Journal of Sedimentary Research, 2019, v. 89, 1096–1108
MICROBIAL STRUCTURES AND DINOSAUR TRACKWAYS FROM A CRETACEOUS COASTAL ENVIRONMENT (DAKOTA GROUP, COLORADO, U.S.A.)
NORA NOFFKE,1 JAMES HAGADORN, 2 AND SAM BARTLETT 3
1 Department of Ocean, Earth & Atmospheric Sciences, Old Dominion University, Norfolk, Virginia 23529, U.S.A.
2 Department of Earth Sciences, Denver Museum of Nature & Science, Denver, Colorado 80205, U.S.A.
3 Friends of Dinosaur Ridge, 16831 West Alameda Parkway, Morrison, Colorado 80465, U.S.A. e-mail: nnoffke@odu
Microbially induced sedimentary structures may help preserve unique glimpses of ancient shoreline habitats, but are little known from Mesozoic epicontinental settings. To help fill this knowledge gap, we describe a diverse suite of microbial structures from the Upper Cretaceous ‘‘J’’ Sandstone (South Platte Formation, Dakota Group) that are spectacularly exposed at Dinosaur Ridge in Morrison, Colorado, USA. Structures include ‘‘tattered’’ bed surfaces and ferruginous sand chips in supratidal flat facies. A large over-flip structure is preserved in a channel locally known as Crocodile Creek. In upper-intertidal facies, multidirectional ripple marks occur. Perhaps the most well-known microbial structures are exposed on extensive bedding surfaces known as ‘‘Slimy Beach,’’ where lower supratidal-flat facies are dominated by decimeter-scale erosional remnants and pockets. Morphologies and superposition of the structures allows identification of three generations of erosional pockets. Generation A of these erosional pockets exhibit size similarities to ornithomimid, sauropod, and ornithopod dinosaur tracks from adjacent bedding planes, raising the question of whether initial disturbance of the mat-bound surface could have been from track making. Generation B erosional pockets are older and record continuous erosion of the initial pockets until they were eventually overgrown and sealed by microbial mats. Generation C pockets are the oldest ones, exposing wide areas of barren sediment that could not be overgrown by microbial mats anymore. In concert, the microbial structures point to seasonally variable meteorological conditions along the coastline of the Western Interior Seaway and indicate that the ‘‘Slimy Beach’’ bedding plane represents a multi-year record of dinosaur locomotion.
Large bedding planes representing facies deposited along the margins of epicontinental seas are rare, yet such exposures can provide key insights into the characteristics and paleontology of ancient coastal habitats. Dinosaur Ridge is one of the best-known examples of such a record for the Cretaceous. This National Park Service National Natural Landmark is recognized for its spectacular exposures of Jurassic to Cretaceous strata that bear dinosaur bones, trackways, fossil plants, ashes, and sedimentary structures (e.g., LeRoy 1946; MacKenzie 1963; Weimer et al. 1972; Lockley 1993; Hunt et al. 2002). In this succession, the ‘’J’’ Sandstone is of particular interest because it includes a little-known suite of microbially induced sedimentary structures (MISS) that, in association with trace fossils and bedforms, preserves a snapshot of Earth history when ornithomimid, sauropod, and ornithopod dinosaurs ambled along the ancient shores of the Western Interior Seaway.
In these types of clastic settings, MISS are formed syndepositionally by benthic microbial communities (e.g., Schieber 1986; Gerdes and Krumbein 1987; Hagadorn et al. 1999; Gerdes et al. 2000; Schieber et al. 2007a; Noffke 2010). Such microbial structures are preserved in a variety of shallow marine, coastal, fluvial, and lacustrine environments. Comparative analyses with modern examples can help constrain ancient paleoenvironmental conditions. At Dinosaur Ridge, microbial structures have a long history of study. MacKenzie (1972) hypothesized that odd-shaped ripple patches occurring in Unit 21 (Weimer and Land 1972) at Dinosaur Ridge were deposited in a tidal flat dominated by ‘‘algal’’ mats. This microbial interpretation was later corroborated by Reineck (1979) and validated by Schieber (2007b).
Microbial communities like these are important because they can foster preservation of dinosaur-track-bearing surfaces, by enhancing beddingplane cementation, and altering sediment rheology. For example, Kvale et al. (2001) noted erosional remnants and pockets comparable in size to those at Dinosaur Ridge (see also Lockley et al. 1986) at a tracksite formed on a Middle Jurassic tidal flat in Wyoming. Cariou et al. (2014) observed similar phenomena in a Late Jurassic tracksite of the Jura Mountains, France, and Marty et al. (2007, 2010, 2013) in the Jura Mountains in Switzerland and the High Atlas Mountains in Morocco. In an Early Cretaceous sauropod–theropod tracksite in Sichuan, China (Xing et al. 2016), desiccation of mat-overgrown tracks and early carbonate cementation catalyzed by microbial mats is thought to have stabilized tracks (sensu Marty et al. 2009; Cuadrado et al. 2011). Microbes have also been invoked as catalysts for enhanced track formation. For example, microbial processes are hypothesized to have enhanced the formation of Early Cretaceous ornithopod tracks in clastic strata of Qijiang, China, by maintaining sub-mat sediment pliability while simultaneously reducing the sediment’s proclivity to collapse from formation of deep tracks (Dai et al. 2015). In an Early Jurassic carbonate tidal flat from Italy, Avanzini et al. (1997) inferred that cyanobacterial mats helped sediments to behave plastically to semi-elastically, fostering formation of better-defined sauropod and theropod footprints.
To help refine our understanding of the paleoenvironments and preservational conditions represented by the ‘‘J’’ Sandstone at Dinosaur Ridge, we mapped and analyzed previously known and new MISS from the succession. We describe these structures below and suggest how their distribution, mode of preservation, and association with trace fossils contributes to our understanding of the paleoenvironment and dinosaur activity.
The Dakota Group contains a transgressive succession including facies thought to reflect transition from fluvial systems (channels and floodplains of the Lytle Formation) to swamps, marshes, and tidal flats (Plainview Sandstone Member). These facies are overlain by delta-plain and shallowbay marine facies (Skull Creek Member). A subsequent shallow deltaic facies (Kassler Sandstone) is capped by tidal flat and channel (‘’J’’ Sandstone) facies that locally may be overtopped by marsh, swamp, and beach facies (Lane 1963; MacKenzie 1968, 1972; Weimer 1970; Chamberlain 1976; MacMillan and Weimer 1976).
Studied outcrops are in the ‘‘J’’ Sandstone Member of the South Platte Formation (Dakota Group), exposed along a roadcut on the eastern flank of Dinosaur Ridge (e.g., Weimer 1970) (Fig. 1). These sandy strata were deposited at approximately 458 north paleolatitude (see synthesis in van Hinsbergen et al. 2015) along the coastline of an epeiric sea that blanketed much of western North America during the Late Cretaceous (e.g., Waage 1955; Weimer and Haun 1960; MacKenzie 1965; Kaufman 1984). Our observations of the sedimentary characteristics of the succession are consistent with those of previous workers, and with the hypothesis that the upper part of the ‘‘J’’ Ss was deposited in a shallow, tidally dominated marine setting composed of a mosaic of intermittently emergent sand flats, tidal channels, and swampy to estuarine paleoenvironments.
To facilitate comparison with field guides by MacKenzie (1968), Lockley and Marshall (2014), and others, we have organized our contribution into a stratigraphic framework comparable to theirs. We key our graphic log in Figure 1 and our descriptions to Weimer and Land’s (1972) unit numbers, which indicate well-known outcrops along West Alameda Parkway at Dinosaur Ridge.
For this study, we conducted a lithological facies analysis of a 27-mthick stratigraphic section from the upper Kassler Sandstone to the lower Mowry Shale, with detailed focus on the bedding planes on which we observed microbial structures (Fig. 1). Under permit by Jefferson County Open Space, samples from seven rock beds were taken for thin-sectioning to search for microbial textures. Petrographic examination showed that the rocks are too oxidized to bear carbonaceous laminae, but hematite-rich textures may occur. The distribution of structures on a large (~ 150 m2) bedding surface at ‘‘Slimy Beach’’ (22.4 m in Fig. 1) and at a smaller exposure at ‘‘Crocodile Creek’’ (18.7 m in Fig. 1) were gridded, mapped, and photographed with the aid of a drone.
Stratigraphy and Facies
There are six main units in the ‘‘J’’ Sandstone at Dinosaur Ridge, known from bottom to top as Units 19 to 24 (Fig. 1). Strata are described in detail by Weimer and Land (1972), MacKenzie (1968, 1972), and MacMillan and Weimer (1976), and trace fossils are discussed in Chamberlain (1976). At the base of our studied succession is Unit 19, which is interpreted to have been deposited in a supratidal flat. It is characterized by very fine-grained sandstone that bears a variety of symmetrical, current, and interference ripples, vertical and horizontal bioturbation, local ferruginous crusts and woody and leafy plant debris, and clay laminae that are locally thicker and bear polygonal cracks. Unit 20 is thought to have been deposited in
estuary-tidal or distributary channels and consists of medium- to finegrained sandstone that fines upward, locally has local mud clasts at channel bases, and contains abundant tabular and trough cross-stratification, ripple cross stratification, leaf and stick impressions, Diplocraterion, and other vertical burrows. Unit 21 likely represents an upper-intertidal flat and is dominated by thin-bedded fine-grained sandstone that bears a diverse array of ripple cross stratification and a variety of trace fossils. Unit 22 is hypothesized to represent deposition in a tidal channel that incised supratidal flats, and is characterized by medium- to fine-grained sandstone that fines upward with trough cross stratification and ripple cross stratification, with rare impressions of plant debris and vertical bioturbation. Unit 23 is thought to represent deposition in lowersupratidal- flat and swamp settings, and is dominated by very fine to fine
sandstone with minor siltstone and shale, trough and ripple cross stratification, channels, root marks, logs/branches, and sticks, and a variety of horizontal and vertical trace fossils including Diplocraterion. Unit 24 may represent a channeled tidal flat and is trough cross stratified fine- to medium-grained sandstone with thin, polygonally cracked shale beds, capped by a thin conglomeratic sandstone. Unit 24 is overlain by gray marine mudstones of the Mowry Shale.
Weimer and Land (1972) suggest that these units were generally sourced from the west and deposited in a broad, shallow delta complex. Microbial structures are preserved in all of the non-channelized parts of these Units 19–24. Considered together, they represent the most diverse suite of microbial structures yet known from dinosaur tracksites, and are described below.
Microbially Induced Sedimentary Structures
Observation of Multidirected Ripple Marks.—Two large bedding planes of fine-grained hematitic sandstone of Unit 21 bear patches of ripple marks of various directions (Figs. 1, 2). The basal bed is 9.5 cm thick and bears ripple-mark cross stratification, in vertical section made visible by hematite-rich laminae. It overlies a 85-cm-thick, cross-stratified finegrained sandstone. The upper bed is 3–5 cm thick and has rare ripple cross stratification. These beds are locally incised by meter-scale channels of the overlying rock unit.
The upper surfaces of both sandstone beds are covered by asymmetrical ripple marks (Fig. 2A). The basal bed has an average ripple index of 10.5 (4 to 16) whereas the upper bed has an average ripple index of 8.5 (4 to 12).
The ripple marks on the lower bedding plane surface are widely exposed on the left portion of the outcrop (Fig. 2A). They are arranged into decimeter-sized patches where each patch includes a group of ripples of the same direction. However, the ripple directions between the patches vary. In a representative study area (Fig. 2B) three main directions, labeled as a, b, and c, can be distinguished (Fig. 2C). The ripple valleys of each patch group show a different degree of infill (leveling).
Interpretation of Multidirected Ripple Marks.—Such multidirected ripple marks can result from the interference of episodic strong flood currents with microbial-mat development on a sandy tidal surface (Noffke 1998, 2010) (Supplemental Fig. 1). In the Dinosaur Ridge example illustrated in Figure 2, the sharply defined ripples of suite ‘‘c’’ cover the largest part of this representative study area. The relative large surface part occupied by those ripples and their unaltered morphologies suggests that they represent the youngest generation of ripples on the overall bedding plane. The ripples merge with those of ripple suite ‘‘b,’’ which are interpreted to represent an older generation of ripples. Finally, ripple suite ‘‘a’’ covering the smallest area of the bedding plane, is inferred to be the oldest. The valleys of suite ‘‘a’’ ripples are filled in almost entirely by ancient mat coverage (Noffke 1998).
Water depths and current velocities inferred from ripple indices are internally consistent with these interpretations. For example, the basal bed’s mean ripple index (10.5) suggests that it experienced current ripples formed in water depths of 20–100 cm, and under maximum currentvelocities of 30–50 cm/s. The upper bed’s ripple index (8.5) suggests it experienced water depths of 10–20 cm and maximum current velocities of 15–20 cm.
In modern tidal flats, multidirectional ripple marks form in a stepwise fashion over the course of weeks (Noffke 1998). At first, the surface of freshly deposited, ripple-marked sand is colonized by benthic microorganisms forming a thin biofilm within a few hours. Such initial growth stages of microbial mats tend to form patches instead of extensive sheets.
The microbial patches protect the underlying sandy substrate against erosion, and the ripple marks (analogous to those of generation ‘‘a’’ at Dinosaur Ridge) are also protected against reworking compared to areas not yet colonized by microbenthos. Subsequently, the barren areas surrounding the mat-stabilized patches are reworked again by subsequent currents. Here, a second generation of ripple marks is formed (analogous to generation ‘‘b’’ at Dinosaur Ridge) that may have a different direction than the older ripple marks of the established mat patches. Over time, the initially established mat patches expand laterally and now stabilize the
second generation of ripple marks as well. A third reworking event affects the remaining, still barren sedimentary surface, and a third generation of ripple marks forms. This last generation of ripple marks would be analogous to those of generation ‘‘c’’ at Dinosaur Ridge. The older a microbial mat is, the thicker it may become. Mature, epibenthic microbial mats may fill in ripple valleys. The ripple marks that were formed at first therefore have levelled (‘‘smooth’’) valleys, filled in by a mature microbial mat, whereas younger ripples show unmodified (‘‘sharp’’) valley-crest morphologies.
Observation of Sand Chips.—Two exposed bedding planes in Unit 22 display flat clasts scattered across the surfaces of heavily bioturbated, silty fine- to medium-grained quartz sandstones (Figs. 1, 3A). The chips are 3 to 12 cm long and 3 to 4 mm thick, and have unusual, irregular shapes with rounded margins that contain many lobes and embayments. They are composed of carbonate-cemented, fine-grained quartz sand. Their burnt yellow to rusty red color is caused by limonite staining (Fig. 3B).
Interpretation of Sand Chips.—These sand chips are interpreted as fossil fragments of ancient microbial mats (sand chips; Pflueger and Gresse 1996). Microbial sand chips observed in modern shoreline settings have distinct lobed shapes like the examples from Dinosaur Ridge (Fig. 3C). Their shapes can be described using two indices (Noffke 2010). Index 1 is the longest axis of a chip divided by the axis, which runs through the central point of the long axis at a 908 angle.
Index 2 is the length of the circularity of the chip divided by the longest axis. In outcrop, four representative chips were measured; they have an index 1 of ~ 1.8, and an index 2 of ~ 0.31.
Examples of modern microbial sand chips show similar index 1 values of ~ 1.75 and index 2 values ~ 0.53 (Noffke 2010). Microbial sand chips originate by erosive water currents undermining the edges of a microbial mat and removing the substrate beneath. After some time, the edge of the microbial mat hangs down like a tissue and becomes fringed. Eventually, centimeter-scale pieces of microbial mat are ripped off their parent site from the mat margin and then transported and redeposited at sites of lower current velocities.
In modern tidal flats of Mellum Island, Germany, the transport distance of centimeter-scale chips commonly is less than 200 m from the point of their origin (Noffke 2010). These field measurements are consistent with boundary conditions observed for experimentally produced chips (Hagadorn and McDowell 2012).
In modern settings, the behavior of cyanobacteria in MISS can be observed. If a mat chip flipped over so that its foremost top layer formed by photoautotrophic cyanobacteria now faces downward, the microbes move towards the new top of the chip in order to receive sunlight. Once the microbes deceased, their organic matter remains and is within hours
diagenetically mineralized by heterotrophic bacteria that foster precipitation of pyrite (Noffke 2000; Schieber 2007b; Newman et al. 2016). This pyrite may later be oxidized to limonite. Such microbial behavior and subsequent diagenetic alterations may explain why the rims of some
Dinosaur Ridge sand chips are stained by limonite.
Observation of Over-Flip Structure.—At least four discrete tidal channels (numbered 1–4 in Fig. 4A) occur in Unit 22 (Fig. 1). In all channels, only the stoss-sides are preserved. The sides are . 458 steep. Such an angle is unusually high for unconsolidated sand, which commonly builds only up to 308 slope angles. Along the stoss side of channel 1, ripple marks (arrow ‘‘rm’’ in Fig. 5A) are oriented almost perpendicular to the inferred water line.
The stoss side of channel 1 displays a slump structure of 0.5 m length3 15 cm width resembling an ‘‘overturned fold’’ (Fig. 4A, arrow ‘’over-flip’’). The close-up in Figure 4B and sedimentological evidence reveal that the slump structure is a 1–1.5-cm-thick layer of sandy sedimentary rock. Along this same channel slope, scratch marks of crocodile claws are visible (Lockley 1993) (Fig. 4B, arrow ‘’b’’).
Interpretation of Over-Flip Structure.—The morphological appearance of this structure is highly unusual for slumped beds of sand (Fig. 4B). First, it is unusual that an originally uncemented sand layer formed a wavy coherent sheet only a centimeter in thickness while having a relatively wide lateral extent. Second, whereas in a slump structure the apex of the ‘’fold’’ normally would point downslope, this example here has an apex pointing uphill (Fig. 4B, marked with ‘‘a’’). Therefore, we interpret this ‘’slump’’ structure as a fossilized microbial mat that was flipped over (Fig. 4C). Such over-flip structures have been observed in other ancient deposits (e.g., Donaldson et al. 2002; Noffke et al. 2008), and a continuum of such flipped-over structures have been produced experimentally (Hagadorn and McDowell 2012). If the levee and stoss sides of the channels at Dinosaur Ridge were lined by a microbial mat, it would have stabilized the sandy slope sufficiently to support this steep angle of this channel slope.
Crocodile scratch marks (marked with ‘‘b’’ in Fig. 4B) that are associated with this over-flip structure suggest a scenario in which a crocodile may have slid down into this channel and pulled some of the microbial-matbound lining with it.
Observation of Erosional Remnants and Pockets.—In Unit 23, a large, planar sandstone surface, known to park visitors as ‘‘Slimy Beach,’’ displays an array of round to ellipsoidal depressions, many with irregular outlines (Figs. 1, 5). The depressions vary widely in size, ranging from ~ 10 cm to several meters wide. They are 1–4 cm deep with a ca. 208 to 908 steep slope all around. Their bottoms are often covered by ripple marks, and they cover about a third of the planar bedding surface. In thin section, the sandstone forming the planar surface area has more carbonate cement than the more hematite-rich depressions. The sedimentary surface is also littered with sand chips. Most sand chips are 1–5 cm wide, but some reach 30–40 cm in diameter, such as those in the lower right corner of the outcrop, close to the ‘‘Slimy Beach’’ interpretive sign (Fig. 5).
Interpretation of Erosional Remnants and Pockets.—Earlier studies noted these rippled depressions, referring to them as ‘‘patchy ripples’’ (MacKenzie 1972; Schieber 2007b), with Reineck (1979) comparing the morphological similarity of the surface bed with ‘‘erosional remnants and pockets.’’ Such erosional remnants and pockets form in the lower supratidal zone of modern sand flats like those of Okracoke Island (North Carolina, USA) (Supplemental Fig. 2), and Mellum Island in the North Sea (Reineck 1979; Gerdes et al. 1985; Noffke 1999, 2010). Our 1:1 scale survey of the surface (Fig. 5) yielded no evidence inconsistent with
interpreting these structures as ‘‘erosional remnants and pockets,’’ as suggested by the earlier workers. The elevated, planar areas of the sandstone surface are interpreted as fossilized epibenthic microbial mats. Monitoring the formation of modern erosional remnants and pockets in equivalent shoreline settings showed that a small obstruction or damage to the sediment-sealing microbial mat is sufficient for erosion to start. At this initial local spot of disturbance, water currents remove the substrate beneath the microbial mat to form an erosional dent. Starting from here, scouring continues to undercut the margins of the surrounding mat and a depression forms growing laterally over time (Noffke 1999; Hagadorn and McDowell 2012). Because the sand in such depressions is not bound by microorganisms, the grains remain loose and ripple marks may form. At Dinosaur Ridge, a similar paragenetic sequence is inferred (Supplemental Fig. 3).
Measurements of the geometry of the depressions allow the reconstruction of the direction of the tidal currents forming the topography. In general, erosional remnants and pockets in a tidal system derive from the energy-rich spring flood current ascending the tidal flat and deforming the sandy surface; the weaker ebb current drains the surface leaving fainter structures behind. The surface relief of this Dinosaur Ridge bedding plane records a dominant current directed north to south and a somewhat subordinate current of southwest to west direction (Fig. 6A, B). The ripple direction and the shape of the ripples (short ripple slope pointing landwards) suggest that the ancient shoreline at Dinosaur Ridge was located somewhere towards present-day southwest, which is towards the lower left edge of the outcrop (downdip and downhill of the exposure). Log impressions of mangrove tree trunks, presumably deposited with a receding tide or as flotsam along a strandline, support the interpretation of a nearby location of the shoreline (Fig. 5, Supplemental Fig. 4). Hypothetically, because two current directions are visible, the local shoreline could have been a small embayment causing reflection and separation of the incoming flood water into two currents. A similar situation is described from Mellum Island, Germany (Noffke 1999) (Fig. 6C).
Observations of Tattered Surfaces.—Two widely exposed bedding planes in Units 19 and 24 have a strongly weathered appearance (Figs. 1, 7A). The surfaces of the quartz-rich fine-grained sandstones display a pitted and ‘‘tattered’’ morphology. This appearance of these surfaces is caused by flat clasts that have highly irregular, ragged outlines (Fig. 7B). Most clasts are 1–2 mm thick, and their diameters range from 1 to 20 cm. Holes a few millimeters in diameter are common in the clasts. The clasts are light tan, whereas the remaining rock bed surface is dark brown. Many clasts are connected with each other, whereas others are solitary. The clasts seem to have been ‘‘shaved off,’’ with many having only little contact with the underlying bedding plane (Fig. 7A, B). Some clasts overlie one another in such a way that they appear to have been transported or redeposited (Fig. 7D). Whereas surfaces with such a weathered appearance may be typical for fissile mudstone, the two rock beds are, however, sandstone.
Interpretation of Tattered Surfaces.—Comparable structures are known from modern sandy supratidal flats like those of Portsmouth Island, North Carolina, USA (Noffke 2010). Here cyanobacteria-dominated microbial mats start to decompose in early fall, when growth conditions become less favorable (Fig. 7C, E). The mats decompose and shed into jagged chips similar in morphology and size to the ones at Dinosaur Ridge.
These modern examples exhibit the same type of embayments and lobes of their margins as well as perforation in their centers. In modern sites that contain such ‘‘shredded’’ mats, the mat chips are still connected to the parent mat, whereas others were ripped off completely by erosion, and then transported a few centimeters or decimeters before being redeposited. In light of the morphological similarities with the structures at Dinosaur Ridge, we suggest that these ancient bedding planes record such tattered surfaces during seasonal degradation of microbial mats.
Microbial structures at Dinosaur Ridge not only enhance our understanding of the prevailing paleoenvironmental conditions that produced the ‘‘J’’ sandstone but also provide a useful analogue for understanding how other large dinosaur tracksites might have formed. For example, MISS help constrain the hydraulic conditions in which strata were deposited. Collectively, physical sedimentary structures and MISS suggest minimum depositional depths for the ‘‘J’’ Sandstone ranging from zero to a few decimeters deep for the facies represented by Units 19 and 22 (interpreted as having been produced in the lower-supratidal flats) and extending a meter to several meters in depth for Unit 20 (distributary channels). Dinosaur Ridge’s erosional remnants and pockets point to a tidally dominated system with a dominant north–south oriented current and a subordinate southwest–west current, with a shoreline located toward present day southwest. Ripples suggest that current velocities at least ranged from 15 to 50 cm/s. In concert with ripple morphology in Unit 21, the tidal flats were probably ascending at first towards present day southeast (towards the lower-right edge of the outcrop). During the deposition of Unit 23, however, the shoreline must have been located in present day southwest (the lower left of the outcrop). Multidirectional ripple marks and erosional remnants and pockets may have been products of episodic strong landward winds pushing flood currents far onto the intertidal and supratidal flats. The tattered bedding surfaces and the release of microbial sand chips point towards a seasonally variable ecosystem.
Considered together with previous studies of carbonate cements (Grotzinger and Knoll 1999; Dupraz et al. 2009; Noffke 2010; Warden et al. 2016) and ferruginous staining like that found in sand chips and erosional remnants at Dinosaur Ridge, microbially bound strata were deposited along a shallow to intermittently emergent coastline that fostered carbonate mineral and hematite precipitation.
Given the diversity of microbial structures at Dinosaur Ridge and the paucity of such large bedding plane exposures in the region (i.e., a near prerequisite to recognition of such structures) it is possible that they are more prevalent in the ‘‘J’’ sandstone elsewhere. If so, it is worth considering that microbial mats may have been prevalent in the upperintertidal to lower-supratidal zones elsewhere in the Western Interior Seaway, along which the ‘‘J’’ Sandstone was deposited.
Formation and Taphonomy of Dinosaur Tracks
Microbial communities, like those that are inferred for the ‘’J’’ Sandstone, may have fostered the preservation of dinosaur tracks at Dinosaur Ridge. Track preservation can be supported initially by increased plasticity of the fresh sediment, followed by continued growth of mats and microbe-mediated mineral cementation during consolidation of trackbearing surfaces. Evidence for multiple generations of post-dinoturbation microbial growth in the lower-supratidal-flat facies (Unit 23) at Dinosaur Ridge (Fig. 5) is consistent with these processes. Interestingly, such enhanced preservation is also known from lacustrine to palustrine facies in Early Cretaceous strata from Cuenca, Spain, including from a site that bears crocodilian tracks akin to those at Dinosaur Ridge’s ‘’Crocodile Creek’’ (Moratalla et al. 2017).
Based on thicknesses of sand chips and tattered surface fragments, Dinosaur Ridge’s microbial mats were at least 1 to 4 mm thick. They episodically became desiccated based on their occurrence in intermittently emergent facies—a change that altered sediment elasticity and saturation of the underlying sediment with water. Such changes in the tensile strength of comparable modern sandy mats (e.g., Amos et al. 2004; Schieber 2007c;
Friend et al. 2008; Noffke 2010; Hagadorn and McDowell 2012) allow for the possibility that Dinosaur Ridge’s mats could have actually inhibited formation of small tracks and subdued the details of some trackmaker’s anatomy (sensu Marty et al. 2009, 2010; Cariou et al. 2014).
Conversely, some of the observed microbial structures, such as the erosional pockets, may have actually been triggered by animal behavior, such as dinosaurs walking across the ‘‘J’’ sandstone tidal flat. Kvale et al. (2001), for example, illustrated comparable modern ripple patches from Pine Island, Florida, whose formation was triggered by animals walking across the tidal flat. To assess this possibility, we evaluated the paragenetic sequence represented by the erosional remnants and pockets in Unit 23 (Fig. 5; Supplemental Fig. 5). There are three generations: (i) small, round erosional pockets 10 to 50 cm in diameter (Generation A) of which some show an about ~ 1-centimeter-high rim. The bottom of these pockets may include structures akin to deformed and broken-up microbial mats. The large clasts near the interpretive sign ‘‘Slimy Beach’’ may have been mat fragments ripped off their substrate and dragged upward by a dinosaur foot until falling back onto the surface. (ii) Larger, mostly ellipsoidal erosional pockets that reach 50 to 1.20 m diameters and that display ripples of wellpreserved morphology, sometimes with a reddish color (Generation B); and (iii) very large (up to several meters in diameter) erosional pockets with a highly irregular outline (Generation C) that have ripple marks with rounded crests and valleys; sometimes a multidirected ripple pattern exists on the bottoms of those erosional pockets. The rounded crests and valleys point to an exposure of the originally loose sand to wind erosion and desiccation. The irregular outer shape of these depressions was caused through the continuing lateral erosion of the original pockets until neighboring depressions finally became connected with each other. Smaller depressions that are very shallow and of smooth morphology can be interpreted as footprints that were overgrown by microbial mats before any further erosive destruction (Supplemental Fig. 5). The crosscutting relationships, lateral arrangements, and sizes of the depressions lead to the suggestion that the different generations of erosional pockets record a temporal succession of individual events of dinosaurs crossing this tidal flat area; each event may have been several weeks or months apart from the next. The reasons for this hypothesis are that erosional remnants and pockets in modern supratidal flats are flooded only on occasion, when strong landward winds support a spring high tide, and that microbial mat regeneration takes a few months.
Other sites, such as the Lower Cretaceous ripple-patch-bearing tracksites of the Sousa Basin (Carvalho et al. 2013) might be amenable to such inquiry, and could help to test these hypotheses. Sedimentological studies of geologically ‘‘young’’ shallow to emergent dinoturbated strata is growing, and many marginal lacustrine and marginal marine Cretaceous and Jurassic strata bear signs of microbial influence (Silvia Filho 2009; Porchetti et al. 2016). Integrating data from sites like Dinosaur Ridge with the growing body of work on modern and ancient trackmaker–microbial interactions (e.g., Marty et al. 2009; Cuadrado et al. 2011; Carvalho et al. 2013) offers a direct path for deconvolving the paragenetic and taphonomic history of ancient tracksites.
The ‘‘J’’ Sandstone at Dinosaur Ridge bears a diverse array of clastic microbial structures. The rock succession includes typical MISS like microbial sand chips, erosional remnants and pockets, tattered surfaces, multi-directed ripple marks, and over-flips. These structures help constrain local hydraulic conditions, identify the location of the paleoshoreline, and refine our understanding of the amount of time represented by Dinosaur Ridge’s bedding planes.
Sedimentological studies of these types of dinoturbated strata are growing, illustrating that many marginal lacustrine and marginal marine Cretaceous and Jurassic strata were microbially influenced.
Supplemental files are available from the SEPM data archive: https://www.sepm.org/supplemental-materials.
We thank our colleagues at The Friends of Dinosaur Ridge for their welcome and support, as well as Matteo Belvedere, Martin Lockley, the JSR Editors, and an anonymous reviewer for constructively helpful reviews. We are grateful to Jefferson County Open Space for permission to conduct fieldwork on these outcrops. Brent Breithaupt, Norb Cygan, Lauren Henderson, Gary Johnson, Martin Lockley, and Bob Raynolds generously shared knowledge that contributed to our understanding of these outcrops. Kathy Honda searched for difficult-to-obtain references, and Chelsea Bitting helped draft Figures 5 and 6.
AMOS, C.L., BERGAMASCO, A., UMGIESSER, G., CAPPUCCI, S., CLOUTIER, D., DENAT, L., FLINDT, M., BONARDI, M., AND CRISTANTE, S., 2004, The stability of tidal flats in Venice Lagoon: the results of in-situ measurements using two benthic, annular flumes: Journal of Marine Systems, v. 51, p. 211–241.
AVANZINI, M., FRISIA, S., VAN DEN DRIESSCHE, K., AND KEPPENS, E., 1997, A dinosaur tracksite in an Early Liassic tidal flat in northern Italy: paleoenvironmental reconstruction from sedimentology and geochemistry: Palaios, v. 12, p. 538–551.
CARIOU, E., OLIVIER, N., PITTET, B., MAZIN, J.M., AND HANTZPERGUE, P., 2014, Dinosaur track record on a shallow carbonate-dominated ramp (Loulle section, Late Jurassic, French Jura): Facies, v. 60, p. 229–253.
CARVALHO, I.S., BORGHI, L., AND LEONARDI, G., 2013, Preservation of dinosaur tracks induced by microbial mats in the Sousa Basin (Lower Cretaceous), Brazil: Cretaceous Research, v. 44, p. 112–121.
CHAMBERLAIN, C.K., 1976, Field guide to the trace fossils of the Cretaceous Dakota hogback along Alameda Avenue, west of Denver, Colorado, in Epis, R.C., and Weimer, R.J., eds., Studies in Colorado Geology: Colorado School of Mines, Professional Contributions, v. 8, p. 242–250.
CUADRADO, D., CARMONA, N.B., AND BOURNOD, C., 2011, Biostabilization of sediments by microbial mats in a temperate siliciclastic tidal flat, Bahia Blanca estuary (Argentina): Sedimentary Geology, v. 237, p. 95–101.
DAI, H., XING, L., MARTY, D., ZHANG, J., PERSONS, W.S., HU, H., AND WANG, F., 2015, Microbially-induced sedimentary wrinkle structures and possible impact of microbial mats for the enhanced preservation of dinosaur tracks from the Lower Cretaceous Jiaguan Formation near Qijiang (Chongqing, China): Cretaceous Research, v. 53, p. 98–109.
DONALDSON, J.A., MUNRO, I., AND HILOWLE, M.A., 2002, Biofilm structures, trace fossils and stromatolites in Early Paleozoic quartz arenites and carbonates of the Ottawa region, Ontario: 2th Canadian Paleontology Conference, Program and Abstracts, p. 12.
DUPRAZ, C., REID, R.P., BRAISSANT, O., DECHO, A.W., NORMAN, R.S., AND VISSCHER, P.T., 2009, Processes of carbonate precipitation in modern microbial mats: Earth-Science Reviews, v. 96, p. 141–162.
FRIEND, P.L., LUCAS, C.H., HOLLIGAN, P.M., AND COLLINS, M.B., 2008, Microalgal mediation of ripple mobility: Geobiology, v. 6, p. 70–82.
GERDES, G., AND KRUMBEIN, W.E., 1987, Biolaminites: Heidelberg, Springer, 193 p.
GERDES, G., KRUMBEIN, W.E., AND REINECK, H.-E., 1985, The depositional record of sandy, versicolored tidal flats (Mellum Island, southern North Sea): Journal of Sedimentary Petrology, v. 55, p. 265–278.
GERDES, G., NOFFKE, N., AND KLENKE, T., 2000, Microbial signatures in peritidal siliciclastic sediments: a catalogue: Sedimentology, v. 47, p. 279–308.
GROTZINGER, J.P., AND KNOLL, A.H., 1999, Stromatolites in Precambrian carbonates: evolutionary mileposts or environmental dipsticks?: Annual Review of Earth and Planetary Sciences, v. 27, p. 313–358.
HAGADORN, J.W., AND MCDOWELL, D.C., 2012, Microbial influence on erosion, grain transport and bedform genesis in sandy substrates under uniflow directions: Sedimentology, v. 59, p. 795–808.
HAGADORN, J.W., PFLUEGER, F., AND BOTTJER, D., 1999, Unexplored microbial worlds: Palaios, v. 14, p. 1–2.
HUNT, A., LOCKLEY, M., AND WHITE, S., 2002, Historic Dinosaur Quarries of the Dinosaur ridge Area: Friends of Dinosaur Ridge and the University of Colorado at Trackers Research Group. 12 p.
KAUFMAN, E.G., 1984, Paleobiogeography and evolutionary response dynamic in the Cretaceous Western Interior Seaway of North America, in Westerman, G.E., eds., Jurassic–Cretaceous Biochronology and Paleogeography of North America: Geological Association of Canada, Special Paper 27, p. 273–306.
KVALE, E.P., JOHNSON, G.D., MICKELSON, D.L., KELLER, K., FURER, L.C., AND ARCHER, A.W., 2001, Middle Jurassic (Bajocian and Bathonian) dinosaur megatracksites, Bighorn Basin, Wyoming, U.S.A.: Palaios, v. 16, p. 233–254.
LANE, D., 1963, Sedimentary environments in Cretaceous Dakota Sandstone in northwestern Colorado: American Association of Petroleum Geologists, Bulletin, v. 47, p. 229–256.
LEROY, K., 1946, Stratigraphy of the Golden–Morrison area, Jefferson County, Colorado: Colorado School Mines, Quarterly, v. 41, p. 46–55.
LOCKLEY, M.G., 1993, A Field Guide to Dinosaur Ridge: Friends of Dinosaur Ridge, Morrison, Colorado, 29 p.
LOCKLEY, M.G., AND MARSHALL, C., 2014, A Field Guide to the Dinosaur Ridge Area: Friends of Dinosaur Ridge, Morrison, Colorado, 56 p.
LOCKLEY, M.G., MCCREA, R.T., BUCKLEY, L.G., LIM, J.D., MATTHEWS, N.A., BREITHAUPT,
B.H., HOUCK, K.J., GIERLIN´ SKI, G.D., SURMIK, D., KIM, K.S., XING, L., KONG, D.Y., CART,
K., MARTIN, J., AND HADDEN, G., 2016, Theropod courtship: large scale physical evidence of display arenas and avian-like scrape ceremony behaviour by Cretaceous dinosaurs: Scientific Reports, v. 6, p. 189–152.
MACMILLAN, L.T., AND WEIMER, R.J., 1976, Stratigraphic model, deltaic plain sequence, J Sandstone (Lower Cretaceous), Turkey Creek area, Jefferson County, Colorado: Colorado School of Mines, Professional Contributions, v. 8, p. 228–241.
MARTY, D., AYER, J., BECKER, D., BERGER, J.P., BILLION-BRYANT, J.P., BRAILLARD, L., HUG,
W.A., AND MEYER, C.A., 2007, Late Jurassic dinosaur track sites of the Transjura Highway (Canton Jura, NW Switzerland): overview and measures for their protection and valorization: Bulletin of Applied Geology, v. 12, p. 75–89.
MARTY, D., STRASSER, A., AND MEYER, C.A., 2009, Formation and taphonomy of human footprints in microbial mats of present-day tidal-flat environments: implications for the study of fossil footprints: Ichnos, v. 16, p. 127–142.
MARTY, D., BELVEDERE, M., MEYER, C.A., MIETTO, P., PARATTE, G., LOVIS, C., AND THURING, B., 2010, Comparative analysis of Late Jurassic sauropod trackways from the Jura Mountains (NW Switzerland) and the central High Atlas Mountains (Morocco): implications for sauropod ichnotaxonomy: Historical Biology, v. 22, p. 109–133.
MARTY, D., MEYER, C.A., BELVEDERE, M., AYER, J., AND SCHAFER, K.L., 2013, Rochefort– Les Grattes: an Early Tithonian dinosaur tracksite from the Canton Neuchatel, Switzerland: Revue de Paleobiologie, v. 32, p. 373–384.
MACKENZIE, D.B., 1963, Dakota Group on west flank of Denver basin, in Katich, P.J., and Bolyard, D.W., eds., Geology of the Northern Denver Basin and Adjacent Uplifts: Rocky Mountain Association of Geologists, 14th Annual Field Conference, Guidebook, p. 135– 148.
MACKENZIE, D.B., 1965, Depositional environments of Muddy Sandstone, western Denver basin, Colorado: American Association of Petroleum Geologists, Bulletin, v. 49, p. 186–206.
MACKENZIE, D.B., 1968, Studies for students: sedimentary features of Alameda Avenue cut, Denver, Colorado: The Mountain Geologist, v. 5, p. 3–13.
MACKENZIE, D.B., 1972, Tidal sand flat deposits in the lower Cretaceous Dakota group near Denver, Colorado: The Mountain Geologist, v. 9, p. 269–277.
MORATALLA, J.J., MARUGAN-LOBON, J., MARTIN-ABAD, H., CUESTA, E., AND BUSCALIONI, A.D., 2017, A new trackway possibly made by a trotting theropod at the Las Hoyas fossil site (Early Cretaceous, Cuenca Province, Spain): identification, bio-dynamics, and palaeoenvironmental implications: Palaeontologia Electronica, v. 20, 3:59A, p. 1–14.
NEWMAN, S.A., KLEPAC-CERAJ, V., MARIOTTI, G., PRUSS, S., WATSON, N., AND BOSAK, T., 2016, Experimental fossilization of mat-forming cyanobacteria in coarse-grained siliciclastic sediments: Geobiology, v. 15, p. 484–498.
NOFFKE, N., 1998, Multidirected ripple marks arising from bacterial stabilization counteracting physical rework in modern sandy deposits (Mellum Island, southern North Sea): Geology, v. 26, p. 879–882.
NOFFKE, N., 1999, Erosional remnants and pockets evolving from biotic–physical interactions in a Recent lower supratidal environment: Sedimentary Geology, v. 123, p. 175–181.
NOFFKE, N., 2000, Extensive microbial mats and their influences on the erosional and depositional dynamics of a siliciclastic cold water environment (lower Arenigian, Montagne Noire, France): Sedimentary Geology, v. 136, p. 207–215.
NOFFKE, N., 2010, Microbial Mats in Sandy Deposits from the Archean to Today: Heidelberg, Springer, 196 p.
PFLUEGER, F., AND GRESSE, P.G., 1996, Microbial sand chips: a non-actualistic sedimentary structure: Sedimentary Geology, v. 102, p. 263–274.
PORCHETTI, S.D., BERNARDI, M., CINQUEGRANELLI, A., FARIA DOS SANTOS, V., MARTY, D.,
PETTI, F.M., CAETANO, P.S., AND WAGENSOMMER, A., 2016, A review of the dinosaur track record from Jurassic and Cretaceous shallow marine carbonate depositional environments, in Falkingham, P., Marty, D., and Richter, A., eds., Dinosaur Tracks: The Next Steps: Indiana University Press, p. 380–390.
REINECK, H.E., 1979, Rezente und fossile Algenmatten und Wurzelhorizonte: Natur und Museum, v. 109, p. 290–296.
SCHIEBER, J., 1986, The possible role of benthic microbial mats during the formation of carbonaceous shales in shallow Proterozoic basins: Sedimentology, v. 33, p. 521–536.
SCHIEBER, J., 2007a, Microbial mats in the siliciclastic record: a summary of diagnostic features, in Eriksson, P.G., Alterman, W., Nelson, D., Mueller, W.U., Catuneanu, O., and Strand, K., eds., The Precambrian Earth: Tempos and Events: Amsterdam, Elsevier, Developments in Precambrian Geology 12, p. 663–672.
SCHIEBER, J., 2007b, Ripple patches in the Cretaceous Dakota Sandstone near Denver, Colorado, a classical locality for microbially bound tidal sand flats, in Schieber, J., Bose, P.K., Eriksson, P.G., Banerjee, S., Sarkar, S., Altermann, W., and Catuneanu, O., eds., Atlas of Microbial Mat Features Preserved within the Clastic Rock Record: Amsterdam, Elsevier, p. 222–224.
SCHIEBER, J., 2007c, Flume experiments on the durability of sandy microbial mat fragments during transport, in Schieber, J., Bose, P.K., Eriksson, P.G., Banerjee, S., Sarkar, S., Altermann, W., and Catuneanu, O., eds., Atlas of Microbial Mat Features Preserved within the Clastic Rock Record: Amsterdam, Elsevier, p. 248–257.
SCHIEBER, J., BOSE, P.K., ERIKSSON, P.G., BANERJEE, S., SARKAR, S., ALTERMAN, W., AND
CATUNEANU, O., 2007, Atlas of Microbial Mat Features Preserved within the Siliciclastic Rock Record: Elsevier, Amsterdam, Atlases in Geosciences, 311 p.
VAN HINSBERGEN, D., DE GROOT, L., VAN SCHAIK, S., SPAKMAN, W., BIJL, P., SLUIJS, A.,
LANGEREIS, C., AND BRINKHUIS, H., 2015, A Paleolatitude Calculator for Paleoclimate Studies: PLoS One, v. 10, p. 6.
WAAGE, K.M., 1955, Dakota group in northern Front Range foothills, Colorado: U.S. Geological Survey, Professional Paper 274, p. 15–19.
WARDEN, J.G., CASABURI, G., OMELON, C.R., BENNETT, P.C., BREECKER, D.O., AND FOSTER, J.S., 2016, Characterization of microbial mat microbiomes in the modern thrombolite ecosystem of Lake Clifton, Western Australia using shotgun metagenomics: Frontiers in Microbiology, v. 7, p. 1064.
WEIMER, R.J., 1970, Dakota Group (Cretaceous) stratigraphy, southern Front Range, South and Middle parks, Colorado, in Dakota and related rocks of the Front Range: The Mountain Geologist, v. 7, p. 157–184.
WEIMER, R.J., AND HAUN, J.D., 1960, Cretaceous stratigraphy, Rocky Mountain region, USA: International Geological Congress, Copenhagen, Report, v. 12, p. 178–184.
WEIMER, R.J., AND LAND, C., 1972, Field guide to Dakota Group (Cretaceous) stratigraphy, Golden–Morrison area: Mountain Geologist, v. 9, p. 241–267.
WEIMER, R.J., LAND, C.B., JR., MACKENZIE, D.B., HARMS, J.C., AND WALKER, T.R., 1972, Environments of sandstone deposition, Colorado Front Range: The Mountain Geologist, v. 9, p. 239–267.
XING, L., LOCKLEY, M.G., MARTY, D., ZHANG, J., WANG, Y., KLEIN, H., MCCREA, R.T.,
BUCKLEY, L.G., BELVEDERE, M., MATEUS, O., GIERLINSKI, G.D., PINUELA, L., PERSONS, W.S.,
WANG, F., RAN, H., DAI, H., AND XIE, X., 2015, An ornithopod-dominated tracksite from the Lower Cretaceous Jiaguan Formation (Barremian–Albian) of Qijang, south-central China: new discoveries, ichnotaxonomy, preservation and palaeoecology: PLoS One, 10, e0141059.
XING, L., LOCKLEY, M.G., YANG, G., CAO, J., MCCREA, R.T., KLEIN, H., ZHANG, J., PERSONS,
W.S., AND DAI, H., 2016, A diversified vertebrate ichnite fauna from the Feitianshan Formation (Lower Cretaceous) of southwestern Sichuan, China: Cretaceous Research, v. 57, p. 79–89.
Received 16 March 2018; accepted 30 July 2019.