A microfacies analysis of arid continental carbonates from the Cedar Mesa Sandstone Formation, Utah, USA

Arid continental environments are typically dominated by siliciclastic aeolian, alluvial and fluvial deposits. Despite their common recognition within these environments, carbonate deposits are often overlooked, yet they can provide vital insight into the depositional history, climate and tectonic controls of a sedimentary basin. This work presents a detailed microfacies analysis of the carbonates found within the Cedar Mesa Sandstone Formation of the Western USA. The Cedar Mesa Sandstone Formation is an early Permian, predominantly aeolian succession, exposed across much of the Colorado Plateau of southern Utah and northern Arizona. The formation is dominantly clastic erg deposits, that grade into a mixed carbonate/clastic sedimentary succession interbedded with carbonate and evaporitic units, interpreted to represent sabkha or sabkha‐like deposits. While many authors have worked within the aeolian dominated facies and have proposed various facies schemes for the siliciclastic components, comparatively little attention has been paid to the mixed evaporitic/clastic/carbonate aeolian‐sabkha transition zone. In this work the microfacies of the carbonates present within the Cedar Mesa Sandstone are analysed, in order to: (a) develop a record of, and interpret carbonate components, (b) propose depositional mechanisms and (c) identify evolutionary trends that stand alongside the formation's clastic depositional story. Six microfacies are presented: (MF1) Clastic Influenced Carbonate Wackestone; (MF2) Laminated Carbonate Wackestone/Packstone; (MF3) Microbial Laminated Fenestral Bindstone; (MF4) Rounded Mudclast Wackestone; (MF5) Laminated Bioclastic‐Ostracod‐Carbonate Wackestone and (MF6) Microcrystalline Quartz. The microfacies have been interpreted to document the development of carbonate interdune, lacustrine and continental sabkha environments influenced by localized fault control juxtaposed across a wetting and drying climate cycle and provide useful comparisons for other mixed evaporite/carbonate and clastic sequences.

The predominantly aeolian successions of the Cedar Mesa Sandstone Formation are exposed across much of the Colorado Plateau of southern Utah and northern Arizona, and they represent an early Permian, northeast-southwest trending desert system bounded by a palaeoshoreline to the northwest (Blakey, 1988;Blakey et al., 1988;Huntoon et al., 2000; Figure 1). In the southeast corner of present day Utah, the Cedar Mesa erg deposits grade into mixed evaporite/carbonate and clastic sediments that are interpreted as sabkha deposits of an erg-marginal transition zone (Blakey, 1988;Condon, 1997;Huntoon et al., 2000).
While many authors have worked within the aeolian erg-dominant facies of the Cedar Mesa Sandstone Formation and have proposed various facies schemes for the siliciclastic F I G U R E 1 (A) Simplified 1:500,000 scale geological map of Utah (modified after Hintze, 1980) Log localities are superimposed and highlighted by blue boxes. State outline of Utah is indicated by dashed lines, roads are marked by solid red lines, whereas modern settlements are indicated with a red circle. (B) Stratigraphic setting and depositional setting of the study area from Pennsylvanian to Triassic. Unconformities are marked with an undulating line (after Barbeau, 2003). (C) Palaeogeographic reconstruction of the early Permian Cedar Mesa Sandstone Formation (after Blakey, 1988). The aeolian dune field location is shown in dark yellow, the sabkha facies are shown in dark grey. Red indicates location of faults, ticks indicate the downthrown side. Modern state outlines are superimposed, shown by dashed line 2 | METHODOLOGY Sedimentary logs were recorded at approximately 3 km intervals from north to south through the study area using eastwest orientated canyons which cut perpendicularly to the general north-south strike of the strata. The sedimentary logs cover 30-150 m of succession, with the carbonates appearing sporadically throughout six of the logged sections (logs 1.2, 1.3, 1.4, 1.5, 1.7, 1.8) (Figure 1).
The carbonates are interbedded between thick successions (up to 10 m) of either aeolian, fluvial/lacustrine derived sandstones or evaporites. Samples were collected from the middle of each available carbonate unit, or from the top and bottom of the unit where thicker deposits (40-50 cm) allowed. This resulted in 65 samples (rock samples and thin sections) collected from the log localities ( Figure 1). Unstained thin sections, 30 μm thick, were produced from each sample and subsequently investigated using a Nikon Eclipse LV100N POL microscope, at Keele University, UK. The microfacies of the carbonate units were analysed and classified following the modified Dunham (1962) scheme of Lokier and Al Junaibi (2016), with the percentage of clastic components calculated using comparison charts. Five microfacies and one diagenetic alteration texture have been identified and are presented here. For continuity and ease of reading the diagenetic alteration texture is referred to as a microfacies, although it is acknowledged that this is against the standard definition of a microfacies.

| RESULTS
In the field, carbonates of the Cedar Mesa Sandstone Formation are generally dark grey to blue, and appear homogenous. Individual carbonate units are generally 20-50 cm in thickness and present as either isolated lenses up to 3 m in width ( Figure 2B,F) or laterally continuous units over 10's of metres Figure 2C,E). Outcrop specimens with a wavylaminated ( Figure 2A) or nodular texture ( Figure 2D) are classified as a wackestone, however, some sand-grade-grain supported carbonates are classified as packstones ( Figure  2B,F). The carbonates sporadically appear interbedded with gypsum ( Figure 2D) or with chert nodules, but weather to form predominantly blocky units in outcrop ( Figure 2B,E) between 20 and 40 cm thick.

| MF1: Clastic influenced carbonate wackestone
This facies crops out as isolated, dark grey to blue, siliciclastic, massive fine-grained wackestone ( Figure 2B). The microfacies has a high clastic component (approx. 10 to >50%) within a darker, homogenous dark brown carbonate mud matrix. Some specimens show poorly defined wavy laminations ( Figure 3A), but otherwise most examples lack any sedimentary structures. The carbonate mud matrix is composed of micrite with sparse microspar crystals. There are occasional isolated rounded intraclasts of mudstone with a higher microspar component that float within, and appear slightly lighter than, the background quartz-micrite-rich matrix ( Figure  3B,C). Quartz grains are dominantly well-rounded to subrounded, well sorted and fine to medium-grained ( Figure  3A-C). The majority of intraclasts have sporadic calcified tube-like structures, clotted micrite textures, and are found in association with micrite envelopes. A thrombolytic texture is sometimes preserved, where darker micrite appears to have enveloped 'cauliflower'-like structures ( Figure 3C).

Wackestone/Packstone
Microfacies 2 is a siliciclastic-rich (10%-40% sand grains), dark grey to blue, fine-grained wackestone to packstone, found as horizontally and laterally restricted lenses, approximately 2 m in width. The microfacies is characterized by beddingparallel laminations of dark-brown carbonate mudstone matrix, alternating with either laminations of quartz grains or undulose laminations of light-brown to grey clotted microbial fabrics ( Figure 4A,B). Clotted micrites are typically thrombolytic with peloidal and some protostromate features (calcified tubes) present. Quartz grains are moderately sorted and have a rounded to sub-rounded texture ( Figure 4C). Ostracods occurrences are infrequent (less than 1% of the microfacies) and randomly distributed ( Figure 4B), but otherwise the microfacies is barren of metazoan skeletal grains.
In one sample, horizontal laminations (0.5-0.8 mm thick) of fine-grained quartz grains alternating with a thinner (0.1-0.2 mm), flat-to undulose, brown carbonate-mud matrix were observed. The laminations of quartz grains show slight normal grading from medium to fine grained ( Figure 4C). The laminar fabric is supported by the sand-grade grains (which are considered as extraclasts within the carbonate) resulting in a packstone classification.

| MF3: Microbial laminated fenestral bindstone
This facies crops out over laterally continuous distances between 5 and 10 m as a dark-grey to blue-grey, laminated, lime-bindstone. Beds of MF3 measure between 20 and 40 cm in thickness and are enclosed primarily between evaporitic gypsum deposits (up to 5 m thick), as well as wave-rippled sandstones and palaeosols (0.5-2 m thick). The microfacies can be sub-divided, MF3a is characterized by a dominant laminoid fenestral (LF) fabric, consisting predominantly of type LF-A (horizontally linked lateral fenestral fabrics), but with some isolated areas of type LF B-II (horizontal cavities with laminoid fabrics) with elongate fenestrae and strings of regularly spaced birdseye-like voids between laminated, peloidal and oncoidal clotted fabrics. Microfacies 3b is characterized by thinner and more irregular fenestrae within laminated micrites that typically show an undulose habit. See Tebbutt et al. (1965) and Müller-Jungbluth and Toschek (1969) for further information on laminoid fenestral fabrics. The MF3a is characterized by fenestrae that are arranged concordantly to the stratification. The dominant fabric consists of elongate fenestrae ( Figure 5A) which sit parallel to local contortions within the grain-supported sediment (LF-A). In places these fenestrae appear as strings or chains of regularly spaced birdseye-like voids (approximately 30 μm in diameter) (LF-BII) ( Figure 5A). Both of these void types appear morphologically related; the thickness of the voids are the same (30 μm), they regularly occupy similar positions in sections (e.g. sit along the same laminations) and both are frequently encrusted (possibly scaffolded) by laminated, cyanobacterial and/ or microbial structures ( Figure 5B,C). In places, encrusting elements have been reasonably well preserved, with tube and chamber like structures observed ( Figure  5C,E). Encrusters are reminiscent of organisms such as Rothpletzella and Girvanella ( Figure 5E). The sedimentary framework of MF3 is dominated by oncoids and peloids ( Figure 5D). Peloids occur as several microns to tens of microns wide mictite-grains that are often devoid of internal structures, however, there are examples where tubular and rounded calcified tube-like structures are present ( Figure 5D). The porostromate features are more prominent in the larger (up to 300 µm in diameter) oncoids ( Figure 5D), which typically display laminated features including similar tubular and rounded structures to those of the peloids. These grains are morphologically related (only differing in size), and are thus grouped here as oncoids. The oncoids are often found in association with LF B-II fabrics and can be observed as internal sediments to the voids in places ( Figure 5C). The MF3b is characterized by laminated, 10-20 µm thick, undulose micrites conspicuous as alternating lighter and darker laminae, with elongate fenestrae ( Figure 5A,E). These voids are both fewer in number and thinner (several to tens of microns), than their MF3a counterparts. The micrite laminations typically contain tube, chamber, sausage and beanshaped structures and form the lighter laminae. As in MF3a, showing the high quartz content and poorly defined laminations. (B) Close up of MF1 showing the dark brown carbonate mud matrix and occasional mudstone grains (white arrow) supporting clastic quartz grains (brown arrow). Clastic grains form up to 50%-60% of the sample and are well sorted, with a sub-rounded to rounded texture. (C) Increased matrix/clast ratio and mudstone grains with microspar components (white arrow), quartz grains also highlighted (brown arrow). The mudstone grains appear to have micritic envelopes and occasional protostromate features are discernible these encrusting organisms resemble known encrusting forms like Rothpletzella ( Figure 5E).
Rounded lithic clasts (consisting of both carbonate and quartz grains) are found locally, exhibiting thin (20-30 µm) micritic envelopes consisting of tubular structures. They are observed more typically near to the boundary between the carbonate units and the underlying clastic sediments, with the tops the units devoid of clastic grains. Evaporite pseudomorphs and casts are observed sporadically throughout this microfacies.

| MF4: Rounded Mudclast Wackestone
Microfacies 4 crops out as laterally continuous dark grey to blue mudstone to wackestone ( Figure 2). The matrix of this microfacies is a dark brown carbonate mud matrix. Rounded intraclasts of slightly lighter microspar mudstone are dispersed throughout ( Figure 6). The microfacies contains very few occurrences of bioclasts with only minor occurrences of ostracods (between 1% and 5% of the microfacies) irregularly distributed throughout the samples ( Figure 6C), although more abundantly than in MF2. Isolated quartz grains are sporadically distributed. The matrix is generally homogeneous although evidence of poorly defined clotted textures has been observed ( Figure 6C). The mud clasts are commonly ~1 mm in size with a few larger examples (up to 3 mm) present. The clasts account for up to 40% of the microfacies. The surfaces of these clasts are often the nucleation point for stylolites found throughout this microfacies, with some clasts surrounded by the compressional fractures ( Figure 6A). The stylolites are typically oriented parallel to bedding and occur spaced sporadically throughout the samples.

Carbonate Wackestone
This facies crops out as laterally continuous dark grey to blue coloured, fine-grained carbonate mudstone to F I G U R E 4 (A) Representative photomicrograph of MF2, the microfacies is characterized by horizontally laminated dark brown carbonate mudstone matrix (white arrow) alternating with laminated quartz grains (brown arrow) with occasional slight undulose laminations of light brown to grey microbial clotted fabrics (grey arrow). Quartz grains are reasonably well sorted and show a rounded to sub-rounded texture. (B) The sample shows one example of a Podocopa ostracod oriented with laminations (red arrow), but otherwise is barren of skeletal grains. Brown arrow shows sub-rounded quartz grains, microbial laminations are also present (grey arrow). (C) This sample is dominated by bedding parallel laminations of quartz grains (brown arrow), these alternate with thin flat-to-undulose brown carbonate mud matrix (white arrow). The quartz grains are moderately sorted and show a well-rounded to sub-rounded texture. The quartz laminations show a slight normal grading with the thicker quartz bands being composed of coarser grains which normally grade upwards to finer material (green arrows point in the direction of fining) wackestone ( Figure 2E). The microfacies is characterized by a brown, laminated carbonate mud with a clotted textured matrix. Interspersed laminations of microspar and some peloidal and clotted areas with lighter grey/green thrombolytic textures are also present. The thrombolytic textures typically consist of clumped and tangled protostromate features. Laminations or envelopes of micrite are typically associated with these tube-like structures. The bioclasts are dominantly composed of cross sections through carapaces and individual valves of Podocopa ostracods (comprising up to 25% of the microfacies) alongside some larger isolated shell-like fragments ( Figure 7A). Complete ostracods are mostly aligned with bedding and are between 30 and 60 µm in length. Ostracod tests appear white and the  Figure 7C). Isolated fenestral cavities are present, particularly within the more micritic layers; several stromatictis-like cavities were observed ( Figure 7B,E), exhibiting flat, sedimentfilled bases with an undulous cavity roof, a thin isopachous cement rim, and with later blocky calcite cement fill ( Figure  7E). There is some evidence for 'wavy' laminations ( Figure  7D), but much less frequently than in MF3. The microfacies is largely devoid of clastic grains, with minimal occurrences of isolated quartz grains.

| MF6: Microcrystalline Quartz
This facies crops out as either isolated nodular bands of dark red chert or nodules. Samples are dominantly composed of microcrystalline quartz and show a variety of habits from microflamboyant quartz (cf. Milliken, 1979), to random fibrous granular microcrystalline quartz ( Figure 8A), and to rimmed, radial, and undulose megaquartz ( Figure 8B). Evaporite inclusions are typically present ( Figure 8C), and frequently show evidence for the displacement of the original carbonate material ( Figure 8D) in association with filled fractures of fibrous microquartz ( Figure 8C).

SEDIMENTARY RELATIONSHIPS
Key sedimentary relationships and the nature of the interbedded clastic deposits have been examined ( Figure 9) along with a schematic plot of the microfacies types against sample location within the sedimentary successions ( Figure 10). The sedimentary relationships indicate that certain microfacies are more commonly associated with different clastic sediments, and the positioning of the various microfacies sheds light on some of the local environmental conditions that it is not possible to glean from analysis of the siliciclastic sediments alone.
The MF1, with a total of 21 occurrences, is the most common microfacies. It occurs in relationship with aeolian clastic deposits, both laterally and stratigraphically. The clastics comprise primarily trough-cross-bedded, medium-grained, sandstone sets (Stxb) arranged into cosets (up to 10 m thick) and massive sandstone (Sm). Foresets of the cross-bedding display alternating grainflow and grainfall couplets indicating that the sediments are the product of migrating sinuous-crested aeolian duneform trains. Massive sandstone units (Sm) range between 0.2 and 1 m thick and comprise poorly defined wind-rippled strata with a limited grainsize range that results in a massive appearance. Rhizoliths, up to 1 m in length, are typical. They F I G U R E 6 (A) Photomicrograph of MF4. This sample is characterized by a dominant background matrix of massive dark brown carbonate mud with a few mud grains, often lighter in colour (white arrow). Compressional fractures (stylolites) are also present (black arrow). (B) This sample shows the dominant dark brown carbonate mud matrix with few mud grains (arrowed).
(C) This sample shows a homogenous matrix of brown carbonate mud with minimal skeletal Podocopa ostracod grains (red arrow) between 30 and 70 μm. Isolated clotted microbial textures are also present (grey arrow) branch along horizontal planes (up to 50 cm in width) and fine to a point towards the base of the unit. In addition to these facies, MF1 occurs sporadically in relationships with units of wave-rippled sandstone (Swr), palaeosol (Sfo) or gypsum (G), all no greater than 0.5 m in thickness.
The MF2 shares similar sedimentary relationships with clastic facies to those of MF1. The facies is found typically in association with planar-cross-bedded sets of medium-grained sandstone (Sxb) that sporadically form cosets 1-5 m thick. Foresets of the cross-bedding display alternating grainflow and grainfall couplets indicating that the sediments are the product of migrating straight-crested aeolian duneform trains. Massive sandstone units (Sm), up to 1 m in thickness, are found typically in associations with MF2, as are wave-rippled (Swr) sandstones and palaeosols (Sfo), although in F I G U R E 7 (A) Photomicrograph of MF5 showing the abundance of ostracods (red arrow) and occasional shell fragments (grey arrow), light clasts are shown by the orange arrow. (B) This sample contains abundant cross sections through carapaces and individual valves of Podocopa ostracods (red arrow) and potential stomatactis-like cavities (white arrow) (see E). (C) The crudely laminated carbonate mud matrix interspersed with clasts of a lighter grey/green carbonate mud. Red arrow shows several complete ostracods (30-70 μm) arranged in a bedding parallel fashion, the broken skeletal grains show a dominant convex upwards arrangement along a horizontal plane (yellow). (D) The lighter clasts highlighted in (C) are shown in more detail here (orange arrow). These clasts show evidence of laminations and of a clotted (sometimes thrombolytic) texture, the example highlighted here is reminiscent of the undulous microbially dominated fabric observed in MF3b. Shell fragment is highlighted by grey arrow. (E) A stromatactis-like cavity, the flat base and undulous roof is apparent, as is the sediment fill at the base of the cavity (red arrow). Late blocky calcite cement fills the interior of the cavity (blk.), whereas a rim of smaller calcite cement lines the cavity. It is this rimming cement that distinguishes these cavities from 'true' stromatactis, voids with bare isopachous, fibrous rims much higher frequency than for microfacies MF1. Lowangle, planar cross-bedded, 0.2-0.5 m thick sandstone sets with erosional bases (Sfxb), interpreted to be fluvial sheetflood deposits, are also present in association with this microfacies.
The MF3 occurs typically in stratigraphical and lateral association with gypsum deposits (G) or with gypsum-bound sandstone (Gspl). The gypsum deposits are characterized by multiple enterolithic growth structures and tepee structures, and the gypsum-bound sandstone comprises a pastel-blue fine-grained sandstone that is moderate to poorly-sorted and contains a gypsum-rich matrix and cement, often with multiple gypsum nodules distorting primary sedimentary textures. Palaeosol units (Sfo) and wave-ripple sandstone (Swr) units are also sporadically present in association with MF3.
The MF4 is deposited primarily in association with sheet flood deposits (Sfxb) or wave-rippled sandstones (Swr). The microfacies is also present in relationships with fining-upwards beds of structureless, dark-grey to black silts (Ssl) which range between 0.2 and 1 m in thickness.
The MF6 has seemingly no stratigraphic or lateral association with a distinct set of coeval clastic deposits. As this 'microfacies' is not carbonate, and is probably the result of secondary alteration, its spatial relationship might suggest a non-discriminatory alteration process.

PALAEO-ENVIRONMENT
With the microfacies described, environmental interpretations are made for each of the five carbonate microfacies (MF1-MF5) and placed into context of the larger sedimentological framework of the Cedar Mesa Sandstone Formation.

| MF1
The high content of detrital well-rounded and sorted quartz material indicates deposition occurred in close proximity to a relatively mature clastic sedimentary system. The carbonate mud matrix along with the occasional carbonate grains may represent many depositional environments, however, the close lateral and stratigraphic proximity to sub-aerial deposits (Stxb, Sm) probably indicates extremely shallow (few centimetres) quiet waters. The carbonate grains formed of micro-spar may originate from reworked carbonate material, potentially transported into the system with the clastic component (cf. Lokier et al., 2017). The general lack of evaporites or of any desiccation structures in association with this microfacies suggests sufficient water to maintain long-standing pools/puddles of water suitable for carbonate development. The lack of bioclasts, along with lensoidal geometries suggest an isolated depositional setting (Driese, 1985), lacking input from, or connectivity with larger bodies of water.  Table 1

| MF2
The sedimentary framework of alternating laminations of fining upward, sub-rounded moderately-sorted quartz and clotted micrite fabrics within a more homogenous carbonate mudstone matrix indicates periodic influxes of clastic material, followed by a hiatus in the input of detrital material. This resulted in clotted micrite carbonate precipitation, most Flow of saline fluid and subsequent precipitation of gypsum in the pore space of sediment around the margins of saline lakes as water evaporated at the ground surface

Saline Pan
Note: Each facies is given a code and described in terms of its lithology and texture and sedimentary structures present. Interpretation is based on depositional process and linked to related sedimentary associations.

| 53
likely mediated by microbial communities, as evidenced by the protostromate remains and structures that are most likely to be Rothpletzella and Girvanella. Normal grading of moderately sorted quartz shows suspension settling of clastic sediment from a more immature detrital source than indicated for MF1. Isolated and lensshaped geometries indicate a restricted setting. Rare occurrences of skeletal grains (ostracods) could suggest periodic connection to wider environments. However, the scarceness and random distribution of skeletal grains could favour windblown dispersal (Chaplin and Ayre, 1997).
The presence of clotted and laminar microbial growth also indicate an environment that was wetter than that for MF1. The features are similar to those observed from stromatolites (see Warke et al., 2019) which form under conditions of generally deeper, and longer-lived, standing water than features observed within MF1.
The high clastic content of MF1 and MF2, and their stratigraphical relationships to coeval aeolian clastic deposits (Figure 9), suggest deposition within wet interdune areas. Wet interdunes form when there is a perennial water table in contact with, or above, the depositional surface to form ponds and lakes between dunes (Ahlbrandt and Fryberger, 1981). These long-lived ponds can feature, as observed here, wave-rippled and laminated siltstones, as well as evaporites and carbonates (see Mettraux et al., 2011 for an example of modern equivalents). Wet interdunes have, or exhibit, a variety of morphologies, which are controlled by the shape and style of migration of the aeolian dunes. Highly sinuous dunes result in isolated interdunes with limited lateral extent, whereas straight-crested dunes may develop and preserve large corridors of laterally continuous interdune. The shape and style of dunes, and therefore interdunes, is highly reliant on climate and sediment supply and availability (Rubin, The type of deposit is explained in the legend 1987; Mountney, 2006;Rubin and Carter, 2006). The deposits of MF1 are the most abundant microfacies observed, however, field observations indicate that they are limited in lateral extent (<1 m) and occur typically in close proximity, both laterally and stratigraphically, to aeolian dunes. The high clastic content, limited lateral extents, lack of skeletal metazoan clasts and occurrences of relatively few microbial mediated carbonates indicate a depositional environment typical of the restricted and isolated setting of wet interdune areas between sinuous-crested migrating dunes. Evidence for such duneforms is reported here, and within the formation by previous authors (Mountney and Jagger, 2004;Langford and Massad, 2014). The clastic sediment observed within the microfacies is supplied by wind action from the nearby dunes (cf. Driese, 1985). The relative high abundance of MF1 may be related to a high preservation potential as interdune areas were eventually covered and preserved by migrating dune complexes (Driese, 1985).
Microfacies 2 shares a similar clastic content to that of MF1. However, clotted and laminar microbial growth is present and clastic sediment is more angular and more poorly sorted. Detrital clastic material occurs in regular fining upwards bands, with occasional ostracods present. Interbedded bands of clastic sediment indicate periodic influxes of detrital sediment or localized reworking, suggesting larger interdune bodies with significant periods of interconnectivity between them, and possibly with connection to wider environments. An increased lateral continuity in outcrop of MF2 (compared to MF1), coupled with associations with clastic deposits of straight-crested migrating duneforms, supports an argument for larger, better-connected interdunes (Figure 11). These settings are typically associated with periods of limited aeolian sediment supply, such as those found on the very edge of the erg, or those typical of higher water tables during humid periods, where clastic input to interdune areas is from fluvial influx, and sediment can be reworked (Howell and Mountney, 1997;Mountney, 2006).

| MF3
Microfacies 3 is rarely preserved within the study area, but where present MF3 is laterally extensive and often in association with evaporites. The sedimentary framework of this microfacies is dominantly micrite exhibiting pelloidal or oncoidal features (MF3a) with common evidence of laminar, encrusting modes of formation either within grains or as laterally persistent laminations (MF3b). These grains are interpreted to be benthic peloids and oncoids, forming from similar mechanisms and processes (i.e. biochemical precipitation triggered by microbial activity), but growing to various sizes. The encrusting nature of the laminar structures indicates that these are primary grains, rather than reworked components (e.g. MF3b). The laterally continuous laminations of MF3b are interpreted to be algal and microbial mats, similar to those observed in modern day Abu Dhabi . Undulations observed for these laminated components appear to be primary; there are no microstructures present to indicate secondary compression and the orientation of individual laminations can be seen to be columnar, bulbous and wavy in places ( Figure 5B).
Fenestrae in this microfacies are interpreted as primary cavity networks. Sediments within cavities indicate that the cavities were part of a network through which currents were flowing at the time of primary deposition. The elongate voids common to MF3a are often sheathed in microbial structures. These are related to the primary construction of the voids, as the chain-like cavities often display the same sheath like envelope. These encrusting features then join from the base and roof of the voids to form column-like structures that appear preserved as rows of spaced birdseye-like voids. Encrusting forms such as Rothpletzella and Girvanella have been observed to act as constructors of cavities in other microbially dominated carbonates (Rogers, 2018). Laminations and laminoid fenestrae indicate shallow-water to sub-aerial exposure, and they are often used as indicators of sea level. However, Bain and Kindler (1994) demonstrated that fenestrae should only be associated with sea level where other features associated with intertidalor peritidal characteristics exist. Here, no further evidence for tidal influence is found. Birdseye cavities also generally occur in shallow (intertidal) marine, lacustrine and even in eolianite environments where rainwater induces cavities (Bain and Kindler, 1994). Shinn (1968) reported that birdseye cavities never form in the subtidal zone. It is therefore likely that MF3 was deposited in extremely shallow water and the sediments typically experienced a certain amount of sub-aerial exposure. The cavities may have formed as desiccation-related structures that were subsequently colonized by encrusting cyanobacteria, resulting in their preservation. The lack of skeletal metazoan grains suggests restricted depositional environmental conditions that were inadequate, or too stressed for other biota.
These most likely represent the deposits of laminated microbial mats (Van Gemerden, 1993;Riding, 2000;Lokier et al., 2017). Microbial mats are present in many environments, however, the spatial relationships seen here indicate a likely evaporitic depositional setting, either a marginal marine, or continental sabkha. Marginal marine sabkhas are characterized by pelodial and bioclastic carbonates within the lower intertidal zones, microbial mats within the middle to upper intertidal zone, and evaporites mixed with clastic deposits within the supratidal zones as demonstrated by Court et al. (2017) in the Abu Dhabi Marine Sabkha. Continental sabkhas, such as those interpreted here, share many of these key features, however, the formation mechanisms are different. Saline pans form evaporites from the desiccation of desert lakes which are subsequently sub-aerially exposed as crusts mixed with clastic sediment derived from the suspension and settlement of floodwaters (Lowenstein and Hardie, 1985). Variations in saline ground waters promote phreatic growth of evaporite crystals and nodules around these salt pans forming saline mudflats (Warren, 1983;Lowenstein and Hardie, 1985). These saline mudflats are a recognized habitat for microbial life in desert environments as the interaction between the salt flat and the groundwater provides sheltered habitats for microbial life (McKay et al., 2016).
Microfacies 3 shows abundant evidence of shallow, restricted deposition. Undulous laminations associated with laminoid fenestrae are common and microbial mediation of micrite can be interpreted from the peloidal, thrombolytic and protostromate features observed. Microfacies 3 is therefore interpreted to belong to a shoreline of a continental playa-lake environment.
Microbial mats typically have a very low preservation potential  and are regularly destroyed within marginal marine settings by tidal processes and storm events, preservation therefore favours low energy environments. This further indicates that the microfacies likely formed within a continental sabkha setting rather than a marginal marine setting, around the edges of low energy saline lakes, increasing preservation potential. The presence of coeval lacustrine clastic sediments, and the framework of a continental playa-lake discussed by other authors (Langford and Massad, 2014) also support this argument. The distorted and fensetral form of the microfacies suggests lateral growth Lokier et al., 2017), loading and degassing. This could be driven by the encroachment of aeolian dunes, which would enhance preservation potential (as seen within MF1) during an arid climate (cf. Kocurek, 1981;Driese, 1985). F I G U R E 1 1 Depositional models for arid or humid climates which show the interpreted depositional environment for each microfacies and relationships with coeval clastic environments. Arid conditions are shown in model A, humid conditions in model B. The location of each microfacies is marked with a white circle, with the number representing the corresponding microfacies. Model A shows arid times and large sinuous aeolian dunes, with small isolated wet interdunes shown in blue. Sabkha deposits forming saline mudflats and pans are depicted around a small playa lake. Model B indicates more humid conditions than model A, smaller straight crested dunes are present with larger interconnected flooded interdune areas. A large desert lake is shown in front of the model

| MF4
The rounded mud clasts within this microfacies have similar features (laminated micrites, protostromate features) to some of the other microfacies described here (particularly MF3 and MF5). It is suggested that these clasts may be reworked from partially lithified sediments from the proximal sedimentary environment. The reworking of this deposited material was coeval with the deposition of this sediment. The mudstone clasts composed of microspar grains may indicate reworking of a feature where carbonate sands may be present. The rounding of the clasts indicates transportation of the grains. The lack of sediment lamination and the absence of cavities suggests that this microfacies occurred in water that was deeper than microfacies MF1, 2 and 3. The lack of laminar cavities is interpreted to indicate a lack of primary microbial mats and puckering/desiccation associated with sub-aerial exposure. The lack of Birds-eye cavities suggests that the microfacies was deposited in a setting permanently below the surface of the water. Ostracod occurrences, although in low numbers, are more frequent than the previously described microfacies (ostracods were absent from MF1 and MF3). This may indicate a slightly less restricted or isolated depositional environment than those interpreted for MF1, MF2 and MF3. Stylolites within this microfacies are not associated with primary tectonism, they are not regularly spaced and are oriented parallel to bedding, indicating they formed due to loading due to an applied sedimentary load and burial. Microfacies 4 lacks laminations and is homogeneous in appearance, with the exception of rounded mud clasts, this is interpreted to represent an environment which was deeper, or more persistent than those previously described (ostracods within these sediments also indicate larger, possibly better connected or more pervasive water). The rounded mud clasts, which have some shared features with the microfacies interpreted to be from extremely shallow environments (e.g. MF3), were likely transported from these shallower areas, possibly similar to the transport of sediment in sabkha environments through higher-energy events as described by Lokier et al. (2017). The common stratigraphic association of MF4 with fluvial deposits (Sfxb) likely provided the transport mechanism for the reworking of clasts (Figure 9).

| MF5
The sedimentary framework of this microfacies is dominantly micrite with few microbially encrusted laminations or cavities present. The microfacies is similar in appearance to MF4. Relatively abundant ostracods are present indicating a less restricted environment than the microfacies previously described, indicating deeper water and/or better connected (less restricted) water bodies. Cavities are interpreted to be the result of gas bubbles (or to have a biological origin) rather than desiccation (and subsequent encrusting biota construction) related, this may explain why they are less frequent than in MF3. The presence of stromatactis-like cavities is not indicative of any one setting as the formation mechanism of the cavities is still somewhat enigmatic (Monty, 1995;Aubrecht et al., 2002;Hladil, 2005). Two main hypotheses exist suggesting either a purely biological origin (Tsien, 1985;Flajs and Hüssner, 1993) caused by the collapse (Bourque and Gignac, 1983) or syndiagentic shrinkage of sponge bodies (Delecat and Reitner, 2005) with the coexistence of stromatactis in sediments with bioclastic sand, large oncoids and calcareous algae indicative of shallow-marine environments (Stenzel and James, 1995). Alternatively a physical origin has been proposed (Kukal, 1971;Wallace, 1987) for stromatactis cavities, due to filling of cavity systems with cement and sediment (Bathurst, 1980) or formation during turbulent deposition and separation of unsorted clastic material within dispersed suspension clouds (Hladil, 2005;Hladil et al., 2007Hladil et al., , 2006. However, the presence of stromatactis-like cavities does indicate that the depositional environment was quiet and sub-aqueous. The low diversity, but high productivity of the microfacies suggests a restricted environment, and it is likely that salinity played a restrictive role. This high salinity, restricted environment and lack of body fossils would make the formation of these stromatactis-like cavities by sponges seem unlikely. The laminated nature of the microfacies with disarticulated ostracods along bedding planes, could indicate periodic reworking and detrital input into standing bodies of water during higher energy events, creating turbulent conditions and subsequent settlement, resulting in bedding plane parallel ostracods and turbulent flow generated stromatactis-like cavities (cf. Hladil, 2005;Hladil et al., 2006Hladil et al., , 2007. The common association with lacustrine suspension-settle deposits (Ssl) further supports this argument, with turbulent flows created from the input of fluvial deposits (Sfxb) (Figure 9).
Bioclasts and ostracods are abundant only in MF5 with rare or sporadic occurrences within MF2 and MF4. The microfacies are laminated to varying degrees, except MF4; which has clear evidence of reworking of grains. Microfacies 4 and 5 are both interpreted to belong to a desert lacustrine system.
Microfacies 5 shows a lot of features similar to MF3, however, the microbial mats are broken up and disjointed indicating remobilisation and modification of sediments from previously deposited mats. This is possibly related to the flooding of these saline lake edges due to lake contraction and expansion related to shifting environmental controls (i.e. arid and humid periods, see Mountney, 2006). The layered appearance of MF5 indicates deposition formed by suspension settling within calm long standing bodies of water, with occasional turbulent conditions generating stromatactis-like cavities. The abundance of ostracods and larger shelly fragments ( Figure 7D) indicate conditions more conducive to bio-productivity than the previously described microfacies and could indicate lower salinity levels. Microfacies 5 most likely represents deposition within long lived desert lacustrine systems (Figure 11). Stromatactis-like cavities indicate there was no sub-aerial exposure. Desert lakes, often related to topographic lows, frequently deposit parallel laminated and ripple laminated silts and mudstones from suspension settlement, as well as fresh water carbonates, when the availability of clay-size particles is limited, primarily as a result of climate fluctuations (Tanner and Lucas, 2010). Deposition likely occurred within deeper parts of lakes away from the clastic input of dunes and fluvial systems, with occasional higher energy fluvial input into the lake during humid periods (cf. Gierlowski-Kordesch, 1998) resulting in turbulent conditions mobilising sediment and generating Stromatactis-like cavities through settling (cf. Hladil, 2005;Hladil et al., 2006;Hladil et al., 2007). The relative rarity of MF5 may be due to a combination of the effects of preservation potential or infrequent conditions for deposition within an arid climate. Flooding, increased clastic input or desiccation coupled with rare periods of humidity and lower salinities results in only a limited window for carbonate generation. The occurrence of MF5 only within log 1.7 could indicate a more central lake location, far enough away from clastic input.
The reworked appearance of MF4 is probably due to expansion of these desert lakes over the microbial mats of MF3 fed by increased fluvial discharge during climate fluctuations, acting as a primary influence on lacustrine sedimentation within the area.

| MF6
The MF6 is not a carbonate and therefore not a specific depositional microfacies. The microfacies represents the replacement of carbonate and evaporitic minerals by silica, most likely as a product of burial diagenesis (Scholle and Ulmer-Scholle, 2003). These features are typically associated with silica fabrics in chert nodules, which have replaced evaporite minerals (Milliken, 1979;Hesse, 1989). MF6 is a post-depositional process, therefore it offers little help in determining the depositional story of the formation.
Sulphate or chlorides are the most probable minerals to have been replaced, as these occur as either cements, displacive or replacive nodules, or as interbedded strata in carbonate rocks. Whether these minerals relate directly to the precipitation and deposition of primary evaporites from concentration of saline waters, or they relate to the migration of evaporitic brines into underlying or adjacent stratigraphic units as displacive or replacive nodules that are unrelated to evaporitic conditions, is unknown (Scholle and Ulmer-Scholle, 2003). Even after deposition and substantial burial, evaporite minerals can be remobilized and precipitated in distant and stratigraphically unrelated units (Scholle and Ulmer-Scholle, 2003).
The seemingly random distribution of the microcrystalline quartz facies (MF6) shows this facies was probably related to a selective diagenetic process, however, the formation mechanism remains somewhat equivocal, although most likely attributed to the alteration of evaporitic material (Hesse, 1989). Whether specific facies are more susceptible to alteration is unclear as the primary fabrics are obliterated.

| DISCUSSION
The interpretation presented above supports a wholly continental depositional setting for the sabkha-like sediments of the Cedar Mesa Sandstone Formation. They were most likely deposited within playa-lake and lake-marginal settings along the edge of an aeolian erg, with their stratigraphical relationships probably governed principally by climate fluctuations.
During arid times MF1 formed within isolated wet interdunes sitting in front of sinuous crested aeolian dunes. The MF3, exhibiting excellent preservation of microbial mats, formed around the edges of saline-rich playa lakes, within saline mudflats, occasional encroachment of dunes over the mats resulted in degassing, contorting and deformation of the mats, possibly exacerbated by the lateral growth of the mats into one another. Laminoid fenestrae preserved within the microbial mats indicate probable desiccation of the mats (forming cavities) followed by the subsequent colonisation of said cavities by encrusting cyanobacteria.
During humid times, MF2 formed in interconnected wet interdunes formed between the crests of in-phase straight crested dunes, with frequent fluvial influx transporting and depositing clastic material. Larger perennial desert lakes flooded over previously deposited microbial mats, reworking them. Within the middle of these deeper desert lakes, MF5 formed in fresh-brackish conditions away from clastic input of the dunes and fluvial systems.
Notwithstanding this interpretation, it is worthy of note that many of the individual features of the microfacies described here are shared with heavily marine-influenced, mixed evaporite, carbonate and clastic successions, such as those of the Zechstein of Northern Europe (see Tucker, 1991;Peryt et al., 2012). These include frequent wavy microbial laminations (Steinhoff and Strohmenger, 1996), microbial crusts, and encrusting modes of formation (Kiersnowski et al., 2010;Peryt et al., 2012) indicative of shallow sub-aqueous to temporally sub-aerial environments (Peryt et al., 2012).
However, the typically isolated and restricted spatial extent of individual facies, coupled with the spatial and temporal distribution of associated clastic sediments and the limited distribution of the sabkha-like strata, argue against a marine influence. Nevertheless, a wholly continental interpretation, as is presented here, does present two further questions. Stanesco and Campbell (1989) present a marine influenced interpretation based upon oxygen, carbon, and sulphur isotopic analysis. Although this result was generated from materials susceptible to recycling of marine signatures, the likelihood of such marine recycling requires some reflection. Furthermore, if the sediments examined in this work are the deposits of a continental sabkha on the edge of an aeolian dunefield, then this rather localized area must have remained reasonably wet through time, even during periods of relative aridity in the desert system. A localized control for the wet area within the Cedar Mesa erg is required to explain the presence of the deposits.
Localized tectonics that generate tectonic lows in which water can pool and subsequently evaporate is a recognized primary control on sabkha formation (cf. Mertz and Hubert, 1990). It is possible that the arrangements of the inverted normal faults that control the Comb Ridge and Raplee Ridge monoclines ( Figure 1) may have provided this tectonic low. These faults have been dated as inherited Precambrian basement structures with multiple movement throughout geological time (Kelley, 1955;Huntoon, 1993). Pre-inversion, the two antithetic extensional faults form a graben-like structure ( Figure 12) and topographic low that coincides geographically with the sabkha deposits. It may explain the location and distribution of the continental sabkha sediments, with potentially deeper facies (i.e. MF5) occurring near the point of maximum fault displacement.
In addition to channelling surface water into a topographical low, it is conceivable that the arrangements of faults may have provided further water to the sabkha low by channelling ground water to surface. If this was the case then it is conceivable that groundwater may have been in contact with the underlying marine salts of the Pennsylvanian Paradox Formation and recycling of the marine signature from the Paradox may result in the marine geochemical signature recognized by Stanesco and Campbell (1989). Significant additional analysis of geometries and structural relationships within the sub-surface coupled with further in-depth geochemical analyses are required to further this argument.

| CONCLUSIONS
Five primary microfacies and one diagenetic 'microfacies' have been described for the first time within the Cedar Mesa Sandstone Formation of the Cutler Group, Utah, USA. The microfacies show evidence of preservation of ancient microbial mats and record an erg-marginal sabkha within an arid continental setting that is responding to climate variation.
The controlling factors on the location of the sabkha within a dominantly arid aeolian formation remain somewhat equivocal, but some explanation may be provided by the geometric relationships of normal faults present at the time of sediment deposition and now inverted for the Comb and Raplee ridges.
The interpretations made here highlight the importance of a holistic sedimentary approach to the interpretation of mixed sedimentary successions that considers both the carbonate component as well as the clastic component. As such, the work complements and expands upon depositional models proposed by previous workers and provides examples of well-preserved carbonate material from a depositional environment where preservation potential (i.e. ancient microbial mats) is perhaps better than previously thought.