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Palaeogeography, Palaeoclimatology, Palaeoecology

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Environmental drivers of cyclicity recorded in lower Permian eolian strata,Manitou Springs, Colorado, western United States

James D. Pike, Dustin E. Sweet⁎

Department of Geosciences, Texas Tech University, 125 Science, Lubbock, TX 79409-1053, United States

A R T I C L E I N F O

Keywords:Ingleside FormationEarly PermianEolian stratigraphyLate Paleozoic climateAncestral Rocky Mountains

A B S T R A C T

Ancient eolian deposits are effective records of environmental conditions and commonly exhibit cyclicity.Accordingly, these deposits can reveal information that allows assessment of the earth system drivers controllingthat cyclicity. Here, we assess those drivers in the lower Permian Ingleside Formation exposed near ManitouSprings, Colorado. The unit is characterized by two major eolian depositional intervals, exposed as topographicridges, and punctuated by alluvial facies, exposed as a valley between the two ridges. Eolian facies are mod-erately sorted, sub-rounded, fine-grained, sub-arkosic sandstone. Sets of cross strata are up to 10.5 meters thick,and foresets exhibit internal laminae. Eolian cosets are regularly truncated by parallel to sub-parallel boundingzones composed of laterally continuous massive to planar laminated muddy sandstone. Rhizoliths are locallypresent in these zones. Thin section petrography demonstrates that clay and carbonate cement are abundant inthese zones when compared to eolian cosets. Bounding zone thicknesses range from surfaces (with no thickness)to 1.7 m. The bounding zones cut across all other surfaces within the eolian stratigraphy, and are inferred to besuper bounding zones (SBZ). Based on similar relationships to interior erg deposits of the coeval Cedar MesaSandstone and in the context of late Paleozoic glacioeustasy, we propose the following model for creation of theSBZ. Dry eolian dunes mobilized and climbed during regression when abundant sand was available. Duringtransgression, the encroaching shoreline trapped sediment resulting in deflation due to the loss of sediment flux.Relative sea level rise also raised the water table and when deflation reached a depth near the capillary fringe ofthe water table, the deflationary surface stabilized resulting in the SBZ.

Three levels of cyclicity are observed in the Ingleside Formation that are inferred to reflect changes in localand global paleoenvironmental conditions. The first is the long-term gradation from the predominantly fluvialupper part of the Fountain Formation to the predominantly eolian Ingleside Formation. This trend is inferred tobe the result of long-term aridification of the Pangaean interior. The second scale of cyclicity is denoted bypunctuation of the two eolian ridges by alluvial facies comprising the intervening valley. We propose that thisdry—wet—dry environmental change is driven by relative global ice volume during the early Permian. Highervolumes of global ice, occurring as Gondwanan ice sheets, appear to correlate to the eolian intervals, whereas,the alluvial interval may be coeval with feckless or complete loss of Gondwanan ice sheets. The third scale ofcyclicity is indicated by the SBZ-coset couplets and is inferred to be the result of glacioeustatic variations in sealevel resulting from the waxing and waning of relatively large Gondwanan ice sheets.

1. Introduction

Eolian depositional systems and have been studied since at least the1930s (Shotton, 1937; Bagnold, 1941). Eolian systems record a varietyof paleoenvironmental parameters and are sensitive to changes in thoseparameters (Lancaster, 1997; Forman et al., 2008). Water table varia-tions resulting from changing environmental conditions has long beenthought to impart some control on the architecture of eolian deposits(Stokes, 1968; Adams and Patton, 1979; Loope, 1981; Loope, 1984;

Kocurek, 1988; Kocurek and Havholm, 1993). Thus, the sensitivity ofeolian systems makes them well suited for relating the rock record topaleoclimate and paleoenvironmental variability. For example,through-going surfaces, termed super surfaces, have commonly beenattributed to a significant event that results in stabilization of a dunefield, such as an elevated water table produced from increased pre-cipitation or rising relative sea level (Stokes, 1968; Loope, 1984;Kocurek, 1988). Moreover, super surfaces are often cyclical, defined bycosets bounded by super surfaces. If super surfaces are present, the

https://doi.org/10.1016/j.palaeo.2018.03.026Received 16 November 2017; Received in revised form 22 March 2018; Accepted 22 March 2018

⁎ Corresponding author.E-mail address: dustin.sweet@ttu.edu (D.E. Sweet).

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cyclicity can provide a temporal archive of relative precipitation, fluvialinundation, changes in sand flux and/or water table variations (e.g.,Loope, 1981; Langford and Chan, 1988; Langford and Chan, 1993;Mountney, 2006).

The Ingleside Formation of the early Permian is exposed along theeastern flank of the modern Front Range of Colorado, and consists ofmixed eolian and shallow marine facies (Maughan and Wilson, 1963;Maughan and Ahlbrandt, 1985). Regionally, several paleoenviron-mental studies of eolian sandstone units have been undertaken (e.g.,Kocurek, 1981a; Loope, 1984; Blakey et al., 1988; Langford and Chan,1988; Havholm et al., 1993; Lawton et al., 2015); however, eolianpaleoenvironmental data along the modern Front Range is under-re-presented. The Ingleside Formation at Red Rock Canyon Open Space,near Manitou Springs, Colorado is well exposed, and presents an ex-cellent opportunity to study the drivers of cyclicity in eolian strata;specifically, assessing the stratigraphy for potential SBZ and assessingthe drivers of recorded SBZ may provide a framework with which toanalyze the paleoenvironmental variation through time, and ultimatelyfurther understand early Permian climate.

2. Tectonic, stratigraphic and paleoclimatic setting

The Ancestral Rocky Mountains (ARM) were a series ofPrecambrian-cored uplifts in the interior of western equatorial Pangaeaduring the late Paleozoic (McKee, 1975). Evidence of significant erosionis recorded in coarse arkosic deposits adjacent to Precambrian base-ment, with structural offset between basins and uplifts up to 10 km(McKee, 1975; Kluth and Coney, 1981; Ye et al., 1996; Soreghan et al.,2012). During the Early Pennsylvanian (late Bashkirian to early Mos-covian), one of the core ARM uplifts, termed the ancestral Front Range,shed sediments eastward into the Denver Basin (Hubert, 1960; Suttneret al., 1984; Sweet and Soreghan, 2010a). The ancestral Front Rangewas likely oriented northwest-southeast, and at least partially separatedfrom the Ute Pass uplift to the south by the Woodland Park trough inthe Manitou Springs region (Fig. 1; Sweet and Soreghan, 2010a). ThePennsylvanian to lower Permian strata in this area record fan-delta,braid plain and eolian depositional environments (Suttner et al., 1984;

Sweet and Soreghan, 2010a; Sweet et al., 2015).Globally, the early Permian is characterized by multiple third-order

transgressive-regressive events, which developed during a long-termsea level fall and emergence of continents (Golonka and Ford, 2000). Adrying trend persisted throughout the Permian (Parrish, 1993; Mackand Dinterman, 2002; Tabor and Montañez, 2004). High-frequencyglacioeustatic cycles commonly characterize lower Permian strata, butbecame less significant toward the end of the early Permian (Veeversand Powell, 1987; Golonka and Ford, 2000; Rygel et al., 2008). A steeptemperature gradient is commonly inferred between the equator andthe poles (e.g., Golonka and Ford, 2000); however, other studies sug-gest at least periodic equatorial cold conditions during the Pennsylva-nian through the early Permian (D'orsay and van de Poll, 1985; Duttaand Suttner, 1986; Suttner and Dutta, 1986; Sweet and Soreghan, 2008,2010b; Giles, 2012; Keiser et al., 2015; Soreghan et al., 2014; Feulner,2017). Dry continental interiors and high-latitude wet belts existed andthe Central Pangaean mountain belt may have created a significant rainshadow at low to mid-latitudes (Golonka and Ford, 2000). Large vo-lumes of Pennsylvanian-Permian loess are preserved in equatorial ba-sins (Soreghan et al., 2008) indicating suspended eolian deposition wascommon. That loess sediment was largely derived from mountainsacross Laurentia, such as the ARM uplifts and the Central Pangaeanmountain belt (Dubois et al., 2012; Sweet et al., 2013; Giles et al., 2013;Foster et al., 2014).

In the early Permian of equatorial Pangaea, there are two likelyatmospheric circulation patterns. West-to-east winds are indicative ofmonsoonal circulation and cross-equatorial flow, whereas, east-to-westwinds are indicative of subtropical zonal circulation (Parrish andPeterson, 1988). During the northern hemisphere summer, the con-tinental interior of Pangaea heated, resulting in the formation of stronglow-pressure cell systems, while the southern hemisphere formed ahigh-pressure cell over the continental landmass (Patzkowsky et al.,1991). The pressure difference between cells resulted in cross-equa-torial flow and a monsoonal effect similar to the one seen in southernAsia today, though larger in magnitude due to greater interior heatingof the supercontinent, giving rise to the term “supermonsoon” (Parrishand Peterson, 1988). The earliest evidence of supermonsoonal

Fig. 1. Early Permian paleogeography of Colorado.The inset map depicts the location of the study areawithin the Pangaean landmass. Highstand position isestimated using Rascoe and Baars (1972) and Blakey(2009). LFS= Loveland Filtration Site (wind direc-tion from Sweet et al., 2015), WPT=Woodland ParkTrough, RRCOS=Red Rock Canyon Open Space(wind direction from this study), UPU=Ute PassUplift. Location of the equator from Loope et al.(2004).

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conditions can be found in the Pennsylvanian and extending into theTriassic (Robinson, 1973; Patzkowsky et al., 1991; Parrish, 1993;Soreghan et al., 2002; Loope et al., 2004; Soreghan and Soreghan,2007; Soreghan et al., 2014). Supermonsoonal conditions were mostpronounced in western equatorial Pangaea, as the Central Pangaeanmountain belt diminished the effect to the east (Parrish and Peterson,1988).

The Ingleside Formation is exposed along the eastern flank of themodern Front Range, and consists of eolian and shallow marine sand-stone and carbonate strata (Maughan and Ahlbrandt, 1985). In thestudy area, the Ingleside Formation is 215meters thick, and was pre-viously mapped as the Lyons Formation (Trimble and Machette, 1979);however, the contact between the Fountain Formation and the over-lying eolian Permian strata studied in the paper is conformable. Sweetet al. (2015) coupled these observations with sedimentologic andavailable biostratigraphic data to suggest a new lithostratigraphic cor-relation that places the Ingleside Formation conformably atop theFountain Formation (Fig. 2; Sweet et al., 2015). Hereafter, we utilizethe term Ingleside Formation, rather than Lyons Formation, for thepredominantly eolian unit atop Fountain Formation in the study area.

3. Methods

Field data collected includes a measured stratigraphic section, strikeand dip measurements from foresets and super surfaces, lithologicsamples, and panoramic (Gigapan©) images. The stratigraphic sectionwas measured using a Jacob staff and Brunton compass. Samples werecollected for grain-size and petrographic analysis. Strike and dip offoresets were measured for sediment transport direction.

A high-resolution panoramic photograph of the sandstone ridgesfound in the study area was produced. From this photograph and field

observations, a hierarchical framework of cross-cutting relationshipswas developed and based on the work of Brookfield (1977). Strike anddip of eolian foresets (S1) were measured at various stratigraphic in-tervals. Additionally, strike and dip of super surfaces (S0) that likelyapproximate horizontal surfaces at time of deposition and are consistentwith strike and dip of outcrop trends at map scale were measured. Inthe study area, strata are incorporated into a monocline that dips in theDenver Basin to the east resulting in steeply dipping strata. Removal ofthat tilt was accomplished by rotating S0 surfaces to horizontal with anequal area stereonet and accordingly untilting the S1 surfaces. Afteruntilting, the dip magnitude for all foresets was below the angle ofrepose. Stereonet 9.5© developed by Richard Allmendinger was used tountilt the data. Down-dip azimuths were plotted on an equal-area rosediagram using GeoRose 5.1© from Yong Technology Inc.

Lithology samples were disaggregated for grain-size analysis usingthe citrate-bicarbonate-dithionite (CBD) method (Mehra and Jackson,1960; Janitzky, 1986). Samples were wet sieved through a 62 μm meshto assess only the grain-size distribution of the sand fraction. Grain-sizedistributions were analyzed with a Beckman-Coulter LS-13230 laserparticle-size analyzer.

4. Ingleside Formation stratigraphy, sedimentology and transportdirection

4.1. Eolian sandstone facies

The best exposed and most voluminous facies of the InglesideFormation in the study area crops out as two large ridges composed ofsandstone along the Red Rock Canyon Trail and Quarry Pass Trail(Figs. 3, 4). The facies is a well to moderately sorted, subrounded,quartzarenite to subarkose with distinctive large sets of cross bedding.

Fig. 2. Late Paleozoic stratigraphic chart of the study area. See text for discussion of age control. Modified from Sweet et al. (2015). Time scale is from Gradstein et al.(2012). Glacial periods from Frank et al. (2015).

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After removal of the steeply dipping eastward tilt, cross beds can bedivided into low-angle (< 10°) and high-angle (up to 30°) sets. In-dividual cross-beds of the high-angle sets are up to 20m in length andup to 10.5 m high. Cross beds are both normal- and reverse-graded,with locally present wind ripples, especially on the low-angle sets.Commonly, dip angles of the individual high-angle sets shallow to a fewdegrees in the down-dip direction, due to interbedding of wind rippleand grain-flow strata near the base of the slipface (Pike, 2017). Cosetsof eolian strata are separated from one another by zones of horizontallylaminated to massive sandstone (up to 1.7m thick) or less commonlytrue surfaces with zero thickness (Fig. 5). When a bounding zone se-parating the cosets is present, it is typically more poorly sorted andfiner grained than foreset-bearing strata (cf. Fig. 6A, B). Bounding zonesare heavily cemented, both by carbonate and clay cements (Fig. 6), andcommonly exhibit rhizoliths, bleaching, and mottling (Fig. 5). In-dividual bounding zones or surfaces are laterally continuous andmaintain relatively constant thickness through the study area, ap-proximately 1.3 km (Fig. 7).

The cross-bedded sandstone facies represents an ancient eolian dunefield. The high-angle cross beds are inferred to represent the lee sides ofdunes, or foresets, as indicated by internal laminae reflecting gradingattributed to grain flows and the down-dip shallowing of the foreset(Kocurek, 1991). The low-angle sets of cross beds commonly containwind-ripple laminations, and are inferred as the basal portion of fore-sets (Kocurek, 1991). Based on trough-like geometry of foresets andpredominately uniform orientations, individual dunes are interpretedas being transverse/crescentic dunes, where the orientation of the dunecrestline is within 15° of the orientation of dominant wind directions(Kocurek, 1991). Second-order surfaces that separate sets were identi-fied in most of the eolian intervals of both ridges (Fig. 7). These second-order surfaces are interpreted to reflect the migration of individualdunes across a draa, likely as climbing dunes (e.g., Rubin and Hunter,1984; Kocurek, 1991; Rodríguez-López et al., 2014). The western ridgecould reflect minimal dune climbing, as fewer second-order surfaceswere identified. The bounding zones are interpreted as analogs of superbounding surfaces, which represent intervals of dune field deflation,bypass or fluvial inundation (Stokes, 1968; Loope, 1984; Kocurek,1988; Langford and Chan, 1988). Given that the majority of the supersurfaces described in this paper are zones, we utilize the term superbounding zones (SBZ), hereafter.

4.2. Alluvial facies

The two sandstone ridges are punctuated by a region of poorlyoutcropping strata. These strata are 64meters thick, and of varyingcharacter, but predominantly siltstone and fine-grained sandstone.

Various sedimentary structures include crudely stratified sandstone,planar stratified sandstone, laminated mudstone, trough cross-beddedsandstone, or massive mudstone (Figs. 4, 5). Grain-size ranges from siltto granule. Paleosol horizons defined by rhizoliths and local calciteconcretions are present. Locally, well preserved rip-up clasts occurabove scoured surfaces.

The lack of outcrop does not allow a better interpretation of therecessive facies other than alluvial. Planar and trough cross-bedding inconjunction with basal course lags and scouring are typically strongindicators of fluvial transport (e.g., Miall, 1977). Paleosol horizonslikely formed in interfluves. Crudely stratified granule deposits canoften be found in alluvial systems and are common to the underlyingFountain Formation (e.g., Sweet and Soreghan, 2010a).

5. Hierarchical surface framework of dune stratigraphy

A high-resolution panoramic photograph of the eastern sandstoneridge was digitally mapped and field checked in order to build a hier-archical framework of eolian strata (Fig. 7). During mapping, surfaceswere defined as SBZ, first-order, second-order, third-order, or contactsbetween individual foresets after the work of Stokes (1968), Brookfield(1977), and Kocurek (1988). First-order surfaces represent the move-ment of individual dunes or draas across an area, and are locallyoverlain by interdune/interdraa deposits (Brookfield, 1977). Second-order surfaces are moderately inclined surfaces, which represent thepassage of dunes across draas (Brookfield, 1977), or more recently assurfaces separating climbing dune sets (e.g., Rubin and Hunter, 1984;Mountney, 2006; Rodríguez-López et al., 2014). Third-order surfacesbound bundles of cross beds within sets and represent reactivation ofthe lee slope (Brookfield, 1977).

Bounding zones up to a few meters thick, as well as true surfaceswith no thickness could be traced across the entire outcrop along bothridges (Figs. 6, 7). These are interpreted as SBZ, and their importanceand character are discussed in the subsequent section. Alternatively, thethrough-going zones could reflect interdune/interdraa deposits atopfirst-order surfaces, which are commonly characterized by irregularbasal surfaces that result in microtopography due to water or windscouring (Kocurek, 1981b). Associated interdune deposits in wetterconditions commonly exhibit root structures, adhesion features, brec-ciated laminae, evaporative structures, contorted structures, wavy la-minae, water ripples, and subaqueous cross bedding (Kocurek, 1981b).Interdune deposits are laterally variable, both parallel and perpendi-cular to wind direction, and often grade from wet to dry interdunedeposits (Kocurek, 1981b; Hummel and Kocurek, 1984). Interdunedeposits commonly pinch out as lenticular bodies when viewed per-pendicular to paleowind direction, and rise irregularly relative to

Fig. 3. Geologic map of study area. Note the two large ridges, western and eastern, separated by a recessive topographic valley. Strata dips steeply (60–70°) to theeast. Location of measured stratigraphic section shown by red lines. Underlying imagery is from Google Earth ©2017 Google. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of this article.)

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paleohorizontal when viewed parallel to paleowind direction (Kocurek,1981b). Spacing of interdune strata varies across a dune field (Kocurek,1981b). While the bounding zones show some sedimentary featurescommonly associated with interdune deposits (rhizoliths, wavy la-minae), they are continuous over the entirety of the outcrop, andmaintain constant stratification style laterally. Thicknesses of boundingzones remains relatively constant, and no apparent microtopography onthe basal surface of the bounding zones was observed. Additionally, thebounding zones do not show a bimodal grain-size distribution (Fig. 6A)observed in some interdraa deposits (Clemmensen, 1989; Langfordet al., 2008). These observations bolster the interpretation that thebounding zones in the study area are SBZ, rather than first-order sur-faces.

The reason for the lack of first-order surfaces and interdune depositsis uncertain. Based on the style of stratification, eolian strata in the

study area could represent a dry dune field (Kocurek and Havholm,1993). Dry eolian systems range from those where dunes migrate acrossa barren substrate (Finkel, 1959), to those where interdune flats areabsent, and replaced by depressions between dunes (Sweet et al., 1988).Dunes are often built in dry systems at the expense of interdune de-posits (Kocurek et al., 1992), and in many cases accumulation in drysystems does not occur until bedform growth has progressed to thepoint where interdune flats have been eliminated (Wilson, 1971).Maybe most importantly, interdune deposits within dry eolian systemsare rarely preserved in the rock record (Hummel and Kocurek, 1984).

5.1. Character of super bounding zones

Six SBZ were identified in the strata of the western sandstone ridge,and are defined by numbers 1–6. These range in thickness from 0.4 to

Fig. 4. Stratigraphic section of the Ingleside Formation in the study area. Corresponding breaks in section indicated on column indicate the end of individualmeasured sections shown as red bars on Fig. 3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of thisarticle.)

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1.7 m. Cosets or individual sets bounded by SBZ range in thickness from1.5 to 15m. SBZ 2, 3, 4, and 6 are mottled, recessive, finer grained thaneolian strata, and exhibit rhizoliths up to 60 cm in length. Rhizoliths inthese zones are calcified and calcite cement fills almost all pore space inthese zones similar to those found by Loope (1988) (Fig. 6). SBZ 1demonstrates a fining-upward trend through the interval, a scouredgranule lag, granule lenses, and is capped by a well-sorted, fine-grained,planar-stratified sandstone. SBZ 5 is coarser than surrounding eolianstrata, and shows rhizoliths and stabilization toward the top of thezone. SBZ 2–5 exhibit calcic paleosol formation indicating relativelystable intervals. Only SBZ 1 locally exhibits signs of fluvial reworking.

In the eastern sandstone ridge, eight surfaces or zones are present,defined by the letters A-H, and ranging in thickness from 0 to 1.2mthick. Cosets or individual sets bounded by the SBZ range in thicknessfrom 1.6 to 14.5m. SBZ A and H are true surfaces with no thickness.SBZ B, C, D, E, and G are recessive in nature, truncate eolian strata, andlocally exhibit weakly defined parallel or mottled wavy laminations.SBZ C truncates eolian strata, but does not show the demonstrable li-thological change like other surface zones. Most zones in the easternridge are finer grained than surrounding eolian strata, and pore space isalmost entirely filled with clay cements (Fig. 6A, B).

The SBZ of the eastern ridge are interpreted as forming in a sand flatdepositional system, near the capillary fringe of the water table, similar

to other interpretations of eolian strata (Stokes, 1968; Loope, 1981;Havholm and Kocurek, 1994). Differences in cementation and stabili-zation between the SBZ of the two ridges could be from effective pre-cipitation differences during stabilization events. SBZ of the westernsandstone ridge exhibit calcite cements and calcified rhizoliths, in-dicating at least semi-arid conditions (Mack and James, 1994), whereasthe SBZ of the eastern ridge exhibit more clay content suggestive ofmore chemical weathering and less calcic that is consistent with in-creased precipitation relative to the SBZ of the western ridge. Alter-natively, these differences in cement character of the SBZ from the tworidges could reflect different depths of deflation relative to an anceintwater table, such that the SBZ of the western ridge record less deflationindicated by higher abundance of calcite cement and calcic paleosols.

6. Grain-size and sediment transport data

Samples were collected for grain-size analyses from each cosetbounded by a SBZ. Three samples were collected for each interval of theeastern ridge. One to two samples were collected from each interval ofthe western ridge, as the eolian strata of this ridge is not as well ex-posed. The mean grain size for all intervals is between 2 and 3φ (Fig. 8),indicating fine sand. Interval 2 is the coarsest interval of the westernridge with a mean grain size of 2.32φ, while the finest is interval 3 with

Fig. 5. Photographs of the Ingleside Formation in the study area. A) Multiple foreset orientations inferred as two sets of cross beds separated by a third-order surfaceinferred as lee slope reactivation. Pencils (image center) shown for scale and to highlight orientation of cross bedding. B) Calcified rhizoliths in SBZ 3 of the westernridge, light gray to left of pen. C) SBZ B and C of the eastern ridge truncating grain-flow foresets. Stratigraphic location shown on Fig. 4 (at 175 and 177m). D)Recessive nature of SBZ 3 of the western ridge. E) Trough cross-bedding in coarse sandstone of the alluvial facies housed in the valley between the two eoliansandstone ridges. Cross-bedded interval is just to the right of the scale. Scale is 10 cm long. F) Eastern ridge composed of eolian sandstone facies. Recessive alluvialfacies are represented by the vegetated foreground of the image.

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a mean of 2.83φ. For the eastern ridge, interval C is the coarsest with amean grain size of 2.038φ, while the finest interval is D with a mean of2.74φ. The western ridge is moderately sorted on average, while theeastern ridge is well to very well sorted. For the western ridge, twofining trends are observed, one from surfaces 1 to 3 and the other fromsurfaces 4 to 6 (Fig. 8). For the eastern ridge, grain size is relativelyconstant, though there seems to be a small fining trend from interval Ethrough interval H.

Strike and dip of foresets (n= 760) were measured from the twoeolian ridges. In addition, strike and dip measurements (n=153) ofSBZ were made to estimate a paleohorizontal surface and used to re-store the steeply dipping (up to 70°) strata of the Ingleside Formation.The mean trend of eolian foresets of the western ridge is 196°(N= 222), whereas the mean trend of eolian foresets of the easternridge is 223° (N=538; Fig. 9).

7. Discussion

7.1. Paleowind directions

At the paleolatitude during deposition, the predominantly south-west down-dip directions are indicative of expected dune migrationunder normal subtropical zonal circulation with an epeiric sea locatedto the east of the study area (Fig. 1; Parrish and Peterson, 1988;Peterson, 1988). Topography is often an aerodynamic and accumula-tion control (Mainguet, 1978; Fryberger and Ahlbrandt, 1979). ARMtopography may have imparted some control on local aerodynamicconditions and accumulation of the dune field. To the north, wind di-rection data from coeval deposits demonstrate a slightly more north-to-south wind direction than those of the study area (Fig. 1; Sweet et al.,2015). The slightly more southwesterly transport direction in the studyarea could reflect a lack of topography to the west where the Woodland

Fig. 6. Photomicrographs of the Ingleside Formation. A) and B) cross-polarized and plane light, respectively, images of sample collected of SBZ C (177.8 m abovebase on Fig. 4). Quartz grains are finer than the eolian sets, and nearly all pore space is occupied by clay cement. C) and D) cross-polarized and plane lightphotomicrographs of eolian sets (177.8 m above base on Fig. 4). Quartz grains are subangular to subrounded, with only thin rims of clay coating grains. E) and F)cross-polarized and plane light photomicrographs of SBZ 3 (40.8 m above base on Fig. 4). Sample is almost entirely carbonate cement with few quartz grains present.

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Park trough at least partially segmented the ancestral Front Range fromthe Ute Pass block (Fig. 1; Kluth and McCreary, 2006; Sweet andSoreghan, 2010a).

The orientation of foresets is not always an accurate indicator ofprimary wind direction. Dune fields with multiple superimposed bed-forms and abundant third-order bounding surfaces are indicative ofdune fields with multiple wind directions and non-parallel migrationdirections (Kocurek, 1991). Large, simple sets of grain-flow cross-bed-ding, as seen in the study area, are more likely to reflect regional pa-leowinds (Kocurek, 1991). Parrish and Peterson (1988) comparedmeasurements of Pennsylvanian through Jurassic eolian strata of thewestern US with predicted directions from a global climate model, andfound a good correspondence for at least 75% of the data. Thus, itseems likely that down-dip foreset orientations provided here serve asat least a first-order approximation of paleowind directions, especiallygiven the relative consistency of data derived from coeval localities200 km to the north.

7.2. Drivers of cyclicity in the rock record

Tectonism can drive cyclicity in the rock record (e.g., Ettensohn,1994). In the Manitou Springs area, the Ute Pass Fault facilitated latePaleozoic uplift of pre-Cambrian basement (Suttner et al., 1984).Movement on that fault could have periodically created accumulationspace for the dune field. However, previous studies of late Paleozoicmotion on the Ute Pass Fault show that movement terminated in theMiddle to Late Pennsylvanian (Sweet and Soreghan, 2010a). Elsewhereacross the Ancestral Rocky Mountains, a similar cessation of LatePennsylvanian fault motion have been documented, such that both theWichita and Uncompahgre uplifts were onlapped and buried by earlyPermian strata (Soreghan et al., 2012). Thus, cyclicity present in theeolian facies of the Ingleside Formation resulting from periodic move-ment on the Ute Pass Fault can be discounted because that fault showsno evidence for movement in the early Permian.

In the Permian Cedar Mesa Sandstone, exposed in eastern Utah,super surfaces have been interpreted to be the result of lateral dunemigration during fluvial flooding events, and referred to as flood sur-faces (Langford and Chan, 1988). In this model, dune migration ceasesduring flood events that inundate the dunes and migration of dunesmoves laterally whereby an extensive erosion surface is cut parallel tothe depositional surface. Invoking the flood surface model seems un-tenable for the creation of SBZ of this study as that interpretation re-quires evidence for fluvial facies or erosion typified by a surface that

exhibits paleorelief from fluvial incision. The SBZ in the InglesideFormation have relatively planar and subparallel basal surfaces withminimal, if any, relief (Fig. 6). Moreover, fluvial facies are only ex-hibited in SBZ 1.

Rubin and Hunter (1984) proposed another model for creation ofsuper surfaces that invokes alternating angles of bed climb. In theirmodel, dunes never need to leave the region to form super surfaces.Rather, the climb angle alternates between positive values that allowdunes to climb and near zero or even a negative value that allows dunesto erosional truncate underlying deposits. Their model indicates thepreservation of interdune deposits recorded by finer grained materialand paleosols. Invoking this model requires reinterpretation of the SBZof this study as interdune deposits. However, interdune deposits arelenticular in nature as they record the space between migrating dunes.The SBZ from this study can be traced along an exposure that is orientedsemi-perpendicularly to transport direction for at least 1.3 km. Ad-ditionally, the SBZ maintain a relatively constant thickness along theextent of the outcrop. Thus, to interpret these as interdune depositsrequires dune spacing perpendicular to the migration direction of over1.3 km and coincidental stacking of interdune deposits in the samevertical location through time. For these reasons, we argue that theRubin and Hunter (1984) model is a difficult reconciliation for thecreation of the SBZ in the study area.

In the study area, the eolian strata were deposited at least 70 kmfrom the highstand shoreline as indicated by the nearest marine de-posits (Fig. 1; Rascoe and Baars, 1972; Blakey, 2009). Workers haveplaced eolian systems into a transgression-regression framework(Mountney, 2006; Blanchard et al., 2016). That work theorizes thateolian systems are deposited during lowstand periods, when theshoreface sediment is exposed to wind erosion. In the study area, directinfluence from inundating marine deposits is not recorded. However, inthe core of Cedar Mesa erg ranging from 20 to 80 km from that coevalhighstand shoreline, Mountney (2006) proposed that dune mobilizationoccurred during regression, and stabilized or deflated to a depth near anelevated water table resulting from a nearer shoreline during trans-gression events. Thus, in the Mountney (2006) model, relative sea leveldrives the cyclicity even though no marine inundation occurred. Weinvoke a similar model based on the following similarity of observationspresent in our study and the Mountney (2006) study: 1) deposits occursimilar distances from the coeval highstand shoreline; 2) SBZ exhibitplanar subparallel geometries over similar exposure distances; 3) SBZrecord little to no paleorelief; 4) rhizoliths and calcic paleosols occur inthe SBZ interval; 5) overall abundance of finer grained material and

Fig. 7. Gigapan of the eastern ridge (top) and line drawing (below) demonstrating the heirarchal framework of surfaces. Note the consistent spacing of the SBZ alongstrike of the outcrop.

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lack of fluvial deposits that characterize the SBZ; and 6) the local in-itiation of small, low-angle dunes directly over SBZ that are recorded byrelatively thin cross sets (Fig. 5C). Specifically, this model implies thatas relative sea level rose, the encroaching shoreline trapped sedimentthat was previously available for transport. The reduction of sedimentsupply resulted in deflation until the capillary fringe of a transgression-driven rising water table was intersected, whereby stabilization oc-curred. During this period of stabilization, soils develop across the re-gion. Importantly, this process does not require or preclude significant

changes in precipitation to raise the local water table.Models produced by numerous workers result in varying archi-

tecture recorded in the sedimentary strata. Rodríguez-López et al.(2014) provide a good summary of those variations. The internal ar-chitecture mapped in detail in the Ingleside Formation (Fig. 7) de-monstrates that the SBZ are roughly parallel, maintain constant thick-ness and bound cosets. These relationships are most consistent withdeflation down to the capillary fringe of the water table, rather thanflood surfaces or bypass surfaces (cf. Fig. 4E, F of Rodríguez-Lópezet al., 2014). White sands of New Mexico appear to provide a modernanalogue that demonstrates similar architecture to our study (Simpsonand Loope, 1985; Loope and Simpson, 1992); however, in those studiesprecipitation variation and aquifer recharge from the surroundingmountains raises the water table, whereas we suggest that transgressionresulted in a raised water table.

An important point of disagreement in variously proposed modelsfor the creation of extensive super surfaces through deflation is the timerequired to remove that sand to a downwind location, or sand-disposalproblem, is significant (cf. Loope, 1981; Rubin and Hunter, 1984). Asdeflation occurs, sediment progressively migrates to a successivelyfarther downwind location. Ultimately, though if the sand cannot betransported out of the region, the deflationary surfaces must eventuallymerge into an active dune field. Obviously, the size of the dune field,sediment supply, wind speed, and available time for transport are im-portant factors, but the time scales for large ergs is on the order 400-kyr(Rubin and Hunter, 1984). The SBZ of the Ingleside Formation arepreserved in outcrop in an orientation that is semi-perpendicular to thesouthwesterly dune migration direction. Thus, to test for relationshipsthat demonstrate the transition from a deflationary surface that mergesinto an active dune field, strata would have to be exposed farther to thesouthwest. Unfortunately, these relationships are untestable in thestudy area because uplift of the Front Range during the Laramide or-ogeny eroded all lower Permian strata directly west of the Inglesideoutcrop (Trimble and Machette, 1979).

7.3. Age control for the Ingleside Formation

Biostratigraphic data recovered directly from the InglesideFormation comes from areas to the north of the field area, where thedune field was inundated during transgression (Fig. 1). At the type lo-cality for the Ingleside Formation, in Owl Canyon, Colorado, Triticitesventricosus was recovered from approximately 8m above the base of theunit (Hoyt and Chronic, 1961). Subsequent work indicates that thisfusulinid species is earliest Permian in age (Maughan and Ahlbrandt,1985). The top of underlying Fountain Formation in the region hasyielded Virgilian fusulinids (Gzhelian; Maughan and Ahlbrandt, 1985)indicating little to no lacuna across the formation boundary. To date, adirect upper age constraint recovered directly from the Ingleside For-mation does not exist. The unit is overlain gradationally variably by theSatanka and Lyons formations (Maughan and Wilson, 1960). Calcar-eous foraminera, Pseudoglomospira sp., were recovered from the upperpart of the Satanka Formation (Chen and Boyd, 1997) and is a longranging taxa, but is common to upper Artinskian strata while sparselyrecovered in earlier Permian strata of the midcontinent (Groves, 2000).Based on these age constraints, a middle to late Artinskian age for theupper part of the Ingleside Formation is plausible, and consistent withhistorical age models of lower Permian strata along the Front Range(Thompson, 1949; Maughan and Ahlbrandt, 1985). Sweet et al. (2015)argued that unless time-transgression on the order of 10's of millions ofyears occurred along strike of the depositional system, the age of theIngleside Formation ranged from earliest Asselian (early Wolfcampian)to middle to late Artinksian (late Wolfcampian) (Fig. 2).

7.4. Temporal scale of cyclicity and coupled global processes

Cyclicity and resultant environmental variations recorded in the

Fig. 8. Grain-size distribution of each interval bounded by a SBZ. Each dis-tribution represents the combination of one to two samples for the westernridge and at least three samples for the eastern ridge. Black squares indicatemedian grain size. Black circles indicate coarsest 10th percentile.

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early Permian strata in the study area occurred at three time scales. Thetransition from the predominantly fluvial strata of the upper part of theFountain Formation to the predominantly eolian strata of the InglesideFormation occurs in all exposures of the boundary along the modernFront Range—a distance on the order of 230 km. We infer this wide-spread change across boundary of the two formations to reflect a long-term drying trend, on the order of 10Myr, and is consistent with otherstudies of the early Permian (Parrish, 1993; Tabor and Montañez,2004). Secondly, the two ridges of eolian strata in the Ingleside For-mation are separated by alluvial facies. These alluvial facies are inter-preted as a wet period between two drier periods recorded by the twoeolian ridges. Lastly, SBZ-coset cycles represent the highest frequencycyclicity archived in the strata and may correlate with glacioeustaticcycles, potentially on the order of 400 kyr per cycle (Heckel, 1986; Suret al., 2010).

The long-term drying trend of the Permian is largely considered tobe driven by the slow amalgamation and northward drift of Pangaea(Parrish, 1993; Tabor and Montañez, 2004). The northward drift ofwestern Pangaea resulted in progressive removal from tropical latitudesto mid-latitudes (Loope et al., 2004). Moreover, the assembly of Pan-gaea coincided with a long-term sea level fall through the Permian,which for central Pangaea resulted in a large land mass that was pro-gressively isolated from marine influence.

The SBZ-coset cycles reflect the highest frequency of change re-corded, and as such, the driver needed to be similarly scaled. Well-documented cyclical strata, or cyclothems, characterize late Paleozoicstrata across western Pangaea and are most commonly inferred asglacioeustatic driven (e.g., Wanless and Shepard, 1936; Veevers andPowell, 1987; Boardman and Heckel, 1989; Rygel et al., 2008). Ourmodel driving the SBZ-coset cyclicity is variations to the sedimentsupply and water table position that result during transgression andregression. Specifically, eolian strata are the result of the abundance ofreadily mobile sand that occurs under shoreline regression conditions.Conversely, the SBZ result from reduction of sediment supply and risingwater table during shoreline transgression periods. Therefore, in thecontext of the well-documented glacioeustatically driven, cylcothemicstrata of the late Paleozoic, potentially, each SBZ-coset cycle representsa similar magnitude of time as glacioeustatic cycles observed elsewherein equatorial Pangaea. For the coeval Cedar Mesa erg, a durationequivalent to the long eccentricity cycle (413 kyr) is proposed for timebetween super surface formation (Mountney, 2006). Duration of Middleto Upper Pennsylvanian major cyclothems across the midcontinentrange from 235 to 400 kyr (Heckel, 1986).

Paleoenvironmental drivers for the eolian ridge—alluvial val-ley—eolian ridge cyclicity are more difficult to assess. New insights intothe late Paleozoic “icehouse” suggests that two distinct climate states

best characterize the period (Montañez and Poulsen, 2013). One staterepresents discrete intervals of substantial global ice budget that waxedand waned resulting in the high-frequency glacioeustatic cycles socommon to Pennsylvanian and early Permian strata; whereas, thesecond state represents intervals with minimal global ice budget,minimal eustatic change, and typically warmer conditions. Thus, itseems appropriate to consider the late Paleozoic as intercalated inter-vals of icehouse and greenhouse conditions. Based on climate modelling(Heavens et al., 2015), a global greenhouse state equates to sig-nificantly wetter conditions than an icehouse state for equatorial Pan-gaea. The eolian strata comprising the ridges could reflect buildup ofGondwana ice, where waxing and waning of that ice coincides with thehigher frequency SBZ-coset cycles. Conversely, the intervening wetteralluvial facies, recorded in the valley between the two eolian ridges,may record the loss of Gondwanan ice. Based on this, the eolian stratain the study area could correlate to the P1 and P2 glacial cycles of Franket al. (2015; Fig. 2). When using a 400-kyr time duration for each SBZ-coset cycle, the western ridge spans 2.4Myr while the eastern ridgerepresents a duration of 3.2Myr. The P1 and P2 glacial cycles are ap-proximately 8 and 7Myr, respectively (Frank et al., 2015). These cal-culations assume no error in the Frank et al. (2015) age model, meaningdurations could resolve better if error is present in their age model.Alternatively, it is possible that only the end of the P1 and beginning ofthe P2 are captured in the stratigraphy of the study area, or some of therecord could be missing. If the alluvial facies represent an interglacialperiod, then the interval spans 4 to 5Myr based on the age model ofFrank et al. (2015). The ideas presented here provide a testable hy-pothesis such that if presence and absence of Gondwanan ice drove theeolian ridge—alluvial valley—eolian ridge cyclicity, then coeval eoliandeposits elsewhere in western equatorial Pangaea may exhibit a similarsignal.

8. Conclusions

The lower Permian Ingleside Formation near Manitou Springs,Colorado records two episodes of eolian deposition forming distinctiveridges that are punctuated by an interval of alluvial deposition housedin the valley between the two ridges. Within the eolian intervals,fourteen zones, termed super bounding zones (SBZ), were identified.These zones are laterally continuous, exhibit a consistent stratificationtype throughout the study area, have planar basal surfaces withminimal, if any, relief, and thickness of individual SBZ is relativelyconstant. Intervals between the SBZ represent cosets of climbing dunemigration or more rarely individual sets. Dominant dune transport di-rection was to southwest, which is consistent with expected onshoreprevailing winds from the epeiric sea located northeasterly of the study

Fig. 9. Area-weighted rose diagrams demonstrating azimuths of the down-dip direction after tilting of beds was removed. Data collected from eolian foresets on thewestern ridge (A) and eastern ridge (B). Black arrow is mean resultant vector.

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area. Each SBZ-coset couplet is inferred to be driven by glacioeustaticsea level change, where shoreline regressions resulted in a lower watertable and abundant sediment that was mobilized and transportedsouthwesterly into the study area. Subsequent transgression raised thewater table and trapped sediment resulting in deflation of the dune fieldto the capillary fringe of an elevated water table. The deflation surfacestabilized once the capillary fringe of the water table was intersectedresulting in the formation of the SBZ.

Three scales of cyclicity are observed in the Ingleside Formation.From lowest-to-highest frequency these are: 1) a change from pre-dominantly fluvial deposits of the underlying Fountain Formation to thepredominantly eolian deposits of the Ingleside Formation; 2) thepunctuation of two eolian ridges by an interval of alluvial facies, oreolian—alluvial—eolian cyclicity; and 3) SBZ-coset couplets containedwithin the eolian deposits. Respectively, we infer that the drivers ofthese different scales of cyclicity are: 1) the long-term amalgamationand northward drift of Pangaea; 2) absence or presence of Gondwananice that is characteristic of a dynamic late Paleozoic climate that swit-ched from greenhouse-to-icehouse climate states; and 3) waxing andwaning of global ice volume and associated glacioeustatic changeduring intervals that reflect large ice sheets on Gondwana.

Acknowledgments

This research was made possible by student grants to J. Pike pro-vided by the Colorado Scientific Society and the East Texas GeologicalSociety. We would like to extend sincere gratitude, as their supporthelped to offset expenses incurred during this research. We thank thecity of Colorado Springs for allowing the work at Red Rock CanyonOpen Space, and for giving permission to collect samples. J. Hessert, T.Jackson, and K. Chowdhury provided valuable field assistance. J.Browning helped prepare thin sections. Detailed reviews by R. Langfordand D. Loope greatly helped with clarity and content of this paper.

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