Recognition and Significance of Upper Devonian Fluvial, Estuarine, and Mixed Siliciclastic-Carbonate Nearshore Marine Facies in the GEOSPHERE, V

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GEOSPHERE Recognition and significance of Upper Devonian fluvial, estuarine, and mixed siliciclastic-carbonate nearshore marine facies in the GEOSPHERE, v. 15, no. 5 San Juan Mountains (southwestern Colorado, USA): Multiple incised https://doi.org/10.1130/GES02085.1

16 figures; 3 tables; 1 set of supplemental files valleys backfilled by lowstand and transgressive systems tracts James E. Evans1, Joshua T. Maurer1,2, and Christopher S. Holm-Denoma3 CORRESPONDENCE: [email protected] 1Department of Geology, Bowling Green State University, Bowling Green, Ohio 43403, USA 2Carmeuse Lime and Stone Company, 6104 Grand Avenue, Suite B, Pittsburgh, Pennsylvania 15225, USA

CITATION: Evans, J.E., Maurer, J.T., and Holm- 3Geology, Geophysics, and Geochemistry Science Center, U.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225, USA Denoma, C.S., 2019, Recognition and significance of Upper Devonian fluvial, estuarine, and mixed siliciclastic-carbonate nearshore marine facies in the ■■ ABSTRACT Allen and Posamentier, 1993; Catuneanu, 2006) and transgressive estuarine San Juan Mountains (southwestern Colorado, USA): Multiple incised valleys backfilled by lowstand and depositional systems (Cotter and Driese, 1998; Fischbein et al., 2009; Ainsworth transgressive systems tracts: Geosphere, v. 15, no. 5, The Upper Devonian Ignacio Formation (as stratigraphically revised) com- et al., 2011) in evaluating relative sea-level changes and the influence of allo- p. 1479–1507, https://doi.org/10.1130/GES02085.1. prises a transgressive, tide-dominated estuarine depositional system in the genic controlling variables (eustasy, tectonics, and sediment supply). In outcrop San Juan Mountains (Colorado, USA). The unit backfills at least three bedrock studies, the recognition of paleovalleys is complicated by available exposure Science Editor: David E. Fastovsky paleovalleys (10–30 km wide and 42 m deep) with a consistent stratigraphy versus the scale of the features. Similarly, estuarine facies may be difficult to Guest Associate Editor: Steven Whitmeyer ≥ of tidally influenced fluvial, bayhead-delta, central estuarine-basin, mixed tid- recognize because of lateral variability and extent, compared again to available

Received 15 November 2018 al-flat, and estuarine-mouth tidal sandbar deposits. Paleovalleys were oriented exposure. This study presents a new, integrated interpretation of the Upper Revision received 23 May 2019 northwest while longshore transport was to the north. The deposits represent Devonian sedimentary record for the southern Rocky Mountains based upon Accepted 17 July 2019 Upper Devonian lowstand and transgressive systems tracts. The overlying a depositional systems analysis of the Ignacio Formation. Upper Devonian Elbert Formation (upper member) consists of geographically There have been significant disagreements about the age, stratigraphy, and Published online 9 August 2019 extensive tidal-flat deposits and is interpreted as mixed siliciclastic-carbonate depositional environments of the Ignacio Formation in the San Juan Moun- bay-fill facies that represents an early highstand systems tract. Stratigraphic tains of southwestern Colorado, USA (Fig. 1). The unit has been variously revision of the Ignacio Formation includes reassigning the basal conglomerate considered Cambrian or Devonian (Fig. 2), and depositional interpretations to the East Lime Creek Conglomerate, recognizing an unconformity sepa- have ranged from shallow marine (Barnes, 1954; Baars, 1965; Baars and See, rating these two units, and incorporating strata previously mapped as the 1968) to colluvial fans, braided streams, lagoon-tidal flat, and marine shelf McCracken Sandstone Member (Elbert Formation) into the Ignacio Formation. deposits (Wiggin, 1987) to estuarine and tidal-flat deposits (Maurer and Evans, The Ignacio Formation was previously interpreted as Cambrian, but evidence 2011, 2013; McBride, 2016a). Key to understanding the geologic history of the that it is Devonian includes reexamined fossil data and detrital zircon U-Pb study area are four significant modifications of existing stratigraphic relation- geochronology. The Ignacio Formation has a stratigraphic trend of detrital ships introduced in a companion paper (Evans and Holm-Denoma, 2018) and zircon ages shifting from a single ca. 1.7 Ga age peak to bimodal ca. 1.4 Ga discussed further in this report. and ca. 1.7 Ga age peaks, which represents local source-area unroofing history. First, an enigmatic conglomeratic unit locally overlying Proterozoic base- Specifically, the upper plate of a Proterozoic thrust system (ca. 1.7 Ga Twilight ment rocks and typically considered part of the overlying Ignacio Formation Gneiss) was eroded prior to exposure of the lower plate (ca. 1.4 Ga Uncom- (Baars, 1966; Baars and See, 1968; Wiggin, 1987; Campbell and Gonzales, pahgre Formation). These results are a significant alternative interpretation 1996; Thomas, 2007) has been proposed as a new stratigraphic unit, the East of the geologic history of the southern Rocky Mountains. Lime Creek Conglomerate (Evans and Holm-Denoma, 2018). The East Lime Creek Conglomerate is 0–23 m thick, consists of cobble-boulder conglomerate and thin interbedded sandstone, and has buttressing relationships to ■■ INTRODUCTION the underlying Proterozoic rocks interpreted as paleo–sea cliffs, paleo–wave- cut platforms, and paleo-tombolos. The unit has been interpreted as a rocky This paper is published under the terms of the Over the past several decades there has been increasing recognition of the shoreline depositional system composed of upper shoreface-beachface tabular CC‑BY-NC license. significance of incised valley fills (Vail et al., 1977; Van Waggoner et al., 1990; cobble-boulder gravels and offshore subaqueous debris-flow deposits (Evans

107o45′W 107°30′W

Ti PP 550 Tv Silverton

K JTR PP Ti MDC Pg 18 17 + Study o Ti + + 37 45′N 15 16 N area JTR 14 + 13+ + + 10+ 12 + + + 9 11 PP 7 8 + + + + + 6 + + + + Neogene + Tv volcanic rocks + + + + + + + + PCu + + + Neogene + + + + + + Ti Ti intrusive rocks MDC + + + + + + + + + + Paleogene + + PP + Pg sedimentary rocks + + + + + MDC + 1 5 2 Cretaceous + + + + K 4 + + sedimentary rocks 3 + Triassic-Jurassic JTR sedimentary rocks

550 PP JTR Pennsylvanian--Permian JTR PP sedimentary rocks

JTR K Cambrian--Mississippian K MDC sedimentary rocks

+ + Proterozoic igneous Pg 160 PCu and metamorphic rocks Durango + + 37o15′N Stratigraphic section 10 km locations in Figures 3 9 and 15

Figure 1. Location map showing the study area in southwestern Colorado (USA) and locations of measured sections (numbers refer to sections in Table 1 and Figs. 3 and 15). Locations without numbers were used for samples, paleocurrents, and additional observations but not measured sections. Regional geology is modified from Steven et al. (1977) and Evans and Reed (2007).

GEOSPHERE | Volume 15 | Number 5 Evans et al. | Upper Devonian incised valley sequences, southern Rocky Mountains Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/5/1479/4831251/1479.pdf 1480 by guest on 28 September 2021 Research Paper Cooper (1955) Knight and (2018) Holm-Denoma Evans and (1905a) Cross et al. Fisher (1957) Rhodes and Knight (1957) Baars and See (1968) Baars and (1996) Gonzales Campbell & Evans (2007) (2016a) McBride (1954) Barnes

Schematic section Ouray Limestone (Dyer Formation) Unit C upper mbr. upper mbr. upper mbr. upper mbr. upper mbr. Elbert Fm. Elbert Fm. Elbert Fm. Elbert Fm. Elbert Fm. Elbert Fm. Elbert Fm. Elbert Fm. Elbert Fm. Elbert Fm.

Unit B Upper Devonian

SS Mbr. McCr. SS Mbr. McCr. SS Mbr. McCr. SS Mbr. McCr. SS Mbr. McCr. Lower Devonian(?) Figure 2. Stratigraphic nomenclature chart showing redefinition of the Ignacio Upper Cambrian-- Formation and the recently recognized Lower Ordovician Devonian age of the unit (from Evans and Spud Hill Mbr. Holm-Denoma, 2018). Abbreviations used: Fm.—Formation; Mbr.—Member; Cgl.— Stag Mesa Mbr. Upper Cambrian Conglomerate; W.S. Mbr.—”Weasel Skin Ignacio Quartzite Ignacio Quartzite Ignacio Quartzite Ignacio Quartzite Ignacio Quartzite Ignacio Formation Ignacio Formation Ignacio Quartzite Ignacio Formation Ignacio Formation Member” (never formally proposed); McCr. SS Mbr.—McCracken Sandstone Member. Proterozoic See text for details.

igneous and metamorphic

Tamarron Mbr. rocks conglomerate

sandstone

siltstone and mudstone Mbr. W. S. unnamed unnamed East Lime cgl. unit formation Creek Cgl. carbonates Proterozoic Uncompahgre Formation, Twilight Gneiss, granite

and Holm-Denoma, 2018). Because of poor age constraints, the age of the unit phengite prior to deposition of the overlying units. Finally, erosional reworking was previously considered Neoproterozoic–Cambrian (Wiggin, 1987; Camp- of the top of the unit is indicated by rare sericite-cemented sandstone clasts bell, 1994a, 1994b; Condon, 1995; Gonzales et al., 2004; Evans, 2007; McBride, incorporated into the overlying Devonian units (Evans and Holm-Denoma, 2018). 2016a), although Spoelhof (1976) and Wiggin (1987) proposed that it might Third, although many previous workers have considered the age of the be as young as Devonian. All of these previous age interpretations assumed Ignacio Formation to be Cambrian (Cross et al., 1905a, 1905b; Knight and that the overlying Ignacio Formation is Cambrian (see below). However, the Cooper, 1955; Baars and Knight, 1957; Rhodes and Fisher, 1957; Baars, 1966; unit was found to have a single late early Silurian (436 ± 17 Ma) detrital zircon, Baars and See, 1968; Campbell 1994a, 1994b; Thomas, 2007), there is now and this combined with field relations suggest that the unit is probably Lower strong evidence that the unit is Devonian. Briefly, the key new age consider- Devonian (Evans and Holm-Denoma, 2018). ations are that: (1) individual beds have been found containing the problematic Second, there is a low-angle (<10°) disconformity between the East Lime Cambrian(?) Obulus brachiopods alongside Late Devonian placoderm fish Creek Conglomerate and Ignacio Formation, although it can be difficult to fossils (McBride, 2016a); (2) the unit contains Cambrian and Ordovician detrital observe at some locations due to obscure bedding attitudes in the uppermost zircons (McBride, 2016a); and (3) the Ignacio Formation overlies the Lower(?) conglomerate beds (Evans and Holm-Denoma, 2018). In addition, sandstone Devonian East Lime Creek Conglomerate (Evans and Holm-Denoma, 2018). in the East Lime Creek Conglomerate has sericite (sensu Eberl et al., 1987) In addition, some previous workers suggested the Ignacio Formation could cements, unlike the overlying Ignacio Formation. The sericite cements are be Devonian because of the absence of an unconformity between the Ignacio interpreted as evidence of surficial weathering and early diagenetic alteration of Formation and the overlying Devonian Elbert Formation (Read et al., 1949; primary mixed-layer illite and/or smectite clays to fine-grained muscovite and/or Barnes, 1954; Maurer, 2012).

Finally, strata in the San Juan Mountains that were previously considered TABE 1. OCATIONS OF STRATIGRAPHIC SECTIONS to be part of the overlying Devonian McCracken Sandstone Member of the ocation Informal description atitude ongitude Elbert Formation have been reassigned to the Ignacio Formation (Evans and number Holm-Denoma, 2018). The McCracken Sandstone Member is defined from one 1 Fall Creek 37.50278N 107.5503W exploration well in the Paradox Basin (the Four Corners area) (Cooper, 1955; 2 Canyon Creek 37.85111N 107.7314W Baars and Knight, 1957), and there have been significant disagreements about 3 Bakers Bridge 37.45889N 107.8006W whether or not the unit is exposed in the San Juan Mountains. Baars (1965) 4 Shalona ake railroad outcrop 37.48556N 107.8058W argued that the unit did not extend far enough eastward to appear in the San 5 Rockwood 37.48694N 107.8078W 6 Milepost 53.5 (U.S. Highway 550) 37.67639N 107.7911W Juan Mountains. Other workers have argued that the McCracken Sandstone 7 Milepost 54 (U.S. Highway 550) 37.68056N 107.7861W Member can be recognized in the San Juan Mountains using the criteria that 8 Coal Bank Pass south 37.69722N 107.7775W it is generally whiter in color, harder (due to silica cement), more quartzose 9 Meadow below Coal Bank Pass 37.68556N 107.7606W rich, better sorted, and has better rounded grains than sandstone of the Ignacio 10 West side of ime Creek 37.70889N 107.7592W Formation (Knight and Cooper, 1955; Baars and See, 1968; Campbell and Gon- 11 Type section of the ECC 37.70917N 107.7375W zales, 1996; Thomas, 2007; McBride, 2016a). However, those distinctions are not 12 East side of ime Creek 37.70944N 107.7347W 13 Andrews ake trail 37.71444N 107.7147W statistically robust—on petrofacies plots, there is significant overlap within one 14 Molas Creek waterfall 37.73944N 107.6861W standard deviation (Evans and Holm-Denoma, 2018). McBride (2016a) proposed 15 East of Molas ake 37.73749N 107.6772W remapping the Ignacio Formation and McCracken Sandstone Member as two 16 Sultan Creek–Molas Creek valley 37.75751N 107.6756W coeval, geographically adjacent units. In contrast, Evans and Holm-Denoma 17 Sultan Creek south 37.71001N 107.6753W (2018) reassigned these strata to the Ignacio Formation because of the: (1) 18 Sultan Creek north 37.76501N 107.6753W absence of an unconformity between the units; (2) absence of statistically signif- Note: Coordinates are in reference to North American Datum of 1927. icant petrologic differences; (3) evidence from facies analysis and paleocurrent ocation numbers refer to Figures 1 and 3. ECCEast ime Creek Conglomerate (Evans and Holm-Denoma, 2018). data that the units formed an integrated depositional system (Maurer and Evans, 2011, 2013); and (4) geochronologic evidence that both units are Upper Devonian.

Research Laboratory in Denver, Colorado. Zircon was ablated with a Photon Supplemental Table 1. Petrofacies Summary from Ignacio Formation and former McCracken SS Member (Ev■ans e■t aMETHODSl. 2018) Machines Excite 193 nm ArF excimer laser in spot mode (150 total bursts per Sample No. n Qm Qp Chert P K Lv Lm Ls Mica Accessory Matrix Cement

S-222 308 40.9 12.3 1.0 0 27.9 0.6 0 0 1.0 1.9 0.6 13.6 grain) with a repetition rate of 5 Hz, laser energy of ~3 mJ, and an energy S-223 237 50.2 29.1 0 0 5.9 0.5 0 0 0 0.8 3.0 10.6 S-224 292 53.4 5.5 2.4 0 0.7 0 0 0 0 0.3 0 37.6 2 S-240 295 62.4 14.6 0 0 2.4 0 0 0 0 0.3 0 20.4 Eighteen (18) stratigraphic sections were measured in the field and their density of 4.11 J/cm . Pit depths were typically <20 µm. The rate of He carrier S-252 326 54.9 4.0 0 0 1.0 0 0 0 0 0 0 40.5 S-253 272 72.1 3.7 1.8 0 0 0 0.4 0 0 0 4.0 18.4 02JE2 294 31.6 18.4 0 0 6.8 0 0.3 0.7 1.7 3.1 21.1 16.3 locations recorded using GPS (Table 1). Sixty-six (66) thin sections for sandstone gas flow from the HelEx cell of the laser was ~0.6 L/min. Make-up Ar gas 02JE3 282 65.9 5.0 0 0 0 0 0 0 0 1.1 8.9 8.6 02JE6 212 30.6 12.3 0.5 0 0 0 0 0 0 0 0.5 52.9 02JE7 262 52.3 3.4 0 0 0.4 0 0 0 0 0.8 0.4 42.8 petrography were prepared using standard methods, and composition was (~0.2 L/min) was added to the sample stream prior to its introduction into the 05JE21 291 52.2 19.6 0.7 0 0 0 0.3 0 0 1.7 7.9 18.2 05JE22 241 46.1 18.3 0 0 0 0 0.4 0 0 0 7.5 26.1 05JE26A 225 15.6 9.8 0 0 0 0 0 0 0 0 0 73.3 determined from point counting >300 grains per slide (total n = 18,769) following plasma. Nitrogen with a flow rate of 5.5 mL/min was added to the sample 05JE26B 217 4.6 8.8 0 0 0 0 0 0 0 0 0 86.2 05JE27 288 56.6 24.0 0 0 0 0 0 0 0 0 12.8 4.5 1 07JE5A 271 41.0 11.2 0 0 0 0 0 0 0 1.1 4.8 40.9 the methods of Dickinson et al. (1983) (Table S1 in the Supplemental Materials ). stream to allow for significant reduction in ThO/Th (<0.5%) and improved 07JE5B 274 50.7 16.8 0 0 0.3 0 0 0 0 0.7 9.1 21.1 08JE5 265 62.3 14.3 0 0 0 0 0 0 0.4 0 1.9 18.5 08JE6 286 57.3 12.6 0 0 0 0 0 0 0.7 0 3.1 21.6 Paleocurrent interpretations were based upon 124 unidirectional measurements ionization of refractory Th (Hu et al., 2008). The laser spot sizes for zircon 08JE7 295 56.9 19.6 0 0 0 0 0 0 1.0 0 5.8 18.4 08JE12 322 59.6 9.9 0 0 0 0 0.6 0 0.3 0 10.2 6.5 08JE19 229 14.4 27.5 0 0 0 0 0 0 0.9 7.9 0.4 49.0 of cross-bedding and from 24 bidirectional measurements from the orienta- were ~25 µm. With the magnet parked at a constant mass, the flat tops of the 08JE20 301 16.6 1.3 0 0 0 0 0 0 0 0.7 0 80.7 08JE24 278 47.1 8.3 0 0 0 0 0 0 0.4 0 5.4 31.6 202 204 206 207 208 232 235 238 08JE25 248 65.3 12.0 0 0 0 0 0 0 0 0 0 25.4 tion of the crests of wave ripple marks exposed on bedding surfaces. Vector isotope peaks of Hg, (Hg + Pb), Pb, Pb, Pb, Th, U, and U were 08JE29 225 20.9 3.6 0 0 0 0 0.9 0 0.4 2.7 0.9 67.7 08JE39 334 52.1 15.9 0 0 4.2 0 0 0 0 0.3 3.9 15.9 08JE40 320 61.9 9.4 0 0 10.6 0 0 0 0 0 0.6 18.2 08JE42 302 58.9 9.6 0 0 0.3 0 0 0 0.3 1.7 5.3 21.3 means were calculated and plotted for each location, and rose diagrams were measured by rapidly deflecting the ion beam with a 30 s on-peak background 08JE43 283 53.7 11.7 0 0 0 0 0 0.3 0 0.3 0 33.9 09JE7 259 62.2 9.7 0 0 0.5 0 0 0 0 0 0.8 28.2 09JE8 239 22.6 2.9 0 0 0 0 0 0 0 0 0.4 74.1 created using a nonlinear scale (Nemec, 1988), showing vector mean, circular measured prior to each 30 s analysis. 09JE9 278 23.0 5.8 0 0 0 0 0 0 0 0 0 70.5 09JE10 299 30.1 13.5 0 0 0 0 0 0 0.3 1.3 0 63.1 09JE11 288 5.2 2.1 0 0 0 0 0 0 0 1.4 0 89.2 standard deviation (Krause and Geijer, 1987), and Rayleigh test of significance Raw data were reduced offline using the Iolite 2.5 program (Paton et al., 09JE12 236 52.1 5.9 0 0 1.4 0 0 0 0 1.7 3.0 31.4 10JE3 217 6.0 1.4 0 0 0 0 0 0 0 0 0 88.5 10JE6 273 50.9 10.6 0 0 0 0 0 0 0 0.3 4.4 31.5 (Curray, 1956). The paleocurrent data sets are statistically significant (p <0.05). 2011) to subtract on-peak background signals, correct for U-Pb downhole frac- 10JE7 298 40.9 25.5 0 0 0 0 0 0 0 0 20.1 12.5 10JM1 244 53.7 3.3 0 0 2.5 0 0 0 0 0 6.6 32.0 10JM2 317 51.4 13.6 0 0 10.7 0 0 0 0 0 6.9 17.3 Five samples were collected for detrital zircon U-Pb geochronology. For tionation, and normalize the instrumental mass bias using external mineral 10JM4 265 36.2 15.5 0 0 24.2 0 0 0 0 0.4 2.6 20.8 each sample, 2 kg were disaggregated using standard crushing techniques, reference materials, the ages of which had previously been determined by then zircons were concentrated using heavy liquid and magnetic separation. isotope dilution–thermal ionization mass spectrometry (ID-TIMS). Ages were Zircon grains were mounted in epoxy, polished, and evaluated for zoning and corrected by standard sample bracketing with the primary zircon reference 1 Supplemental Materials. Petrology data; detrital zir- inclusions using cathodoluminescence. material Temora2 (417 Ma; Black et al., 2004) and secondary reference mate- con U/Pb geochronology data. Please visit https:// Analyses of zircon grains were conducted using a Nu Instruments AttoM rial Plešovice (337 Ma; Sláma et al., 2008) and USGS standard WRP-63-08 doi.org/10.1130/GES02085.S1 or access the full-text article on www.gsapubs.org to view the Supplemen- laser ablation–single collector–inductively coupled plasma mass spectrome- (1707 Ma). Reduced data were compiled into a probability density diagram tal Materials. ter (LA-SC-ICPMS) at the U.S. Geological Survey (USGS) Southwest Isotope using Isoplot 4.15 (Ludwig, 2012). 206Pb/238U ages are reported for zircon

analyses 20% were excluded from probability density Formation, and/or combinations of those units was due to a complex history plots (Table S2 [footnote 1]). of deposition, pre-Pennsylvanian block uplifts (Grenadier horst, Sneffels horst, and unnamed horsts and grabens), erosion, subsidence, and further deposition (Baars, 1966; Baars and See; 1968; Weimer, 1980; Baars et al., 1987); according ■■ RESULTS to this argument, stratigraphic relationships required recurrent movement on faults, and even reversals of movement sense on individual faults. Stratigraphic Relationships Most of the evidence for the first proposal is from what is now recognized as the East Lime Creek Conglomerate rather than the Ignacio Formation. Detailed Description examination reveals that neither unit shows (1) evidence for fault controlled proximal-distal trends in lithology, thickness, or grain size; (2) tectonically con- The stratigraphic sections are correlated using the conformable contact trolled changes in facies distributions; (3) changes or reversals in paleocurrent between the Ignacio Formation (as redefined) and the overlying upper mem- patterns; or (4) changes in sediment source areas. Further inspection of the ber of the Elbert Formation (Fig. 3). This upper contact marks a change from hypothesized fault zones failed to show any direct evidence for faults (i.e., drag the dominantly fluvial and estuarine sandstone of the Ignacio Formation to folds, slickensides, or fault gouge), nor evidence for seismogenic features in the marine shale and carbonate of the upper member of the Elbert Formation. the adjacent sedimentary units, such as fluid-escape structures or convoluted Outcrop correlation reveals three important relationships. bedding (e.g., Evans, 1994; Myrow and Chen, 2015). First, the Ignacio Formation is lithologically heterogeneous (sandstone, Misinterpretation of different depositional facies within the Ignacio Forma mudrocks, and minor carbonate, conglomerate, and replaced evaporite) and tion as different stratigraphic units is responsible for the second proposal. stratigraphically complex, with different lithologies dominant at intervals As a laterally extensive depositional system, the Ignacio Formation presents throughout the section. A number of locations have a sandstone-rich interval different depositional facies at different locations. The presence or absence at the top of the section, which was the basis for previous workers calling this of depositional facies at any location does not require faulting. It should be interval the McCracken Sandstone Member. However, sandstone-rich intervals noted that McBride (2016a) used a different stratigraphic approach yet reached are interspersed throughout the Ignacio section, and the uppermost part of substantially the same conclusion about faulting. In summary, this study finds the unit is not sandstone rich at all locations. no evidence for post-Proterozoic but pre-Pennsylvanian (i.e., pre–Ancestral Second, the thickness of the Ignacio Formation is highly variable between Rocky Mountains) faulting in the study area. locations. Where exposed, the basal contact is an unconformity that rests on either Proterozoic crystalline rocks or the East Lime Creek Conglomerate, with erosional relief typically <2.5 m. Overall, the Ignacio Formation varies locally Facies Analysis between 0 and 42 m thick. Third, the relationship of Devonian sedimentary rocks to underlying Pro- Twenty-six (26) lithofacies are observed in the Ignacio Formation (Table 2). terozoic basement shows the importance of paleotopography (Fig. 3). Using Following standard practice, these lithofacies are grouped into seven lithofa- the measured sections and observations at additional locations where the cies associations (FA1–FA7) representing specific depositional environments upper member of the Elbert Formation directly overlies Proterozoic basement, or subenvironments (Table 3). there was ≥65 m of paleorelief on this contact. Uncertainties about paleorelief are due to erosional loss of the East Lime Creek Conglomerate and because the base of the Ignacio Formation is not always exposed. Facies Association 1 (FA1): Fluvial Deposits

Description. FA1 consists of channeliform sandstone bodies interpreted as Interpretation single-story and multistory channel fills, and intervening fine-grained deposits with poorly developed paleosols (Fig. 4). The lower parts of channel-fill suc- Previous workers advanced two arguments to explain lithologic variation cessions consist of red trough cross-bedded sandstone and planar-tabular and complex stratigraphic relations in the lower Paleozoic section in the San cross-bedded sandstone that form sets up to 50 cm thick and stacked cosets Juan Mountains. The first argument is that syndepositional faulting controlled up to 1.5 m thick (Figs. 5A, 5B). Cross-bed sets are separated by internal Ignacio Formation deposition and explains the juxtaposition of the Ignacio For- erosion surfaces and channel lags consisting of thin pebble conglomerate mation and Proterozoic rocks (Baars, 1966; Baars and See; 1968; Thomas, 2007). layers (Fig. 5B), rare mudstone intraclasts, and massive pebbly sandstone. The second argument is that the local presence or absence of the conglomerate, The upper parts of channel-fill successions include ripple-laminated sandstone

1 2 3 4 5 6 7 8 9 Fall Creek Canyon Bakers Shalona Lake Rockwood MP 53.5 MP 54 Coal Bank Pass Meadow below Creek Bridge railroad outcrop (south side) Coal Bank Pass

VVV VVV

VVV

VVV VVV

VVV

Elbert Fm. (upper mbr.)

Ignacio Fm. sandstone shale carbonate Grain-size scale East Lime Creek Conglomerate vfg fg mg cg vcg p cb b ms Proterozoic units

10 m granite Twilight Gneiss Uncompahgre Fm.

Figure 3. Measured stratigraphic sections in the Ignacio Formation. Numbers refer to locations shown in Figure 1. Coordinates are given in Table 1. Horizontal correlation line marks the contact with the upper member of the Elbert Formation. MP—highway milepost; ELCC—East Lime Creek Conglomerate; mbr.—member; Fm.—Formation. Grain-size abbreviations: ms—mudstone; vfg—very fine-grained sandstone; fg—fine-grained sandstone; mg—medium-grained sandstone; cg—coarse-grained sandstone; vcg—very coarse-grained sandstone; pbl—pebble conglomerate; cb—cobble conglomerate; b—boulder conglomerate. (Continued on following page.)

10 11 12 13 14 15 16 17 18 West side of Type section East side of Andrews Molas Creek East of Sultan-Molas South of North of Lime Creek ELCC Lime Creek Lake Trail waterfall Molas Lake fault valley Sultan Creek Sultan Creek

V V V

Symbols:

conglomerate, aser, wavy, or intraclasts lenticular bedding planar-tabular VVV mudcracks cross-bedding

trough mudcrack breccias cross-bedding planar burrows lamination Grain-size scale current ripples surface traces vfg fg climbing ripple ms mg cg vcg p cb b shells lamination

wave ripples bioherm 10 m

Figure 3 (continued).

TABE 2. ITHOFACIES CODES AND DESCRIPTIONS Code ithology Sedimentary structures Interpretation Gm Conglomerate, pebble Massive Thin fluvial or estuarine channel lags Smc Sandstone, cg–vcg Massive, can be pebbly Rapid deposition, destratified, lags Smn Sandstone, fg–cg Massive, normal grading Fallout from suspension Smf Sandstone, vfg–mg Massive Rapid deposition or destratified Sp Sandstone, mg–vcg Planar-tabular cross-bedded Two-dimensional dunes Sph Sandstone, mg–cg Herringbone cross-bedded Two-dimensional dunes with flow reversals St Sandstone, mg–vcg Trough cross bedded, festoon cross-bedded Three-dimensional dunes Sr Sandstone, vfg–mg Asymmetrical ripples, climbing ripples Current ripples Srw Sandstone, vfg–mg Continuously crested symmetrical ripples Wave ripples Sl Sandstone, vfg–fg ow-angle, planar to wavy laminated ow-angle inclined planar bedding Se Sandstone, vfg–vcg Massive with mud intraclasts Mud clasts derived from mudcracks SSl Siltstone Planar laminated Fallout from suspension SSm Siltstone Massive Rapid deposition or destratified SMf Heterolithic (ss–ms) Flaser bedded Ripples with mud drapes SMw Heterolithic (ss–ms) Wavy bedded Ripples with mud drapes SMk Heterolithic (ss–ms) enticular bedded Ripples with mud drapes SMl Heterolithic (ss–ms) Planar laminated Tidalites Fl Mud shale Planar laminated Fallout from suspension settling Fm Mudstone Massive Rapid deposition or destratified Cm Carbonate, mudstone Massive Carbonate mud (micrite) Cp Carbonate, packstone Massive with shell hash Storm layer with shell debris Cf Carbonate, floatstone Massive with mud clasts Mud clasts derived from mudcracks Cb Carbonate, bindstone aminated (planar, wavy, or domal) Biostratification features Cn Carbonate, nodular Disrupted bedding, teepees, hoppers, nodules Desiccation or evaporite dissolution P Pedogenic carbonate Small nodules Poorly developed paleosol Coal Coal fragments Wood debris Abbreviations: vcgvery coarse grained; cgcoarse grained; mgmedium grained; fgfine grained; vfgvery fine grained; sssandstone; msmudstone.

TABE 3. FACIES ASSOCIATIONS Code Facies association ithofacies Ichnofacies Organiation Interpretation FA1 Fluvial channel and St, Sp, Sr, Gm, Smc, Sl, Smf, SSm, Fm Rare (Sk) Broadly lenticular, erosive base and Multistory channel fill floodplain internal erosion surfaces, 3–6 m thick, mostly sandy two- and three- dimensional dunes FA2 Tidally influenced fluvial St, Sp, Smc, Sph, Srw, SMf, SMw, SMk, Fl, Fm Rare (Sk, P) Broadly lenticular, erosive base, flow Estuarine point bar channel reversals, drapes, mud chips FA3 Bayhead delta St, Sp, Sr, Smn, Gm, Sl, SMf, SMw, SMk, Fl, Fm Minor (Sk) Inclined heterolithic strata, climbing Bayhead delta ripples, wave-modified turbidites FA4 Estuarine channel and St, Sp, Sr, Srw, Sl, Smf, SMf, SMw, SMk, Fl, Fm Major: P, Pa, Th, Broadly lenticular sand bodies with mud Central estuarine basin central basin Ro, Di, Mo, Tr, Ga drapes, extensive bioturbation (local firmgrounds) FA5 Tidal flat (mostly Sl, Srw, Smf, Se, SSl, SMf, SMw, SMk, SMl, Fm Major: Ru, Sk, P Sheet like, poorly exposed, with small Estuary margin siliciclastic) channels, mud drapes, mud chips FA6 Peritidal and supratidal Cm, Cp, Cf, Cb, Cn, Fm Biostratification Small bioherms, disrupted bedding, Estuary margin flat (mostly carbonate) teepee structures, nodules, hoppers FA7 Estuarine-mouth tidal St, Sph, Srw, Sl, Smf, Fm Major: P, Th, o Tabular, internal erosion surfaces, may Tidal sand bar sand bar be capped by firmgrounds Note: See Table 2 for explanation of lithofacies codes. Ichnofacies codes: DiDiplocraterion; GaGastrochaenolites; oLockeia; MoMonomorphichnus; PPlanolites; Pa Palaeophycus; RoRosselia; RuRusophycus; SkSkolithos; ThThalassinoides; TrTrichophycus.

Canyon Creek US Highway 550 MP 53.5 Bakers Bridge ms vfg fg mg cg vcg pbl ms vfg fg mg cg vcg pbl ms vfg fg mg cg vcg pbl

upper member (Elbert Fm.) upper member (Elbert Fm.) upper member (Elbert Fm.) MFS VVV MFS MFS VVV Smf FA7 tidal FA6 upper FA7 tidal St sandbars Cb tidal flat sandbars Sp St F/TRS Srw FA7 tidal FA7 tidal SMf FA4 estuarine sandbars channel fill St sandbars Sl F/TRS Fl F/TRS Cm FS Cm FA4 estuarine FA4 estuarine Sph Sl channel fill FA4 estuarine channel fill F/TRS F/TRS VVV St channel fill FA4 estuarine FA4 estuarine St channel fill SMl channel fill F/TRS St FA4 estuarine F/TRS Smc F/TRS SMk channel fill Smc Smf F/TRS FA4 estuary Smf FA4 estuary F/TRS Figure 4. Detailed stratigraphic sections at Canyon central central Creek (location 2 in Figs. 1 and 3), along U.S. High- Smf basin basin St Srw SMl way 550 at milepost (MP) 53.5 (location 6), and at Fl FS Bakers Bridge (location 3). Lithofacies (and their SMk SMl FA4 estuarine TSE codes) are described in Table 2. Facies associations channel fill FA2 tidally (bold italics) are described in Table 3. Abbrevia- Smc VVV tions: FS—flooding surface; F/TRS—fluvial/tidal Smc F/TRS influenced SMl fluvial ravinement surface; MFS—maximum flooding sur- F/TRS Sp, Sph Smc TSE FA4 estuary channel fill face; SB—sequence boundary; TSE—transgressive central basin (?) Fl FS surface of erosion; Fm.—Formation; IHS—inclined SMf heterolithic stratification. See Figure 3 for definitions (poorly exposed) Gm FA1 fluvial of grain-size abbreviations and lithology symbols. multistory channel fill Sl FA3 bayhead coal k St delta (IHS) fragments Gm FA1 fluvial St channel fill Sr FS SB St Symbols: Bakers Bridge Granite FA2 tidally influenced Sp fluvial channel fill conglomerate, flaser, wavy, or St intraclasts lenticular bedding Fl 10 meters planar-tabular VVV mudcracks cross-bedding Sl FS trough mudcrack breccias St cross-bedding Sl planar FA1 multistory lamination burrows Sr fluvial Gm current ripples surface traces channel fill Gm climbing ripple shells Sl lamination St wave ripples bioherm Fm

A Gm

Figure 5. Outcrop photographs of fluvial (facies as- Gm sociation FA1; see Table 3) and tidally-influenced fluvial (FA2) deposits. (A) Thin conglomerate (lithofacies Gm; see Table 2) at the base of fluvial channel-fill with imbricated clasts. Scale bar in centimeters. (B) Cosets of trough cross-bedded sandstone (lithofacies St) and overlying granule D conglomerate (lithofacies Gm). Arrows indicate set boundaries. Scale bar is 15 cm. (C) Proximal overbank sequence of stacked thin, massive, fine-grained red sandstones (lithofacies Smf) and thin, laminated, coarse-grained white sandstones (lithofacies Sl). Fm Subsequent fluid flow through the better-sorted, Smf coarser layers bleached the iron oxides. Scale bar Sr is 15 cm. (D) Single sets of trough cross-bedded sandstone (lithofacies St) and ripple-laminated sandstone (lithofacies Sr) with mud drapes (lithofacies Fm). Hammer is 30 cm. (E) Channel-fill sandstone St with wedge-shaped beds of trough cross-bedded sandstone (lithofacies St) and mud drapes, overlying Fm floodplain mudstones (lithofacies Fm). Hammer is Sl 28 cm. (F) Thinning- and fining-upward (represented by white triangle) channel fill consisting of packages of massive fine-grained sandstones (lithofacies Smf), laminated sandstones (lithofacies Sl), and mud E drapes (lithofacies Fm), overlying floodplain mud- Fm stones (lithofacies Fl). Hammer is 28 cm. Sl St Smf

Fm Fl

and planar-laminated sandstone. Multistory sandstone bodies are 3–7 m thick, Interpretation. FA2 is interpreted as tidally influenced fluvial deposits having an erosive base and internal erosion surfaces. because of the combination of similar appearance, organization, and architec- Interbedded with channeliform sandstone bodies are thin (<2 m) inter- ture to FA1, and the presence of heterolithic flaser, wavy, and lenticular bedding vals of massive red fine-grained sandstone, siltstone, and mudstone (Fig. 5C). (Reineck and Wunderlich, 1968); evidence for flow reversal (herringbone Locally, these are interbedded with planar laminated red sandstone, siltstone, cross-bedding); wave ripples; and trace fossils that may indicate brackish- and mud shale. The red finer-grained deposits are commonly interbedded water conditions (MacEachern et al., 2005). Tidal effects in modern estuaries with thin stringers of coarse-grained white sandstone (Fig. 5C). The white are known to extend upstream into rivers for tens of kilometers (Allen, 1991). color probably represents bleaching of hematite during late diagenetic fluid migration through the coarser-grained and better-sorted layers. Bioturbation is generally absent, but there are rare examples of Skolithos Facies Association 3 (FA3): Bayhead Delta Deposits in the upper parts of complete channel-fill successions. Finally, the upper parts of finer-grained successions show a combination of carbonate content and Description. FA3 deposits are present at several locations in the study area. minor stratal disruption that suggests poorly developed calcisols. Key diagnostic features in these deposits are composite sets of meter-scale, Interpretation. FA1 is interpreted as the deposits of sand bedload to mixed- low-angle sigmoidal trough cross-bedding with intervening complete to partial load fluvial systems that formed broadly lenticular channels with typical mud drapes (Fig. 6A). Similar deposits have been called inclined heterolithic channel depths <7 m. Based on paleocurrent data, the primary channel-fill- stratification (IHS) (Thomas et al., 1987), and modern examples show the ing elements were downstream-accreting two- and three-dimensional dunes interaction of fluvial currents and tidal reversals (Fenies et al., 1999). Steep- with intervening pool fills (thin conglomerate stringers and wedge-shaped sided, small-scale channeling is also notable in FA3 (Fig. 6B), demonstrating cross-bedded sets in sandstone). Lateral accretion surfaces were not observed. greater sediment cohesion in the finer-grained estuarine sediments versus The interbedded finer-grained intervals are interpreted as proximal overbank sandy fluvial sediment in FA1 and FA2. deposits. The presence of overbank deposits, rare mudstone intraclasts, and Repeated facies successions in FA3 include (1) vertically stacked packages local coal fragments suggests some level of incipient bank stability and epi- of low-angle to vertically climbing ripple-laminated, fine- to medium-grained sodic bank failures (e.g., Plint, 1986). However, the interpreted floodplain red sandstone (Fig. 6C); (2) packages consisting of (in ascending order) an deposits were dominantly noncohesive (sand- and silt-sized) materials. This erosion surface; massive, normally graded sandstone; planar-laminated sand- implies that the banks of paleochannels were relatively nonresistant, low, stone; and capping wave ripples (Fig. 6C); and (3) intervals of interbedded and easily overtopped, which is supported by the observed poorly developed herringbone cross-bedded sandstone, heterolithic flaser-bedded sand- paleosols. Finally, although Skolithos is typically found in marine rocks, it is stone-mudstone couplets, laminated siltstone, and thin mudstone drapes. known to occur in fluvial deposits (e.g., Trewin and McNamara, 1994). Locally interbedded with these facies successions are thin pebble stringers or lenses of pebbly coarse-grained sandstone (Figs. 6C, 6D). The upper surface of FA3 deposits is typically an erosional lag deposit (Fig. 4), and the contact Facies Association 2 (FA2): Tidally Influenced Fluvial Deposits between FA3 deposits and overlying estuarine central basin deposits (FA4) is marked by a color change from red to green-gray (Fig. 6E). Description. FA2 deposits resemble those of FA1 as single and multistory Interpretation. FA3 is interpreted as representing bayhead delta depo- channel-fill successions dominated by trough cross-bedded and planar-tabu- sitional environments, based upon diagnostic features and structures seen lar cross-bedded sandstone units separated by internal erosion surfaces with in modern and ancient examples (Aschoff et al., 2018; Simms et al., 2018). interbedded pebble stringers. As with FA1, the deposits are organized into The low-angle, sigmoidal IHS is a common feature in bayhead deltas, rep- broadly lenticular channel bodies up to ~5 m thick. The key difference between resenting river flows into mixed-salinity receiving water bodies. Intervals FA1 and FA2 is evidence for current reversals in FA2, including: (1) multistory of climbing-ripple sandstone are interpreted as delta-foreset or distributary channel deposits with interbedded herringbone cross-bedded sandstone; (2) mouth-bar deposits caused by flow deceleration and rapid deposition of bed- thin interbedded intervals of heterolithic flaser-bedded, wavy-bedded, or len- load (Wright, 1977). The event layers with wave-rippled upper surfaces are ticular-bedded sandstone-mudstone couplets; and (3) thin mudstone drapes interpreted as wave-modified turbidites (Myrow et al., 2002). The presence of between cross-bed sets (Fig. 5D). Partially mud-draped cross-bed surfaces wave-modified turbidites in these deposits is consistent with episodic density suggest remolding by tidally influenced flow modification (Fig. 5E). Fining- underflows at the delta front into relatively shallow water affected by wave and thinning-upward channel-fill sequences are observed (Fig. 5F) and may processes. Turbidity currents are the primary mode of sediment transport from represent channel abandonment and avulsion processes. Rare trace fossils fluvial channels to the delta front in bayhead deltas (Aschoff et al., 2018). The (Skolithos and Planolites), mud-chip intraclasts, and wave-rippled sandstone heterolithic deposits with tidal sedimentary structures are interpreted as tid- are also present in FA2. alites (Longhitano et al., 2012). Intervals of tidalites interbedded with bayhead

IHS

Srw D Sl Figure 6. Field photographs of bayhead delta (facies association FA3; see Table 3) deposits. (A) Compos- Smn ite set of inclined heterolithic strata (IHS) showing sigmoidal trough cross-bedded sandstone (lithofacies St; see Table 2) and mud drapes (lithofacies Smc Fm). Hammer is 28 cm. (B) Steep-sided channel incised into estuarine central basin deposits and Sr infilled with massive fine-grained sandstones (lithofacies Smf) and mud drapes (lithofacies Fm). Scale bar is 15 cm. (C) Climbing ripple-laminated sandstone (lithofacies Sr), eroded and overlain by pebbly massive coarse-grained sandstone (lithofacies Smc) representing a discontinuously bedded, winnowed surface (fluvial-tidal ravinement surface). These are overlain by normally graded massive sandstone (lithofacies Smn), laminated sandstone (lithofacies E Sl), and wave-rippled sandstone (lithofacies Srw), interpreted as a wave-modified turbidite. Scale bar is 15 cm. (D) Interbedded reddish massive fine-grained sandstones (lithofacies Smf) and white pebbly massive coarse-grained sandstones (lithofacies Smc) in EB the delta front. The erosional lags are interpreted as fluvial-tidal ravinement surfaces (arrows). Hammer is 28 cm. (E) Color transition from reddish bayhead delta (BHD) to gray-green central estuarine basin (CEB) and estuarine bar (EB) deposits (person CEB for scale).

BHD

delta-front deposits probably represent progressive infilling of abandoned low-diversity trace-fossil assemblages dominated by Planolites or dominated distributary channels. by a combination of Rosselia and Thalassinoides. Vertical mixing extended Deposits of these episodic depositional events commonly overlie pebble downward ~30 cm in some instances, resulting in beds with indistinct burrow stringers interpreted as erosional lags or bypass surfaces, representing fluvi- mottling and loss of primary features. Most of the trace fossils listed above al-tidal reworking (Frey et al., 1989). The distinctive erosional surface and lag were also observed by Wiggin (1987) and McBride (2016a), although Wiggin deposit at the top of FA3 is interpreted as a fluvial-tidal ravinement surface. (1987) also recorded Arenicolites and Corophoides. The distinctive red to gray-green color change at the bayhead delta–estu- Most fossils in the Ignacio Formation (including the reassigned strata arine central basin transition (Fig. 6E) has been observed from other modern from the McCracken Sandstone Member) are found in FA4. Vertebrate fossils and ancient estuaries (Cotter and Driese, 1998; Boyd, 2010). Reddening in the consist entirely of placoderm fish plates. Previous workers have described Ignacio Formation is facies specific, indicating that it is not due to diagenetic Bothriolepis coloradensis, Bothriolepis canadensis, Bothriolepis major, Bothri- reddening of the unit as a whole. There is no evidence that reddening is olepis leidyi, Holoptychius giganteus, and Holoptychius tuberculatus (Eastman, related to changes in lithology or permeability. We suggest that the reason 1904; Cross and Larsen, 1935; Denison, 1951), although Thomson and Thomas why red colors are specific to the fluvial and deltaic portions of this unit is (2001) argued that B. coloradensis and B. leidyi cannot be distinguished from probably related to the effect of terrestrial weathering in the source areas. In Bothriolepis nitida. These all have late Frasnian–Famennian faunal ages (Thom- other words, the color transition from fluvial and bayhead delta sediments son and Thomas, 2001). Other fossils include poorly preserved, phosphatic to estuarine sediments is due to significant input of red sand and mud into brachiopods equivocally identified as Obulus sp. (Cross et al., 1905a; Rhodes the proximal reaches of the estuarine environment, such as observed in the and Fisher, 1957), Lingulella sp., and Dicellomus sp. (Baars, 1965). The poor modern Bay of Fundy estuary (southeastern Canada). quality of brachiopod samples has precluded better taxonomic determina- Many of the described features are characteristic of marine deltas in general, tions and limited their usefulness for age determinations (Read et al., 1949; such as basinward-directed paleocurrents; mixture of fluvial, wave, and tidal Barnes, 1954; Baars and Knight, 1957; Wiggin, 1987; McBride, 2016a). Although features; trace fossils and fauna indicative of mixed-salinity conditions; and Obulus has been considered late Cambrian–Ordovician in age (Emig, 2002), evidence for shallow water depths (Aschoff et al., 2018; Simms et al., 2018). in the study area these Obulus(?) brachiopods have been recovered from the In this study, the bayhead delta interpretation is additionally based upon the same bedding unit as Upper Devonian placoderm fish plates (McBride, 2016a). relatively small scale of the features (<10 m thick), close association with Interpretation. The basis for interpreting FA4 as estuarine channel and cen- underlying tidally influenced fluvial deposits, the capping erosion surface tral basin deposits includes the finer grain size (cf. FA1-3), variegated colors, interpreted as a fluvial-tidal ravinement surface (e.g., Simms et al., 2018), and prevalence of tidal bedding structures, architecture of estuarine channel fills, the overlying estuarine central basin deposits (FA4). prevalence of bioturbation, and fossils consistent with brackish-marine conditions. The laterally continuous sandstone-mudstone couplets with flaser, wavy, or lenticular bedding are interpreted as tidalites (Reineck and Singh, 1980). Facies Association 4 (FA4): Estuarine Channel and Central Basin The intensely bioturbated surfaces are interpreted as firmgrounds (Ekdale et Deposits al., 1984). These omission surfaces have been recognized as components of ravinement surfaces in other studies (Jordan et al., 2016). Description. FA4 consists of finer grained, variegated gray-green to red, These successions are similar to modern estuarine channel and central heterolithic sandstone and shale, with fossils and trace fossils indicative of basin deposits observed in Willapa Bay (Washington State, USA) where suc- mixed-salinity conditions. The most prevalent deposits are thin, laterally con- cessions consist of (1) bioturbated basal lags, overlain by (2) gently dipping tinuous sandstone-mudstone couplets with flaser, wavy, or lenticular bedding interlaminated sand and mud layers of the accretionary bank, then overlain by (Fig. 7A). These sheet-like deposits are locally incised by broadly lenticular (3) mudflat and supratidal flat deposits (Clifton and Phillips, 1980). In the Igna- channel fills up to ~2 m thick. These channel-fill successions are either sin- cio Formation, FA4 is dominated by tidalites interbedded with mudstone-rich gle-story trough cross-bedded sandstones encased in finer-grained sediment intervals. Wave influence is indicated by wave ripples and by omission surfaces (Fig. 7B) or multistory trough cross-bedded sandstones consisting of single that acted as firmgrounds for benthic communities (Ekwenye et al., 2016). Bra- cross-bed sets 30–50 cm thick overlain by thin mud drapes (Fig. 7C). Most chiopod shell lag horizons are probable evidence for wave reworking (Frey et bedding shows some degree of biological mixing, homogenization, and loss al., 1989). The trace fossils are consistent with those of modern and ancient of primary sedimentary structures (Fig. 7C). In addition, certain horizons are estuarine systems (Howard and Frey, 1973; Hubbard et al., 2004) and represent intensely bioturbated (Fig. 7D). a depauperate mixed Skolithos-Cruziana ichnofacies as observed in other estu- Trace fossils observed in FA4 include Planolites, Palaeophycus, Thalassinoi- arine environments (Ekdale et al., 1984; Hubbard et al., 2004). Both Bothriolepis des, Rosselia, Monomorphichnus, Diplocraterion, Skolithos, Gastrochaenolites, and Holoptychius placoderm fish fossils have been found in Upper Devonian and Trichophycus (Figs. 7D–7G). Most individual bedding surfaces display freshwater, estuarine, and coastal deposits. It has been speculated that these

Srw

Figure 7. Field photographs of estuarine channel and estuarine central basin deposits (facies association FA4; see Table 3). (A) Estuarine central basin deposits of heterolithic flaser-bedded (lithofacies SMf; see Table 2) and wavy-bedded tidalites (litho- P facies SMw). Circled pen is 14 cm. (B) Small sand St dune (lithofacies Srw) and mud drape (lithofacies Fm) in estuarine central basin deposits. Scale bar is 15 cm. (C) Estuarine channel fill consisting of stacked trough cross-bedded sandstone (litho F/TRS facies St), ripple-laminated sandstone (lithofacies E Sr and Srw), and mud drapes (lithofacies Fm). The lower sandstone has been disrupted and mottled St Ro by intense bioturbation (lithofacies Smf). Hammer is 28 cm. F/TRS—fluvial-tidal ravinement surface. (D) Firmground dominated by Planolites (P ). Coin St Fm is 1.8 cm. (E) Firmground dominated by Thalassi- noides (Th), Rosselia (Ro), and Gastrochaenolites Fm (Ga). Scale bar is 15 cm. (F) Firmground dominated Th by Monomorphichnus (Mo). Scale bar is 15 cm. (G) Firmground dominated by Diplocraterion (Di). } Smf Ga Coin is 1.8 cm. F/TRS

fish may have been anadromous (spending part of their life cycle in freshwater of stratal disruption indicative of exposure, early cementation, evaporite dis- and part in the oceans) and that they may have been air breathers (Benton, 2014). solution, and replacement. The transition upward from FA5 deposits to FA6 deposits is gradational, marked by carbonate mudstone and rare shelly packstone interbedded with fine-grained siliciclastic red mudstone. Most carbonate Facies Association 5 (FA5): Siliciclastic Tidal-Flat Deposits mudstone contains small amounts (<5%) of silt- and sand-sized quartz, some of probable eolian origin (McBride, 2016a). The lower portions of FA6 depos- Description. FA5 deposits are fining-upward successions typically 1–2 m its include meter-scale, flat-bottomed, domal carbonate accumulations that thick (Fig. 8A). The base of the succession is transitional to the estuarine are internally massive or have cryptic lamination (Fig. 9A); relatively flat to channel fills discussed above. These are overlain by (in ascending order): wrinkled or pustular very fine-scale lamination (Fig. 9B); carbonate mudstone (1) heterolithic, lenticular-bedded sandstone-mudstone couplets (Fig. 8B); breccias (Fig. 9B); and replaced hopper crystals (Fig. 9C). In contrast, the upper (2) planar-laminated sandstone-mudstone couplets or siltstone-mudstone portion of FA6 deposits includes horizons of disrupted nodular carbonates and couplets (Fig. 8C); and (3) laminated siltstone, mud shale, and massive mud- carbonate breccia (Fig. 9E); poorly developed and truncated synsedimentary stone (Fig. 8D). The planar-laminated couplets show rare examples of double folds in isolated carbonate layers (Fig. 9F); and horizons with upward-tilting mud drapes. The upper mudstone commonly contains mudstone intraclasts carbonate layers thrust over adjacent layers (Fig. 9G). In parts of the study (Fig. 8D), and locally bedding surface horizons are covered with mudstone area, supratidal-flat sequences were dolomitized, possibly by Cenozoic hydro- intraclast accumulations (Fig. 8E). Trace fossils are patchy but locally abun- thermal alteration (Onasch et al., 1994; McBride, 2016b), to produce resistant dant, including Rusophycus (Fig. 8F), Palaeophycus (Fig. 8G), Planolites, and beds that also contain thin silicified horizons (Fig. 9D). Skolithos (Fig. 8H). There is a notable size reduction in the dimensions of Interpretation. FA6 is interpreted as indicating a peritidal carbonate dep- Planolites and Skolithos compared with burrows in the estuarine central basin. ositional environment representing the upper intertidal to supratidal zone. Finally, FA5 successions are capped by carbonate-evaporite intervals of FA6 The small domal structures are onlapped by adjacent strata (Fig. 9A), imply- (see next section). ing topographic relief in the depositional environment. The mostly massive Interpretation. FA5 is interpreted as siliciclastic-dominated tidal-flat or indistinct internal features imply that the domes were either thrombalite deposits. These are found either as lateral facies equivalents to the estuarine (Aitken, 1967), sponge, or microbial mounds in the upper part of the inter- channels and central basin deposits in FA4, suggesting that they are similar to tidal zone. The overlying flat or wrinkled or pustular finely laminated layers accretionary bank deposits (Clifton and Phillips, 1980), or overlying the estua- (Fig. 9B) are interpreted as microbial laminites produced by the trapping and rine channels of FA4, suggesting progradation of the tidal-flat environments binding of sediment by cyanobacteria biofilms (Logan et al., 1964). The car- (Frey and Howard, 1986; Daidu, 2013). Typically, FA5 transitions upward from bonate mudstone intraclasts and mudstone breccias (Fig. 9B) are interpreted subtidal estuarine channel fills and sandflat deposits with flaser-bedded or as accumulations of mud chips derived from mudcracked polygons. The tilted wavy-bedded sandstone and mudstone drapes, to intertidal mudflat deposits and overthrust carbonate horizons (Fig. 9G) are interpreted as teepee struc- with sand-starved ripples and tidal rhythmites, to supratidal carbonates of FA6. tures, which typically form from shrinking-swelling cycles in partly lithified The tidal rhythmites with double mud drapes are organized into tidal bundles carbonate having surface crusts or beachrock (Pratt, 2010), or from wrinkled (Tessier, 1993; Greb and Archer, 1995). The reddish mudstones in the upper microbial mats (Kendall, 2010). part of the intertidal succession show signs of stratal disruption, intraclasts There are multiple features indicative of dissolved or replaced evaporites (Fig. 8D), and horizons with extensive mud-chip accumulations (Fig. 8E). The in FA6, such as hopper crystal casts (Fig. 9C). The continuous horizons of dis- mud-chip intraclasts probably sourced from mudcracked polygons. The trace rupted carbonate nodules and breccias (Fig. 9E) have been observed elsewhere fossils are a highly depauperate mixed Skolithos-Cruziana ichnofacies similar and interpreted as microkarst-collapse features produced due to the dissolution to that of FA4. Smaller burrow dwellings in estuarine tidal flats have been of underlying relatively thin layers of stratiform evaporite crystals (Middleton, observed elsewhere (Bjerstedt, 1987) and attributed to opportunistic or pioneer 1961; Warren et al., 1990; Smith, 1997). Stratal disruption and syndepositional colonizers in a perturbed environment (Ekdale, 1985). Rusophycus is a bilobed folding in individual carbonate horizons are suggestive of replaced evaporites resting trace seen in other Devonian estuarine deposits (Bjerstedt, 1987) and that had enterolithic folding due to displacive crystal growth as part of the attributed to non-trilobite arthropods (Feldmann et al., 1986). transition between gypsum and anhydrite (Kendall, 2010). Silicification in the form of replacement chert produces blebs and anastomosing irregular layers at numerous localities (Fig. 9D), probably representing replacement of primary Facies Association 6 (FA6): Carbonate Supratidal-Flat Deposits evaporite minerals (Milliken, 1979). Overall, FA6 is dominated by laterally continuous carbonate mudstones Description. The uppermost part of tidal-flat successions in the Ignacio that are mostly either internally massive, or microbial laminites, or disrupted Formation are carbonate-evaporite dominated, and show significant signs layers with mud-chip breccias, mudcracks, nodules, teepee structures, and

A B SuT

IT SMk

C D Cm

Figure 8. Field photographs of siliciclastic tidal-flat deposits (facies association FA5; see Table 3). } (A) Typical appearance of 1–2-m-thick prograding Se tidal flats, including a subtidal (ST) channel-fill -se quence of massive and planar-laminated sandstone overlain by intertidal (IT) heterolithic sandstones SMl and mudstones, overlain by supratidal (SuT) carbonates. Scale bar is 1 m. (B) Lenticular-bedded tidalites (lithofacies SMk; see Table 2) overlying mudstone Fm (lithofacies Fl). Hammer is 28 cm. (C) Bundles (brack- ets) of tidal rhythmites (lithofacies SMl) separated by thicker intervals of mudstone (lithofacies Fm). Hammer is 28 cm. (D) Uppermost intertidal sequence showing red mudstones (lithofacies Fm) E F with mudstone intraclasts (arrow; lithofacies Se) below carbonates of supratidal flat (lithofacies Cm). Hammer is 28 cm. (E) Bedding-plane surface show- Se ing mudstone intraclasts (lithofacies Se), interpreted as mud chips from intertidal zone mudcracks. Coin is 2.5 cm. (F) Firmground with Rusophycus (Ru) tracks. Coin is 1.8 cm. (G) Firmground dominated by Palaeophycus (Pa). Scale bar is 4 cm. (H) Firmground Ru dominated by Planolites (P) and Skolithos (Sk). Scale bar is 15 cm.

G H Sk

Sk Pa P

SuT

SB Cf

Figure 9. Field photographs of carbonate peritidal IT (upper intertidal and supratidal zone) deposits (facies association FA6; see Table 3). (A) Small thrombalite or sponge or microbial bioherm (SB) overlying heterolithic fine-grained clastic deposits of the intertidal D zone (IT) and overlain by carbonates in the supratidal zone (SuT). Scale bar is 1.5 m. (B) Laminated microbialite (lithofacies Cb; see Table 2) overlying intraclast floatstone (lithofacies Cf) probably derived from desiccation and brecciation of mudcracks. Scale bar is 15 cm. (C) Infilled cast of a hopper crystal. Coin Cm is 2.1 cm. (D) Vertically stacked tidal-flat sequences (bracket) consisting of subtidal massive sandstones (lithofacies Smc), intertidal sandstones (lithofacies E Smf), and mud drapes (lithofacies Fm), and overlying Smf supratidal carbonates (lithofacies Cm). The carbon- Cn ates at this location have been dolomitized. Scale Smc bar is 1 m. (E) Nodular carbonate (lithofacies Cn) indicated by arrows, overlain by and with infilling arenaceous carbonate mudstone (lithofacies Cm). ] These stratiform breccias are interpreted as surficial paleokarst collapse features overlying dissolved Cm Cn stratiform evaporites. Scale bar is 15 cm. (F) Syn- depositional deformation in carbonates (lithofacies Cn) interpreted as relict structures from enterolithic folding in replaced evaporites. Scale bar is 15 cm. (G) Interpreted teepee structure in supratidal car- Smf bonate (lithofacies Cn). Scale bar is 15 cm.

syndepositional folds. The combination of structures is consistent a carbonate Provenance, Paleocurrents, and Source-Area Evolution peritidal environment representing upper intertidal and supratidal zones (Pratt, 2010). The presence of evaporites, sediment reddening, and incorporated eolian Sediment Composition quartz grains suggests that arid or semiarid paleoclimate conditions affected the uppermost intertidal and supratidal zones. Description. Sandstone in the Ignacio Formation is mostly quartz arenite and quartz wacke (Table S1 [footnote 1]), with typically 75% ± 15% monocrystalline quartz and 20% ± 13% polycrystalline quartz (Fig. 11A). Some locations Facies Association 7 (FA7): Estuarine-Mouth Tidal-Sandbar Deposits have extremely well rounded, “billiard ball” quartz (Fig. 11B), attributed to eolian input by McBride (2016a). There are, however, individual samples with Description. FA7 is composed of sandstone packages 1–2 m thick with up to 32% feldspar (average 4% ± 8%), including both fresh and highly altered low-angle accretional surfaces (Fig. 10A). These are internally organized into individual grains of microcline (Fig. 11C) and plagioclase. Lithics typically com- basal granule lag deposits, overlain by stacked sets of trough cross-bedded prise <5%. Rare lithic clasts include granite (Fig. 11D), quartzite (Fig. 11E), chert sandstone, and capped by wave- or current-rippled sandstone (Fig. 10B). Indi- (Fig. 11F), and mudrocks (Fig. 11G); all of these except chert had local sources. vidual cross-bed sets are typically <10 cm thick and are separated by internal Carbonate deposits are arenaceous or argillaceous limestone or dolostone erosion surfaces. Cosets consist of packages of three to five cross-bed sets (Fig. 11H), and the incorporated well-rounded quartz sand or angular-suban- separated by partial to complete mudstone laminae (Fig. 10C). These delin- gular quartz silt is consistent with eolian input. eate tidal bundles separated by reactivation surfaces (e.g., Greb and Archer, Interpretation. Sandstone point counts plot in the quartzose recycled pet- 1995). There are rare examples of herringbone cross-bedding (Fig. 10D) and rofacies field of monocrystalline quartz–total feldspar–total lithics (QmFLt) wave ripples (Fig. 10E) at the tops of cosets. The tops of sandstone bodies are diagrams (Fig. 12). This is interpreted as reflecting the composition of adjacent commonly intensely bioturbated omission surfaces dominated by Planolites source rocks: Proterozoic granite (granitic rock fragments and microcline), (Fig. 10F) or Lockeia (Fig. 10G). quartzite from the Proterozoic Uncompahgre Formation, and plagioclase from Interpretation. The larger sandstone bodies are interpreted as estua- the Proterozoic Twilight Gneiss. Rare sandstone and mudrock lithics may have rine-mouth tidal-sandbar deposits, showing evidence for tidal current reversals sourced from the East Lime Creek Conglomerate. The source of the chert lith- and remolding in the form of reactivation surfaces, internal erosion surfaces, ics is not presently known, but one possible source would be the Ptarmigan herringbone cross-bedding, wave ripples, and partial to complete mud drapes Chert Member of the Lower Ordovician Manitou Formation. (Dalrymple, 2010). These are similar to successions observed in modern estuary- mouth settings (Clifton and Phillips, 1980). In some cases, these bar deposits are interbedded with estuarine central basin deposits. In other cases, bar deposits Paleocurrents transition upward to siliciclastic tidal-flat deposits (Fig. 4). Intensely bioturbated horizons that are interpreted as firmgrounds that developed in bar-top settings Description. Paleocurrent analysis is based on 124 cross-bed measurements and were re-exhumed by erosional loss of estuarine basin muds that originally and 24 crestline orientations from wave ripples (Fig. 13). The cross-bedding draped the bedform. data show a statistically significant (p <0.05) unimodal pattern with relatively Alternative explanations include a barrier sequence, or inlet channels high statistical dispersion. The vector mean is to the northwest (azimuth with related flood-tide or ebb-tide deltas. The barrier interpretation is 320°). Crestlines of wave ripples are oriented ESE-WNW (azimuth 105°–285°), rejected in this case because these deposits do not resemble back-barrier which is interpreted to show NNE-SSW (azimuth 015°–195°) paleoflow direc- spillover lobes, lacking significant upper-flow-regime plane-bed structures tions. However, when it was possible to measure paleocurrent data from both or foresets that would be in accord with transgressive barrier retreat into cross-bedding and ripple marks at the same locality, the paleoflow directions estuarine central basin settings. There is an absence of characteristic beach, tended to be virtually identical (Fig. 13). Across the study area, paleoflow nearshore bar, rip current, or shoreface deposits (e.g., Castle, 2000). The direction appears to shift from north directed in the southern part to more lateral migration of inlet channels could possibly produce similar channel west and northwest directed elsewhere (Fig. 13). bedforms and successions (e.g., Moslow and Tye, 1985), but again there is Interpretation. The paleocurrent patterns are consistent with an estuarine no evidence supporting the presence of estuarine mouth-barrier deposits. depositional system. Bedload transport in modern estuaries is typically ebb-tide Ebb-tide deltas can be excluded because there is an absence of interbed- oriented throughout the entire estuary, and flow reversals are usually insuffi- ded offshore deposits and features. Flood-tide deltas that prograded into cient to radically affect the dominant orientation of larger estuarine bedforms the central estuarine basin remains a possible alternative explanation, but (Horne and Patton, 1989). In this case, paleoflow in the fluvial and proximal is judged less likely due to a lack of evidence for significant storm-wave estuarine part of the study area is west or northwest directed with a higher range reworking (e.g., Boyd, 2010). of statistical dispersion. This is interpreted as flow in sinuous streams (higher

A Fm

Srw

Figure 10. Field photographs of estuarine-mouth tidal sandbars (facies association FA7; see Table 3). (A) Macroform (tidal sandwave) overlying central estuarine basin mudstones (lithofacies Fm; see Table 2). Person for scale. (B) Multistory packages of trough cross-bedded sandstones (lithofacies St) and wave-rippled sandstones (lithofacies Srw) with internal erosion surfaces and partial mud drapes 5 cm (lithofacies Fm). Hammer is 28 cm. (C) Reactivation surface in cosets of trough cross-bedded sandstones. Scale bar is 5 cm. (D) Herringbone cross-bedding (lithofacies Sph) and overlying wave ripples (litho- D facies Srw). Scale bar 15 cm. (E) Wave ripple marks E (lithofacies Srw) on an exposed reactivation surface within a tidal sand-bar sequence. Pen is 14 cm. Srw (F) Firmground dominated by Planolites (P). Scale bar is 7 cm. (G) Firmground dominated by Lockeia (Lo) and Planolites (P). Scale bar is 7 cm.

G P P

Figure 11. Photomicrographs from the Ignacio Forma- tion, showing that the unit is dominantly subrounded to subangular monocrystalline quartz (A) but also includes “billiard ball” well-rounded quartz (B) (scale bar is 250 μm in both images). Ignacio Formation from locations near granitic source areas can have significant amounts of microcline (C; scale bar is 250 μm) and granitic rock fragments (D; scale bar is 500 μm). Quartzite (metamorphic rock fragments) (E) is probably sourced from the Uncompahgre For- E mation (scale bar is 500 μm). Chert (F) is a minor constituent, and the source area is not known (scale bar is 500 μm). Also shown are a sedimentary rock fragment (mudstone) with ferruginous cement from intertidal zone deposits (G; scale bar is 500 μm) and an arenaceous carbonate mudstone from supratidal zone deposits (H; scale bar is 250 μm).

G H

Qm regional collision event (Yavapai orogeny) between the Mojave and Yavapai Craton terranes (Gonzales and Van Schmus, 2007; Amato et al., 2008; Jones et al., interior 2009). Concurrent with terrane accretion, the Twilight Gneiss was intruded by Ignacio Fm. 1.72–1.68 Ga trondhjemite plutons (Barker, 1969; Hutchinson, 1976; Baars et al., 1987; Gonzales and Van Schmus, 2007; Jones et al., 2009). Quartzose recycled The protolith of the 2.5-km-thick Uncompahgre Formation was quartz arenite with minor shale and conglomerate (Harris and Eriksson, 1990), with a depositional age ca. 1.7 Ga (Jones et al., 2009). The Uncompahgre Formation Transitional continental Transitional underwent multiple phases of deformation linked to the collision of the Yavapai Mixed recycled and Mazatzal terranes (Mazatzal orogeny) starting ca. 1.66 Ga (Shaw and Karl- strom, 1999; Amato et al., 2008; Jones et al., 2009), then was buried to crustal Dissected depths of 10–15 km, metamorphosed to greenschist facies, and intruded by arc Lithic recycled the 1.44 Ga Eolus Granite (Bickford et al., 1969; Hutchinson, 1976; Gonzales and Van Schmus, 2007; Jones et al., 2009). Transitional arc The contact relationship between the Twilight Gneiss and Uncompahgre

Basement uplift Undis- Formation has been controversial, with explanations ranging between: a deposi- sected arc tional contact (Barker, 1969; Gibson and Harris, 1992); a high-angle fault contact F Lt (Baars et al., 1987); the Uncompahgre Formation being thrust southward over Figure 12. Ternary petrofacies plot of monocrystalline quartz (Qm), total the Twilight Gneiss (Tewksbury, 1985); and a model where the Uncompahgre feldspar (F), and total lithics including polycrystalline quartz (Lt) (Dickinson Formation was initially deposited above the Twilight Gneiss, then “decoupled” et al., 1983) showing the mean (black dot) and standard deviation (shaded area) from point counting 66 thin sections (n = 18,769 grains) in the Ig- and thrust northward (Harris et al., 1987). However, field data show that the nacio Formation. The unit plots primarily in the quartzose recycled field. Twilight Gneiss was thrust northward over the Uncompahgre Formation, based on the following evidence: (1) klippen of gneiss sitting on top of the Uncompah- gre Formation; (2) sheath folds in the gneiss where it ramps over the quartzite; dispersion) down the central axis of several paleovalleys (see next section). The (3) fault-zone cataclastic metamorphism on the top of the Uncompahgre For- estuarine bars that are interpreted to have formed at the seaward entrance of mation (forming a carapace of shattered purple quartzite encased in white vein these estuaries have north-directed paleoflow with a lower range of statistical quartz at the top of the lower plate); and (4) retrograde metamorphism from dispersion. This is interpreted to represent the influence of prevailing wave gneiss to schist at the base of the Twilight Gneiss, representing deformation direction and longshore currents at the entrances of these estuaries. throughout the lower part of the upper plate (Evans and Holm-Denoma, 2018). Ignacio Formation detrital zircon U-Pb geochronology data support this new thrust-fault interpretation. The initial ca. 1.7 Ga age spectrum peak represents Source-Area Evolution erosion of the Twilight Gneiss from the allochthonous upper plate, while the ca. 1.4 Ga age spectrum peak represents erosion of the Uncompahgre Formation Description. At Sultan Creek (location 17 in Fig. 3), a stratigraphic succes- from the parautochthonous lower plate. Thus, the stratigraphic trend, showing sion of five samples was evaluated for detrital zircon U-Pb geochronology. The the initial appearance of the ca. 1.7 Ga age spectrum peak and then the sub- lowest sample, a sandstone in the East Lime Creek Conglomerate, has an age sequent development of the ca. 1.4 Ga age spectrum peak, can be interpreted spectrum peak of ca. 1.7 Ga (Fig. 14). In the overlying Ignacio Formation, the to show the unroofing history of this Proterozoic collisional zone (Fig. 14). four samples show the continuity of the ca. 1.7 Ga age peak, but also show a stratigraphically progressive development of a ca. 1.4 Ga age peak, produc- ing a bimodal pattern near the top of the unit (Table S2 [footnote 1]). Similar ■■ DISCUSSION detrital zircon U-Pb age results were obtained by McBride (2016a), although his samples were not collected from a continuous stratigraphic sequence. Tide-Dominated Estuarine Depositional System Interpretation. The Proterozoic basement consists of the Twilight Gneiss, the Uncompahgre Formation, and a series of granitic intrusions. The pro- The distinguishing characteristic of the Ignacio Formation is the prevalence tolith of the Twilight Gneiss was bimodal dacitic-hypabyssal volcanic rocks of tidal sedimentary structures including tidalites with double mud drapes; and associated pelitic rocks that were deformed and metamorphosed to flaser, wavy, and lenticular bedding; cross-bedded sandstone with mudstone hornblende-plagioclase gneiss and amphibolite (Barker, 1969) as part of a drapes and reactivation surfaces; sandstone with herringbone cross-bedding;

107°45′00″W 107°37′30″W

10 km Silverton 360

Sultan Creek

37°45′N Molas Lake 270 090 37°45′N Cascade Ck. Andrews Lake

CBP n = 124 cross-bedding 180 VM = 320° csd = 63.7 360 X −8 Mill Ck. Lime Ck. p = 1.4 10 Figure 13. Map showing the locations of paleocurrent data. Single arrows indicate the azimuth of unidirectional mea- 550 Animas River surements (cross-bedding), and double arrows indicate the azimuths for bidirectional measurements (wave ripples). Columbine Paleocurrent rose diagrams summarize the cross-bedding 270 090 and wave-ripple data. Abbreviations: CBP—Coal Bank Pass; Lake Ck.—Creek; n—number of measurements; VM—vector mean; csd—circular standard deviation (Krause and Geijer, Electra 1987); p—Rayleigh test of significance (Curray, 1956). For Lake bidirectional data, the mode identifies the probable mean paleocurrent direction. 180 n = 24 Haviland wave ripples mode = 285° Lake

Canyon Creek 37°30′N Fall Creek 37°30′N Shalona Lake

Bakers Bridge Lemon Vallecito Reservoir Reservoir

107°45′00″W 107°37′30″W

mudstone intraclasts derived from mudcracked or brecciated muds; and marked by IHS deposits, distributary mouth-bar successions, and wave-modified carbonate-evaporite tidal-flat deposits with crystal casts, teepee structures, turbidites. Estuarine central basin deposits are sandstone rich, which, coupled enterolithic folding, surficial karst, breccias from stratiform evaporite dissolu- with the unimodal west-directed paleocurrent pattern, suggests significant flu- tion, and nodules. Several facies associations demonstrate the importance of vial bedload transport east to west through the full length of the estuary, similar tidal processes, including tidally influenced fluvial environments, siliciclastic to the modern Connecticut River estuary (northeastern USA) (Horne and Patton, and carbonate tidal flats and supratidal flats, and estuarine-mouth tidal sand- 1989). The pervasive red colors in the fluvial and bayhead delta components of bars that demonstrate remolding by tidal current reversals. Accordingly, the this depositional system may be indicative of fluvial transport of large amounts Ignacio Formation estuarine depositional systems is considered tide domi- of red sand and mud into the proximal reaches of an estuarine depositional nated in the classification scheme of Ainsworth et al. (2011). system, again showing the importance of the fluvial component of the deposi- Diagnostic estuarine subenvironments are bayhead deltas, estuarine central tional system. In summary, the estuarine systems in the Ignacio Formation are basins, and estuarine channels. In the Ignacio Formation, bayhead deltas are considered fluvial influenced as a modifier of the tide-dominated classification.

1762 Ma 1608 Ma 16JE31 meters 40 20 1417 Ma n = 121

embayment Elbert Formation facies 10 510 Ma (upper member) single grain analysis FA7 estuary mouth 0 tidal sand bar 0 800 1600 2400 3200 Ma sequence 1758 Ma 30 16JE30 n = 120 30 20 FA4 1435 Ma central estuarine 10

basin (blue) Number of zircons (red) Probability Figure 14. Stratigraphic section at location 17 (see sequence 0 Table 1 and Figs. 1 and 3 for location and symbol 0 800 1600 2400 3200 Ma explanation) showing the stratigraphic position of a FA4 1738 Ma 16JE29 series of detrital zircon U-Pb age spectra determina- estuarine 20 tions (sample numbers in upper right). The vertical channel-fill n = 120 axis is the number of individual grains (blue histo- sequence gram) and relative probability (red curve), while the 20 horizontal axis is preferred age in millions of years V V V 10 (Ma). The data show a stratigraphic trend from a FA4 unimodal 1.7 Ga age peak at the base of the Ignacio central Formation, to bimodal 1.4 Ga and 1.7 Ga age peaks. estuarine These data are interpreted to show the unroofing basin 0 history of a Proterozoic thrust fault system, specif- sequence 0 800 1600 2000 3200 Ma ically that the allochthonous upper plate (1.7 Ga Twilight Gneiss) was eroded prior to exposure of the 1731 Ma 16JE28 parautochthonous lower plate (1.4 Ga Uncompahgre Formation). See text for details. Cgl.—Conglomerate. FA3 20 n = 120 bayhead delta 10 sequence 10 Ignacio Formation

0 FA3 0 800 1600 2400 3200 Ma bayhead 40 1731 Ma 16JE27 delta sequence n = 140 30

East Lime Creek Cgl. 20 0 435 Ma 10 single grain Twilight Gneiss analysis 0 0 800 1600 2400 3200 Ma

The importance of wave processes is suggested by wave ripples, wave modern estuaries (e.g., Dalrymple et al., 1990). In addition, there is a stratigraphic modification of delta-front turbidites, locally developed micro-hummocky trend from fluvial deposits to estuarine deposits to nearshore marine deposits stratification, and evidence for resuspension of estuarine mud to produce lag (Fig. 15). This trend is interpreted as an overall transgressive succession. This surfaces, omission surfaces, and firmgrounds. However, evidence of strong interpretation is supported by the presence of numerous tidal-fluvial ravine- wave action is lacking (e.g., Vakarelov et al., 2012). For example, there are only ment surfaces within the estuarine central basin deposits (e.g., Boyd, 2010). a few occurrences of tempestites, and there is no evidence for estuary-mouth constructional sand bodies such as spits or barriers (Dalrymple, 2010). Accord- ingly, the term wave affected becomes the final modifier of this estuarine Incised Valley Fills depositional system classification. As a tide-dominated, fluvial-influenced, wave-affected (Tfw) estuarine Incised valley systems (IVSs) consist of paleovalleys created during prior depositional system, the depositional environment of the Ignacio Formation base-level fall, then backfilled by fluvial, estuarine, and coastal depositional compares to similar modern estuaries undergoing transgression. There is an systems during subsequent base-level rise (Allen and Posamentier, 1993). east-west trend of fluvial deposits to increasingly tide- and wave-produced Evidence for IVSs in the geological record requires (1) erosion of underlying features and deposits, similar to the proximal-to-distal relationships seen in geologic units (Zaitlin et al., 1994); (2) regional significance of the basal erosion

Paleovalley 1 Paleovalley 2 Paleovalley 3

3 4 5 2 6 7 8 9 10 11 12 13 14 15 16 17 18 Elbert Fm. (upper member) VVVVV VVVVV VVVVV VVVVV VVVVV VVVVV VVVVV VVVVV

VVVVV ? VVVVV Key: FA7 estuarine mouth ?

Ignacio Formation tidal sand bars FA4-6 central estuary basin, ? channel, and tidal flat VVVVV FA3 bayhead delta (IHS) VVVVV

? ? FA1-2 fluvial channel and/or tidally influenced fluvial VVVVV unconformity

? ? ? basal contacts not exposed

Figure 15. Diagram showing the thickness and lateral distribution of deposits of the depositional environments in the Ignacio Formation estuarine depositional system. Numbered columns are as shown in Figures 1 and 3 and Table 1. The basal contact with Proterozoic crystalline basement rocks or the East Lime Creek Conglomerate (dashed line) is an unconformity (question marks indicated covered basal contacts). The sections are correlated on the conformable contact to the overlying upper member of the Elbert Formation. This upper contact is interpreted as a low-relief transgressive surface separating the Ignacio Formation estuarine depositional system (confined to incised bedrock paleovalleys) from an overlying marine embayment system (unconfined by paleotopography). The data suggest that at least three paleovalleys were present during deposition of the Ignacio Formation. Note that paleovalleys show a consistent ascending stratigraphic sequence of fluvial, tidally influenced fluvial, bayhead delta, central estuarine basin and estuary mouth (tidal sand bars) environments, indicating a continuous transgression. IHS—inclined heterolithic stratification. Section 1 is not included because it is located 30 km east of the other sections (in the proximal region of paleovalley 1).

surface, i.e., a sequence boundary (Dalrymple et al., 1994); (3) facies shifts in the basin fill discordant with the underlying geologic units (Van Waggoner et al., 1990); and (4) onlap of the basin fill on the paleovalley walls (Zaitlin et Leadville Limestone al., 1994). A typical IVS deposit transitions vertically from fluvial to estuarine SB to shallow marine environments (Thomas et al., 1987; Fischbein et al., 2009). However, at any specific location, the facies relationships can vary due to the offshore carbonate extent of incision, geographical position in the paleovalley, paleovalley size factory and shape, and paleovalley-fill architecture (Dalrymple et al., 2006). HST

Evidence for erosion of the underlying Proterozoic crystalline rocks and East Ouray Limestone embayment facies Lime Creek Conglomerate has been presented above, and the regional extent of this erosional surface is shown by correlation (Fig. 15). In places, fluvial nearshore mixed bedrock and bayhead-delta deposits in the Ignacio Formation unconformably overlie clastic-carbonate incised the high-energy shoreline deposits of the East Lime Creek Conglomerate. tidal flat valley The units are unrelated because trends in facies distributions or paleocurrent Elbert Formation directions do not match. Finally, basin-fill onlap is recorded in lateral trends (upper member) MFS in paleovalleys (Fig. 15). FA7 estuarine mouth Thickness variations within the Ignacio Formation delineate at least three tidal sand bars paleovalleys (labeled 1–3 in Fig. 15) in the study area. The dimensions of the TST paleovalleys are difficult to estimate because of inadequate geographic dis- FA4-6 central estuary basin, tribution of outcrops, resulting in unknowns such as paleovalley shape, the estuarine channels, mixed tidal flats paleovalley position of any measured section, and whether or not paleovalleys 10 m amalgamate basinward. The paleovalleys were ≥42 m deep and probably up TSE to 10–30 km in width. The infill of each paleovalley shows a consistent strati- FA3 bayhead delta

graphic succession of fluvial, tidally influenced fluvial, bayhead delta, estuarine Ignacio Formation central basin, and estuary-mouth tidal-sandbar deposits. FA2 tidally influenced fluvial LST FA1 fluvial channel Sequence Stratigraphy

Incised valley-fill successions containing estuarine and bayhead-delta East Lime Creek SB deposits have been recognized from both modern and ancient settings as mark- Conglomerate

ers of lowstand, transgressive, and early highstand systems tracts (Aschoff et Proterozoic units

al., 2018; Simms et al., 2018). Three principles are used in this analysis: (1) the Figure 16. Interpretation of the composite sequence stratigraphy of the Ignacio Formation. At IVS basal unconformity is interpreted as a sequence boundary (SB) repre- different locations, different components of the Ignacio Formation estuarine depositional system senting relative sea-level fall, and exposure and erosion of continental and overlie and onlap onto Proterozoic crystalline basement and/or the East Lime Creek Conglomerate. The evidence for an unconformity between the Ignacio Formation and East Lime Creek Conglom- nearshore marine deposits (Miller et al., 2013); (2) IVS estuarine depositional erate is discussed elsewhere (Evans and Holm-Denoma, 2018). The upper member of the Elbert systems commonly include numerous of fluvial-tidal ravinement surfaces Formation also overlies Proterozoic crystalline basement and/or the East Lime Creek Conglom- (Frey and Howard, 1986); and (3) the transition from estuarine depositional erate at different locations. This basal unconformity is interpreted as a sequence boundary (SB). systems to embayment marine facies is a maximum flooding surface (MFS), Fluvial, tidally influenced fluvial, and bayhead delta facies are interpreted as a lowstand systems tract (LST). The upper boundary of the LST both is a fluvial-tidal ravinement surfaces marked by a typically located at the top of the incised paleovalley infill (Miller et al., 2013). pebble lag surface, and represents a flooding surface that puts estuarine facies above fluvial facies; As discussed, there is significant evidence for an erosional unconformity at accordingly, this surface is interpreted as a transgressive surface of erosion (TSE). The upper part the base of the Ignacio Formation, where fluvial deposits overlie marine depos- of the Ignacio Formation is estuarine central basin, estuarine channel, tidal flat, and estuary-mouth tidal sand bar deposits. This is interpreted as a transgressive systems tract (TST). The maximum its of the East Lime Creek Conglomerate. This basal unconformity matches flooding surface (MFS) is placed at the transition from a fluvial-estuarine system confined by criteria used by other studies (e.g., Miller et al., 2013) and is interpreted as a paleotopography, to an embayment depositional system that is unconfined. This corresponds SB (Fig. 16). Above the SB, the Ignacio Formation is restricted to incised pale- to the Ignacio Formation–Elbert Formation (upper member) contact. The upper member of the Elbert Formation and the Ouray Limestone (Dyer Formation) is interpreted as a highstand systems ovalleys: at different locations, each of the different facies components of the tract (HST). There is a significant unconformity separating the Ouray Limestone from the Leadville Ignacio Formation estuarine depositional system onlaps the paleovalley walls Limestone, interpreted as another sequence boundary. Lithology symbols are as shown in Figure 3.

to unconformably overlie the Proterozoic crystalline basement and/or the East this unit and the Ignacio Formation; and reassignment of strata traditionally Lime Creek Conglomerate. called the McCracken Sandstone Member of the Elbert Formation to the Ignacio The lower part of the Ignacio Formation consists of aggradational to ret- Formation. In addition, new fossil evidence clarifies that the Ignacio Formation rogradational stacked fluvial, tidally influenced fluvial, and bayhead delta is Upper Devonian, and detrital zircon U-Pb geochronology data are consistent facies. The upper contact of the bayhead delta facies is typically marked by with that age assignment. a fluvial-tidal ravinement surface, representing delta lobe abandonment and Facies analysis shows that the Ignacio Formation was a tide-dominated, wave reworking. This first flooding surface corresponds to the onset of marine fluvial-influenced, wave-affected, transgressive estuarine depositional system conditions in the paleovalleys, transitioning to estuarine conditions (e.g., Rossi that backfilled at least three incised paleovalleys in western Colorado. The basal et al., 2017). Because this surface corresponds to both reworking and flooding, unconformity represents a sequence boundary. The fluvial-tidal ravinement it is interpreted as the transgressive surface of erosion (TSE) marking the top surface directly above the bayhead delta facies represents a transgressive sur- of the lowstand systems tract (LST). face of erosion, and thus the lower fluvial part of the Ignacio Formation is an The upper part of the Ignacio Formation consists of retrogradational stacked Upper Devonian lowstand systems tract. The overlying estuarine central basin estuarine channel fills, central basin deposits, and estuary-mouth tidal sand-bar and estuarine channel deposits, tidal-flat deposits, and estuarine tidal sand-bar deposits. There are numerous fluvial-tidal ravinement surfaces (F/TRS) and deposits are capped by a maximum flooding surface representing a change firmgrounds representing the effect of river floods or enhanced tidal currents from marginal marine deposits confined by paleovalleys to marine embay- on estuarine deposition. The upper boundary is placed at the conformable ment deposits (upper member of the Elbert Formation and Ouray Limestone) contact between the Ignacio Formation and overlying upper member of the not confined to paleovalleys, thus the upper part of the Ignacio Formation is Elbert Formation. This lithological transition from marginal marine, mixed an Upper Devonian transgressive systems tract, and the overlying units are siliciclastic-carbonate deposition confined to paleovalleys, to marine, carbon- part of a highstand systems tract. ate-dominated deposition unconfined by paleotopography is interpreted as Finally, local sediment sources for the Ignacio Formation were controlled the MFS, and probably represents marine onlap onto interfluve areas of the by the erosion and unroofing history of a Proterozoic thrust-sheet complex. underlying paleotopography (e.g., Rossi et al., 2017). Thus, the upper part of Additionally, the presence of evaporites, eolian sediment input, and source- the Ignacio Formation is a transgressive systems tract (TST). area terrestrial reddening suggest that the surrounding region was affected The upper member of the Elbert Formation is poorly exposed, but likely by arid to semiarid paleoclimate. forms part of an extensive marine embayment in conjunction with offshore carbonate of the overlying Ouray Limestone. Other studies have found that continued transgression transitions a funnel-shaped fluvial-estuarine system ACKNOWLEDGMENTS to a coastal embayment, with proximal marine mud and a distal carbonate The authors have benefited from outcrop discussions with G. Gianniny, E.F. McBride, and C.M. platform (Fischbein et al., 2009; Aschoff et al., 2018). Thus, the upper member Onasch. We thank M.M. Yacobucci for insights into interpretations of the fossil and trace fossil data. Bharat Banjade contributed significantly as a field assistant. A portion of this research was of the Elbert Formation and Ouray Limestone is interpreted as a highstand an M.S. thesis project at Bowling Green State University for J.T. Maurer. The project received systems tract (HST). There is a significant erosional unconformity separating funding support from the Colorado Scientific Society, Ogden Tweto Memorial Fund, Geological the Ouray Formation from the overlying Mississippian Leadville Limestone Society of America, Bowling Green State University, and U.S. Geological Survey. We especially thank P.M. Myrow, one anonymous reviewer, and D. Fastovsky for their positive and constructive (Klink et al., 2013), which is probably another SB. At sites throughout western suggestions. We also thank D. Sweetkind, J.A. Herrick, and J.L. Slate at the U.S. Geological Survey Colorado and Utah, Myrow et al. (2013) found paleokarst in the upper Dyer for their reviews. 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