously described environments, mixing signals and creating the dispersed nature of the gamma ray responses.

Saline lake
Four facies: crystalline gypsum (G), fine-grained carbonate (Lm), gypsum-bound sandstone (Gspl), and dry pedogenic siltstone/sandstone (Sfr), have been identified from the gamma ray logs within the saline lake depositional environment and are characterised by a wide range of gamma ray responses with two thirds of facies showing statistical differences between each other.
Facies G has the lowest average gamma ray response of the depositional environment (48.18 API) and a broad range of API values (16.00-130.00API).Facies Lm and Gspl have similar average gamma ray responses of 59.46 API and 56.72 API, respectively, however, facies Gspl has the narrowest range of gamma ray responses of the whole dataset (46.80-78.80API), whereas, facies Sfr has the highest gamma ray response and broadest range of the dataset, 85.31 API and 26.40-256.80API, respectively.The spectral gamma ray data also reflects similar trends to those observed in the gamma ray responses.The broad range of gamma ray responses observed in facies Sfr, is also reflected in the spectral gamma ray data with the highest potassium and thorium values (3.30%K and 17.40 ppm Th) and the presence of multiple outliers within the spectral data (Table 2, Fig. 5).Facies G has the lowest potassium values of the study area (0.81% K average) which is reflected in the lowest gamma ray responses of the depositional environment.
K:Th cross plots show the saline lake deposits have a strong exponential correlation of average values and cluster between 0.80 and 3.00 K (%) and 1.00 to 13.00 Th (ppm) with average values of 1.18 K (%) and 4.44 Th (ppm).Facies G has a cluster distribution towards the lower lefthand side of the cross plot and has average values of 0.81 K (%) and 3.15 Th (ppm).Facies Sfr has the highest average values of the environment (1.47% K, 5.79 ppm Th) and displays a wide cluster distribution from lower left to upper right of the plot.Facies Gspl and Lm appear visually as an intermediate with average values of 1.29 K (%), 4.61 Th (ppm) and 1.14 K (%), 4.22 Th (ppm) respectively, and a cluster distribution in the middle-to-middle left of the cross plot (Fig. 6G and H).
On average, the log motifs of the saline lake depositional environment have a wide range of gamma ray responses and display serrate bell or cylinder patterns (Table 3).However, when analysing the individual facies within the depositional environment, facies G has primarily a bell pattern with minor cylinder motifs.Log motifs for facies Lm are often represented as a single low gamma ray response trough, however, where the thickness of the facies is greater than 20 cm, facies Lm has a smooth cylinder motif.Facies Gspl rarely occurs in thicknesses great enough to allow for motif analysis, however, where present it is expressed in bell or cylinder patterns.The log motifs for facies Sfr are frequently present in a serrate bell pattern often with very high value peaks.4.2.3.1.Interpretation.The saline lake depositional environment displays the largest range of gamma ray responses due to the varied depositional processes and sediment sources within this environment.
Evaporitic facies (G) formed from the evaporation and desiccation of desert lakes and have overall the lowest gamma ray response (Fig. 5) of any facies.Despite having varied sedimentary forms (bedded, nodular, brecciated, displacive) (see Pettigrew et al., 2021), these differences are indistinguishable from gamma ray values.XRD results from previous studies (Pettigrew et al., 2021) show the evaporitic facies (G) are almost purely composed of gypsum.This lack of feldspar and sources of potassium, thorium or uranium suppress the gamma ray response and make differentiating the sedimentological detail impossible.
Gypsum bound sandstone (Gspl) is rare and has a low-medium gamma ray response.This facies represents the influx of sedimentladen floodwater into shallow ephemeral lakes with subsequent flow of saline fluid and precipitation of gypsum in the pore space (Lowenstein and Hardie, 1985).This is reflected in the gamma ray responses as the gypsiferous matrix and cement supresses the gamma ray response of the lacustrine sourced felsic-rich host sandstone.
Carbonate facies (Lm) are observed in multiple settings in the studied area (see Pettigrew et al., 2020 for full description).Carbonates in their pure state are not radioactive (Rider, 1996) which should be reflected in the gamma ray responses shown.However, unlike previously described chemically precipitated deposits (G, Gspl) the gamma ray response from limestone facies is dispersed and varied.Field and thin section descriptions (Pettigrew et al., 2020) of deposits show significant clastic content within the carbonates sourced directly from the surrounding environments and facies.This incorporation of sediment with varying compositional maturity may explain the range of gamma ray responses shown.
Pedogenic facies (Sfr) relate to the chemical, physical and/or biological modification and stabilisation of the sediment.High gamma ray responses are due to the highest measured thorium values, when compared to the other facies.Within the saline lake setting, pedogenic facies (Sfr) are salt-rich, with frequent nodules and bands of evaporites.The abundance of gypsum present within the facies suppresses the gamma ray response, resulting in the wide range of gamma ray responses.

Summary model
In isolation, spectral gamma ray signals can provide insight into the general lithology and potential depositional conditions of the deposits.However, detailed sedimentological studies are needed for full characterisation of the deposits and depositional processes to determine the effect of the sedimentology and the intrinsic heterogeneities on the gamma ray signals.As a result, idealised logs for each of the depositional environments identified within this study have been built (Fig. 8).
Within a dominant erg-margin setting (Fig. 8A), the deposits have low counts (<60 API) in either a serrate funnel or cylinder pattern, and represent the deposits of aeolian dunes (Sxb, Stxb).Thin peaks of high counts represent thin palaesol deposits, while other peaks of higher than the background counts, represent areas of interdune (Sm) or wind-ripple deposits (Sxr).Saline lake deposits (Fig. 8B) have prograding bell and cylinder patterns of low to very high gamma ray counts (10-250 API).Very low counts are a result of beds of gypsum and/or evaporites (G), which are often interbedded with beds of salt-rich palaesol deposits (Sfr) resulting in very high counts, depending on the dominant component (salt vs palaesol) the gamma ray signals may be suppressed or elevated.Distal fluvial/lacustrine deposits (Fig. 8C) have dominantly aggrading cylinder patterns of medium gamma ray counts (60-100 API) representing the deposits of suspension settlement (Ssl, Sxr).Occasional erratic peaks of very high counts (up to 200 API), are a result of high concentrations of uranium elements due to potential anoxic conditions within standing bodies of water.

Discussion
Distal portions of arid continental basins are often under-explored or are considered commercially unviable in terms of petroleum plays, due to the higher portion of non-reservoir units (lacustrine/saline lake), when compared to the extensive reservoirs found within more proximal settings of the basin (i.e. the Rotliegend of the North Sea (Glennie and Provan, 1990)).However, recent large discoveries within more distal portions of arid basins (i.e. the Cygnus field within the North Sea, UK (Catto et al., 2018), and the Upper and Lower Slochteren formations of the Dutch north sea (Fryberger et al., 2011)), and the relevance these areas have for carbon capture and storage due to frequent intraformational seals and baffles that are useful for capillary trapping of CO 2 plumes (cf Priddy et al., 2023), has resulted in renewed interest in these depositional settings.
Within the subsurface, the sedimentary architecture, geometries, and interactions between the depositional environments of distal arid basins are difficult to predict, particularly when there is a lack of core coverage.However, outcrop spectral gamma ray studies of similar depositional settings can provide useful petrophysical data which can aid in the interpretation of subsurface datasets.
Previous work by North and Boering (1999) has discussed the difficulty in using gamma ray logs to determine stratal differences in arid continental settings.Those authors concluded that within the proximal aeolian-fluvial setting of the Cutler Group, thorium-potassium ratios and spectral data do not help with facies discrimination.Due to the considerable overlap of readings, aeolian dunes and sandsheets or interdunes and palaeosols cannot be accurately separated from each other.However, recent work by Gross et al. (2023) shows that spectral gamma ray values of arid sediments can show statistically relevant difference between aeolian dune and inland sabkha facies, due principally to a greater abundance of potassium feldspar within damp interdune flats and inland sabkha facies resulting in elevated gamma ray responses.
The work presented within this study on the distal arid setting of the Cedar Mesa Sandstone Formation of the Cutler Group shows considerable overlap in the spectral gamma ray signature for individual facies, and concurs with the concluding remarks of North and Boering (1999).However, by combining spectral gamma ray responses and log motifs with detailed sedimentological analysis, identifiable facies of the depositional environments can be determined within a distal arid erg-margin to saline lake transition (Figs.5-7).North and Boering (1999) suggest that the overlap in gamma ray readings between the aeolian and fluvial lithofacies is due to frequent reworking of aeolian deposits by the rivers crossing the erg, and by the wind reworking the fluvial deposits.This reworking of deposits is typically recognised in aeolian/fluvial settings (i.e. Simpson et al., 2008;Cain and Mountney, 2011;Hême de Lacotte and Mountney, 2022).However, within this study, the fluvial deposits (Sfxb) are distinguishable from aeolian dune deposits (Sxb, Stxb) by their elevated total and spectral gamma ray responses and subtle differences in motif style.
Overall, the gamma ray responses within the erg-margin to saline lake transition of distal arid continental basins are more distinct than in previous studies of dryland deposits within more central basinal locations (North and Boering, 1999).This is most probably due to the erg-margins being more compositionally immature and containing more finer sediments than erg-centres (Mountney and Jagger, 2004).Erg-margins are also more likely to be influenced by other environments i.e. lacustrine and fluvial systems (Priddy et al., 2023), or marine margins (Cross et al., 2023).
Despite being more compositionally immature than central-erg counterparts, erg-margin aeolian deposits still have experienced highly abrasive aeolian transport processes compared to the surrounding fluvial/lacustrine and saline lake environments.This has resulted in a greater textural and mineralogical maturity of the aeolian deposits than observed in the coeval depositional environments, resulting in clear, distinguishable signals for each environment.However, this aeolian influence and frequent recycling of sediment between environments could have also suppressed the overall gamma ray signatures observed within these coeval deposits compared to if studied in isolation.
The downstream variations in the fluvial system will also play a vital role in the observed signals.A fluvial system will decrease in grainsize and increase in mudstone/siltstone content downstream (Cain andMountney, 2009, 2011;Hartley et al., 2010;Weissmann et al., 2010Weissmann et al., , 2013;;Owen et al., 2015;Priddy and Clarke, 2021).Therefore, the distal fluvial systems may in general have higher gamma ray responses than aeolian deposits due to the increased fine-grained component, rich in potassium feldspar.The fact that fluvial deposits are rare within the study area, and are likely to be ephemeral in nature, may have also resulted in less reworking of the sediment due to shorter transport distances.The fluvial deposits present are more likely to have fed into the desert lake system, whereas within more central erg areas, fluvial systems commonly form terminal fluvial fans, where channelised flow terminated before a substantial body of water is reached due to extensive transmission losses as a result of evapotranspiration and infiltration to the surrounding dry substrate (Tooth, 2000;Sutfin et al., 2014;Priddy and Clarke, 2021).The fluvial sediment is then reworked and recycled by aeolian processes (Mountney and Jagger, 2004;Cain and Mountney, 2009) resulting in lower API values and overlapping gamma ray responses with the aeolian deposits.
The depositional processes of the lacustrine environment also exhibit a strong influence on the spectral gamma ray responses of the coeval aeolian deposits.During periods of relative humidity and lake expansion, water tables will rise, stabilising or shutting down aeolian dune systems and resulting in thick long lived lacustrine successions predominantly formed of fine-grained clastic sediments -typically clay minerals such as montmorillonite and illite (Brooks and Ferrell Jr, 1970) formed by suspension settlement following influxes of sediment-laden floodwater.During subsequent periods of increased aridity and lake contraction, the sediments are exposed as saline-rich mudflats with surface and phreatic evaporite growth, and as a result, this sediment is subsequently available for aeolian processes.This is especially evident within the sandsheet deposits, which are typically the first aeolian depositional elements to form as the water table begins to subside within the underlying substrate, and where the processes of reptation form wind ripple facies with increased fine-grained components being derived from dried out lake.These interactions with the lacustrine depositional environment resulted in the elevated gamma ray responses seen within this study, in comparison to others focused in more central-erg areas (North and Boering, 1999).

Comparison with subsurface analogues
Spectral gamma ray quantification and characterisation within the distal portion may be crucial for determining gamma ray signals of the whole basin, as the distinct end member environments observed may result in more easily distinguishable gamma ray signals which can be applied and worked backwards into main aeolian fluvial erg/reservoir.These trends in gamma ray responses and log motifs can then be used to help identify facies, associations, and depositional environments in well data without core.
To test if the patterns and log responses observed are reflective of similar depositional settings, downhole log data from the analogous arid Fig. 9. Comparison between gamma ray logs from Cedar Mesa Sandstone (this study) and downhole analogues.Well 49/16-10 and 48/10B-4 modified after Priddy et al. (2023).Well 49/9-B2 modified after Van den Belt and Van Hulten (2011).See Fig. 4 for lithofacies key.Note depths are relative for ease of comparison and do not reflect true vertical depths.continental deposits of the Rotliegend Group of the Southern North Sea (e.g.Glennie and Provan, 1990;Howell and Mountney, 1997;Fryberger et al., 2011) namely the Slochteren, Leman and Silverpit formations, have been analysed (Fig. 9).
Gamma ray logs from three wells across the Southern Permian Basin have been examined.The wells comprise the deposits of the proximal (49/16-10), transitional (49/9-B2) and distal (48/10B-4) depositional environments of the basin.The trends and patterns recognised in the Cedar Mesa Sandstone Formation have been compared to those recognised in the Rotliegend Group and have ultimately provided insights into how the results of this outcrop study can aid in the interpretation of downhole data.
Proximal erg settings within the Leman and Slochteren formations, show aggrading patterns with similar API values to field values from the Cedar Mesa Sandstone (Fig. 9).Within well 49/16-10 a peak likely associated with a palaesol deposit is also recognised.Furthermore, the absence of wind ripple or interdune spikes is noted, suggesting that the deposits most likely comprise amalgamated stacked uniform dune sediments with minimal sandsheet or interdune deposits, most commonly associated with central erg locations with high sediment supply.
Distal desert lake settings (e.g. the Silverpit Formation) also show comparable trends and results.Log signatures show a generally aggrading signature, with occasional erratic spikes (Fig. 9).Logs values on average are slightly higher than those observed within the Cedar Mesa Sandstone, which may be a result of local provenance differences, or increased argillaceous material in a long-lived distal lake.A decrease in API values observed at approximately 28 m in well 48/10B-4 is also similar to a decrease in API values seen at approximately 3 m within log 5 and is most likely related to fluvial input within the lake (Fig. 9).
Studies like this demonstrate the use of gamma ray outcrop studies, which can be used to feed artificial intelligence or machine learning databases to better constrain gamma ray logs where core or sedimentological control is limited, as well as analyse the strata for potential climatic cyclicity, or for correlating over large distances.

Conclusions
A combination of sedimentary and spectral gamma ray outcrop logging of the distal dryland continental sediments of the Cedar Mesa Sandstone Formation of Utah, USA, has been conducted, along with statistical analysis of the spectral gamma ray data, K:Th cross-plot analysis and log motif interpretation.Despite some overlapping gamma ray responses within the facies, 12 facies typical of distal arid continental basins have been distinguished and distinctive spectral gamma ray values and trends identified.This holistic approach has allowed for controlling factors, such as the depositional processes and sediment recycling, and their effects on the signals to be assessed.
The 12 facies have been grouped into 3 end member depositional environments -erg-margin, fluvial/lacustrine, and saline lakebased upon the sedimentology and spectral gamma ray responses.The erg margin environment comprises aeolian dune, interdune and sandsheet associations, that have low gamma ray responses and display serrate funnel or cylinder log motifs.The fluvial/lacustrine environment comprises unconfined flow, stratified water and ponded water associations that have low to medium gamma ray responses with occasional erratic higher spikes, in aggrading serrate funnel log motifs with minor cylinder patterns.Saline lake environments comprise bedded gypsum/anhydrite, displacive gypsum/anhydrite and brecciated gypsum/anhydrite associations that have a wide range of gamma ray responses and display serrate bell or cylinder log motifs.
The gamma ray analysis of the Cedar Mesa Sandstone Formation has been applied to similar subsurface depositional settings of the North Sea, where comparable gamma ray responses and log motifs were identified.Therefore, this work demonstrates the usefulness of spectral gamma ray techniques in dryland continental environments, as unique spectral trends and values can be quantified and subsequently aid in the interpretation of these deposits when subsurface data is limited.

Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Ross Pettigrew reports was provided by American Association of Petroleum Geologists.If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 2 .
Fig. 2. Stratigraphic column of Pennsylvanian to early Triassic deposits of south eastern Utah, focusing on the interfingering relationships of the formations within the Permian Cutler Group, (after Condon, 1997).

Fig. 3 .
Fig. 3. (A-D)The effects of sampling surface and bed geometry upon outcrop SGR measurements (afterLøvborg et al., 1971;Parkinson, 1996;Svendsen and Hartley, 2001).The red area shows schematically the sample volume.If the measurement surface is uneven the tool measures more or less rock (A and B).Counts are influenced by relationship to bedding (C and D).(E) Schematic diagram displaying the gamma ray spectra of the most prominent naturally-occurring radioisotopes, Potassium-40, Thorium-232 and Uranium-238.Spectral gamma ray detectors use these peaks as their control values with which to distinguish and present levels of the elements within the rock (afterCripps and McCann, 2000).(F) The handheld BGO-Super-Spec 230 gamma ray spectrometer (Radiation Solutions, Inc., Canada) used to collect data.(G-H) Collection method following best practice shown in (A-D).

Fig. 4 .
Fig. 4. Sedimentological, gamma ray and spectral gamma ray logs.See key for details.

Facies
Sxr has the highest gamma ray response of the depositional environment, with an average value of 66.23 API and a range of 34.40-112.40API.Facies Sm has an average gamma ray response of 56.43 API and a range of 24.80-116.4API.The spectral gamma ray data also depict similar patterns in spectral values with facies Sxb and Stxb having relatively low K, Th and U counts and facies Sm and Sxr having higher spectral values (Table 4.2.1.1.Interpretation.The low average gamma ray responses of the erg-margin setting when compared to the two other depositional settings are a reflection of the compositional maturity of the wind-blown aeolian sediments.

Fig. 6 .
Fig. 6.K:Th cross plots.(A) Shows all data points, depositional environments are shown by colour with erg-margin in yellow, fluvial-lacustrine in grey and saline lake in pink.(B) Interpreted cluster plots for each depositional environment.Erg-margin is shown in yellow, fluvial-lacustrine in grey and saline lake in pink.Larger filled circle shows statistical average value of each environment.(C-D) Show K:Th cross plots for the facies within the erg margin environment, (C) shows raw data coloured by facies (D) shows interpreted clusters.Average value for each facies is represented by the larger filled circle.(E-F) Show K:Th cross plots for the facies within the fluvial-lacustrine environment, (E) shows raw data coloured by facies (F) shows interpreted clusters.Average value for each facies is represented by the larger filled circle.(G-H) Show K:Th cross plots for the facies within the saline lake environment, (G) shows raw data coloured by facies (H) shows interpreted clusters.Average value for each facies is represented by the larger filled circle.

Fig. 7 .
Fig. 7. Sedimentological and associated gamma ray logs.Gamma ray logs show total counts in API.Red arrows represent log motif style.Sedimentological features seen in gamma log are highlighted in white text boxes.

Fig. 8 .
Fig. 8. Interpreted block model and idealised gamma ray signal from an (A) erg-margin, (B) saline lake and (C) fluvial/lacustrine deposits of the Cedar Mesa Sandstone.

Table 2 Gamma
Tabular bodies with lateral extents over 10 m and vertical extents <1 m-5m.Type of gypsum/anhydrite is indistinguishable by gamma ray measurements and was only identified via thin section and XRF analysis.These associations have been combined in this study for ease of identification and interpretation.

Table 3
Key log motif style for each depositional environment and containing facies, primary motif is displayed in bold.