ABSTRACT The Morrison-Golden Fossil Areas National Natural Landmark, Colorado, USA, including Dinosaur Ridge, is rich in geological and paleontological history, ranking historically as the premier location and type area for Late Jurassic dinosaurs like Stegosaurus and Diplodocus . As the type area for the Morrison Formation, it famously became central in the “Bone Wars” underway in the 1870s. After a brief historical introduction at Stop 1, the trip will explore the ‘mid’-Cretaceous Dakota Sandstone (Stop 2), which yields the top-ranked dinosaur tracksite in the United States, with two type ichnospecies and the most accessible “nest scrape display trace” evidence of dinosaurian sexual display found anywhere. It also an important location for the study of microbial mat in association with dinosaur tracks, now photogrammetrically surveyed in detail. Dinosaur Ridge serves as the type area for the “Dinosaur Freeway” and the transgression of the Western Interior Seaway. Its diverse invertebrate traces have also been described in detail. After lunch at the Dinosaur Ridge Visitor’s Center exhibit (Stop 3), the trip will move a short distance to Golden to view the type localities for the first bird and crocodilian tracks ever reported from the Mesozoic (Stop 4). We will also visit the younger (Late Cretaceous) Laramie Formation, exposed in the Golden clay pits, cut by the Golden fault, and the source of historically famous paleofloras, the first known Ceratopsian tracks, and other type traces now developed as Triceratops Trail (Stop 5), constituting part of the well- documented Fossil Trace and School of Mines Geological Trail complex. The field excursion will involve easy walks and no strenuous climbs.

Abstract Stromatolites are the most intensively studied sedimentary structures built by microorganisms. They are familiar to us as the knobby carbonate domes and mats that occur in lagoons, embayments, and platforms of tropical oceans, as well as in lakes, and even in arctic climates. However, stromatolites also occur in noncarbonate sediments and sedimentary rocks. They have been recognized in siliciclastic settings, where early carbonate mineral precipitation has not occurred but biostabilization, baffling, trapping, and binding have affected the sediments and produced a varied host of microbial structures. These structures are referred to as “microbially induced sedimentary structures,” or, for short, MISS. Presently, 17 main types of MISS have been recognized, all of them differing significantly in appearance from carbonate-hosted stromatolites. The MISS, like stromatolites, record microbial activities in response to environmental parameters. Thus, certain types of MISS develop at specific depositional sites and therefore provide excellent paleoenvironmental indicators. Pioneering studies of MISS structures were conducted by Gisela Gerdes, Wolfgang Krumbein, and Juergen Schieber in the 1970s and 1980s. Their early work showed some of the problems in recognizing MISS in thick, dominantly siliciclastic successions, especially on wide siliciclastic tidal flats. This pioneering work was followed by studies by David Paterson, Lukas Stal, John Stolz, and Alan Decho that concentrated on biostabilization and biofilms. The book on Biostabilization of Sediments (1994) was the result of the ground-breaking Mini-Microbial Mat meeting organized by Wolfgang Krumbein in 1993.

Abstract Microbially induced sedimentary structures (MISS) were studied on siliciclastic and carbonate tidal flats of Texas to investigate the influence of sediment on the types and distribution of these biosedimentary structures. Carbonate MISS were studied on the wind tidal flats adjacent to Laguna Madre, whereas tidal flats next to Christmas Bay and East Matagorda Bay were visited to study siliciclastic MISS. In both depositional settings, microbial mats characteristically occur on the supratidal to intertidal flats. Reticulated surfaces, gas domes, mat desiccation cracks, sieve-like surfaces, and a variety of mat deformation structures are common in both siliciclastic and carbonate settings. However, the knobby surfaces that were common in the upper supratidal areas of the siliciclastic setting were not seen on the carbonate flats. The distribution of these structures is similar on the two types of sediments. Six MISS zones were identified in the siliciclastic settings based on the assemblages of these structures; however, because of the absence of the “knobby surfaces,” which define Zone I, only Zones II through VI were recognized in the carbonate settings. Zones I, II, and III coincide with the upper supratidal areas and thus occur at the shallower topographic levels, whereas most of Zone VI is located in the lower intertidal and subtidal areas. Zones II through IVare characterized by reticulated surfaces, whereas Zones III and IVare distinguished from Zone II by the additional occurrence of the gas domes and mat desiccation cracks, respectively. Mat desiccation cracks also occur in Zone Vin association with sieve-like surfaces. Finally, Zone VI is characterized by a dominance of mat deformation structures and the occasional existence of sieve-like surfaces. This study highlights the similarity between the siliciclastic settings and the carbonate environments in terms of the interaction between the microbial world and the sedimentary processes. This results in similar types of sedimentary structures and similar distribution profiles. Therefore, this study shows that compared to carbonate MISS, siliciclastic MISS can be equally important in terms of environmental interpretation. The results of this study also demonstrate that subtle variations in MISS and their facies changes can be used to indicate small variations in water depth, regardless of the sediment type.

Abstract Organism–matground interactions reflect two somewhat interrelated aspects: (1) the environmental restriction of microbial mats through geologic time and (2) the evolutionary changes in benthic faunas. The history of such interactions may be subdivided into six phases: (1) Ediacaran, (2) Cambrian, (3) Ordovician, (4) Silurian to Permian, (5) Early Triassic, and (6) Middle Triassic to Holocene. Widespread matgrounds in both shallow- and deep-marine deposits during the Ediacaran provided substrates that were available for benthic colonization and the development of various interactions. The most abundant ichnofossils in Ediacaran rocks are very simple grazing trails ( Helminthopsis ichnoguild), representing grazing of organic matter concentrated within microbial mats below a thin veneer of sediment. In shallow-marine environments, interactions were also evidenced by the mollusk-like Kimberella and associated scratch marks ( Radulichnus ) preserved on microbial mats. Interactions are also indicated for vendozoans, as reflected by serially repeated resting traces of Dickinsonia and the related genus Yorgia preserved on biomats. By the latest Ediacaran, simple burrow systems (treptinids) also occur in association with matgrounds. The replacement of matgrounds by mixgrounds was arguably the most significant change at the ecosystem scale in the history of marine life. By the Early Cambrian, branched burrow systems became more complex and common, resulting in increasing disruption of matgrounds in nearshore and offshore settings. While matgrounds were widespread in supratidal and upper- to middle-intertidal environments during most of the early Paleozoic, lower-intertidal deposits were already intensely bioturbated by the late Early Cambrian. The diachronic nature of the Agronomic Revolution is evident in the deep sea, where microbial matground ecosystems persisted during most, if not all, of the Cambrian. In addition to the Helminthopsis ichnoguild, Cambrian deep-marine ichnofaunas also consist of arthropod trackways and sophisticated feeding strategies represented by different Oldhamia ichnospecies, revealing complex architectural designs by undermat miners. In contrast, in deep-marine Lower Ordovician deposits, microbial textures are rare and patchy and typically not associated with trace fossils. Biomats persisted into the late Paleozoic in the innermost, freshwater region of estuarine systems, as well as in fluvio-lacustrine deposits, glacial lakes, and fjords. Ichnofaunas dominated by very shallow tier structures, such as arthropod trackways and grazing trails, locally associated with matgrounds, were common in these deposits. The widespread development of matgrounds after the end-Permian mass extinction sets the stage for the reappearance of feeding strategies linked to the exploitation of biomats. However, subsequent faunal recovery and deep and pervasive bioturbation resulting from the establishment of the Modern evolutionary fauna led to increased restriction of microbial mats. Analysis of ichnofaunas in matgrounds provides evidence of the temporal and environmental restriction of biomats and allows a better understanding of animal–matground interactions, as well as of preservational biases in the trace-fossil record.

Abstract Microbially induced sedimentary structures (MISS) in siliciclastic shallow-marine strata occur in the lower and middle Cambrian and the Silurian of southern Sweden. These are typically transparent wrinkle structures with a wide range of morphologies. They are exclusívely associated with shoreface to lower shoreface environments, characterized by fine-grained sandstone interbedded with mudstone and a Cruziana ichnofacies. Thicker, non-transparent forms with high-relief crinkled surfaces occur in the same paleoenvironments. The landward sand-dominated facies belt with Skolithos ichnofacies (upper shoreface–foreshore) lack preserved wrinkle structures. Evidently, wrinkle structures are more common than previously thought in the Swedish Paleozoic and provide an important tool for understanding paleoenvironments and benthic paleoecology in strata largely devoid of body and trace fossils.

Abstract Field studies in a siliciclastic mesotidal flat in Bahia Blanca Estuary, Argentina, reveal the presence of extensive areas with microbial mats, covering the upper intertidal and lower supratidal areas. Study of recent environments with microbial mats has increased considerably in recent years, not only because of their unique sedimentologic and ecologic characteristics but also because they provide important implications for the understanding of fossil environments. The main purpose of this research was to evaluate the role of microbial mats in the preservation of biogenic structures. We recorded the distribution of recent biogenic structures all over the siliciclastic tidal flat, focusing the analysis on the preservation of bird tracks. Several footprints were selected and photographed; we recorded the morphologic modifications they experienced over the course of 10 months. This study revealed that most of the footprints showed resistance to tide and wind erosion and also to the heavy rains and storms that affected the tidal flat. This resistance is clearly associated with the presence of the microbial mats, which are known to biostabilize the sediment. In addition, microscopic analysis of the tidal-flat sediment revealed the presence of zeolites, indicating early cementation, which may have favored the consolidation of the footprints. Mat thickness also affected the morphology of the footprints; in areas with thick microbial mats overlying water-saturated sands, the tracks were deeply impressed and did not show fine details. On the contrary, in zones with thin microbial mats overlying relatively stiff muds, the traces were shallow and preserved details such as skin impressions and skid marks. Both types of footprints were affected by mat growth, although in the shallow traces the modification was faster and the fine details were progressively obliterated. This study yields valuable insight into the relationship between microbial mats and the morphology of the footprints and provides key information for the analysis of fossil tracks in equivalent paleoenvironments.

Abstract Even though they are small-scale ecosystems, microbial mats have greatly influenced the development of life on Earth. In order to put together a clear image of how life was on this planet billions of years ago, one has to look to fossils. To see how certain conditions influenced the development of the fossils it is very important to look for modern analogues that may have the potential to create sedimentary structures. Such a possible analogue is the cyanobacterial mat associated with the geothermal spring from Beltiug, Western Plain of Romania. The cyanobacterial diversity was explored by light and electron microscopy, alongside the following culture-independent techniques automated ribosomal intergenic spacer analysis and amplified ribosomal DNA (rDNA) restriction analysis, based on 16S rDNA and 16S-23S internal transcribed spacer markers. Based on the partial 16S rDNA sequences obtained, four cyanobacterial taxons were identified, all belonging to the Oscillatoriales order. Even though the mat started to develop only a few decades ago, its spatial structure and taxon composition are similar to those of modern or fossil sedimentary structures. These observations, coupled with the chemical properties of the water, rich in Ca 2+ and HCO − 2 make the Beltiug mat suitable for the appearance of sedimentary structures.

Abstract The Middle Cambrian Gros Ventre Formation (north-central Wyoming, USA) contains a lagerstätte of Rusophycus (arthropod ichnofossils) preserved in convex hyporelief on the base of sandstone beds or as sand lenses within beds of silty mudstone. Rusophycus was produced by typical trilobite behavior but was preserved only under the particular confluence of conditions that were common during the deposition of the Gros Ventre Formation. These conditions include the background deposition of firm, cohesive muds bound by bacterial mats, which allowed the burrows to maintain their shape until cast by episodic sand deposition. The absence of spreiten and presence of storm event bedding within sectioned Rusophycus specimens support this scenario and contradict previous assertions that Rusophycus was formed within the substrate. Chondrites were also present, extending its range into the Middle Cambrian. Bacterial mats and microbially induced sedimentary structures were present before, during, and after the deposition of the Gros Ventre Formation. Bioturbation indices are comparable to other Middle Cambrian sites, supporting a gradual increase of bioturbation intensity during the Agronomic and Cambrian Substrate Revolutions.

Abstract The Middle Archean Moodies Group (ca. 3.22 Ga), Barberton Greenstone Belt, South Africa, exposes one of the world’s oldest ecosystems. It includes kerogen-rich laminae and thin chert bands interbedded with coarse-grained and gravelly sandstones. The strata record a medium-energy, tidal coastal environment. Analyses of the microscopic structure and chemical composition of the chert bands through petrographic microscopy, Raman microspectroscopy, laser-induced breakdown spectroscopy (LIBS) analyses, C isotopes, and scanning electron microscope (SEM) photography of macerated material, supported by textural observations of hand samples, suggest that these laminae represent variably compressed and early-silicified microbial mats. Internal wavy laminations, amorphous carbon composition, and negative δ 13 C values strongly imply a biogenic origin. Complete HF maceration of chert bands revealed polygonal cell structures in a formerly extracellular polymeric substance matrix. The tuft- and dome-micromorphology of the laminations resembles that of recent photosynthetic filament-dominated microbial mats. Facies interpretations indicate that microbial mats extensively colonized subtidal to intertidal Archean siliciclastic coastlines.

Abstract The Witwatersrand (WWR) ores contain more gold than could have been derived in particulate form by erosion from any conceivable type of source area as proposed by the modified placer hypothesis. In contrast to this, syngenesis goes further to explain a host of observations from those Late Archean Au-U ores. Although recycling, placer processes, and processes of hydrothermal (diagenetic/authigenic) mobilization all contributed, syngenesis was a major factor contributing to ore genesis in this huge metallogenic province. Over 80% of the gold occurs in the Main Reef and Bird Reef of the Johannesburg Subgroup in the Central Rand Group, and about half of this gold is closely associated with carbon derived from microbial remains. In the principal deposits within the WWR basin, the ore is disposed in thin carbonaceous horizons of extensive lateral continuity upon chronostratigraphic unconformities in otherwise unmineralized siliciclastic metasediments. The ore-bearing horizons are not themselves part of the erosion cycle that gave rise to those paleosurfaces but were generated during the initial phase of renewed cycles of deposition after long intervals of nondeposition. They bear little resemblance to placers, their alluvial character seemingly inherited from reworking in fluvial environments. Most of the gold and probably also part of the uranium were made available for transport in solution under relatively low-temperature, chemically aggressive environmental conditions, a situation favored on the emerging Kaapvaal Craton. Intense chemical weathering was made possible by the influence of the same ionizable gases as occur in geothermal systems, and this was a crucial factor leading to metallization. These elements, together with a host of other heavy metals, were then transported to the edge of the depository. A key confluence of conditions was completed with the blooming of microbial communities during hiatuses in sedimentation. Over large areas, microbial mats developed directly on paleosurfaces upon which the goldfields occupy slight depressions, bounded on either side by clean quartz arenites. The resulting metallization was a complex chemical and biochemical precipitation of gold, uranium, pyrite, and associated Co, Ni, Cu, Pb, and As in thin, areally extensive deposits. Metallization was focused at several carbonaceous horizons along the north and northwestern margins of the WWR basin, depending on the availability of metal-rich aqueous fluids coincident with the stillstand of land surface degradation and the consequent proliferation of microbial mats. Biochemical processes supplemented low-temperature geochemistry of the fluids in helping to concentrate a substantial portion of WWR gold in larger particles, which were transported further downslope and then subjected locally to fluvial processes. Gold precipitated outside of the preserved basin by these processes likewise will have undergone alluvial reworking prior to deposition in the conglomerates without the originally associated carbon; recognition of this feature diminishes the source rock problem. Minor remobilization of metals occurred during diagenesis and metamorphism.

Abstract Microbial mat–related sedimentary structures are present in Lower Pleistocene mixed epiclastic-volcaniclastic sediments that host the Cape Vani manganese-oxide (-barite) deposit on NW Milos Island, Greece. Milos Island is a dormant and recently emergent 2 Ma volcano of the active Southern Aegean volcanic arc. The deposit occurs in a 1-km-long marine rift basin floored by a dacite dome. Basin fill is a >60-m-thick sequence of epiclastic glauconite-bearing sediments sandwiched between lower and upper mixed volcaniclastic sandy tuffs and epiclastic sandstones. Host siliciclastics consist of glass shards, lithic fragments, plagioclase, K-feldspar, biotite, pyroxene, and silica and clay cements, overprinted by a barite– silica–K-feldspar–illite assemblage. Manganese (IV)–oxide minerals include dominantly δMnO 2 (vernadite), hollandite group minerals, pyrolusite, ramsdellite, and nanocrystalline todorokite. Microbially induced structures occur in a specific lithofacies referred to as upper “ferruginous and white volcaniclastic sandy tuffs/sandstones” and are characterized by: (1) planar and herringbone cross-bedding, (2) small-scale, vertical fining-upward sequences, (3) flaser, wavy, and lenticular bedding, (4) marine trace fossils similar to Skolithos , and (5) beveling of ripple marks and desiccated silicified mudstone beds. These features, together with the microbially induced structures and the widespread presence of glauconite, reflect a littoral to tidal-flat paleoenvironment. The microbial mat–related sedimentary structures developed in the Mn-oxide ore formation are recognized as: (1) mat-layer structures, (2) growth bedding structures and nodules, (3) wrinkle structures and exfoliating sand laminae, (4) cracks with upturned and curled margins, (5) roll-up structures, (6) fossil gas domes, (7) mat fragments and chips, and (8) mat slump structures, suggesting photoautotrophic, possibly cyanobacterial, mats. The ubiquitous presence of barite, in the host sediments, in the mat-related structures, in feeder-vein and bedding conformable layers, and in the gravel unit that caps the Cape Vani sedimentary rocks, suggests that microbial mats were developed in association with white smokers acting as Mn(II) suppliers, in a sunlit shallow-water or tidal-flat paleogeothermal system. The intimate relationship of Mn(IV)-oxide ore mineralization with the microbial mat–related sedimentary structures, coupled with the presence of Mn mineralized microbial fossils in the ore, strongly suggests the possible role of bacterial photosynthesis in Mn(II) bio-oxidation and Mn(IV)-oxide biomineralization at Cape Vani. It is envisaged that most Mn(IV)-oxide mineralization was synsedimentary and syngenetic and formed due to an interplay among shallow-marine/tidal-flat sedimentation, hydrothermal seafloor to subaerial hot spring activity, which provided Mn(II), and active, possibly photosynthetic, microbial activity. Chemotrophic influence on Mn(IV)-oxide biomineralization cannot be excluded.

Abstract The affinities of the Ediacara biota are a source of continual debate. A case can be made, however, that they represent an assortment of stem and crown group metazoans together with a large proportion of enigmatic forms that are difficult to classify and likely represent extinct multicellular evolutionary experiments. In the backdrop of these complex multicellular organisms are microbial communities, which in the absence of metazoan grazing and active bioturbating, constructed thick mat structures that dominated shallow- to deep-water paleoenvironments throughout the Mesoproterozoic and Neoproterozoic. Microbial mat communities played an important role in Ediacaran ecosystems by creating a firm substrate to which macroscopic organisms could attach. These mat structures are also presumed to have played a vital role in the preservation of soft-bodied Ediacaran fossils. Despite their importance, the study of Ediacaran microbial colonies, especially from deep-water localities well below the photic zone, is limited. As a result of taphonomic difficulties associated with the preservation of microbial colonies in siliciclastic sediments, the proper identification of microbially induced sedimentary structures (MISS) has greatly improved our understanding of Precambrian paleoecosystems. Here we present the oldest evidence of deep-water MISS from the terminal Neoproterozoic Avalon and Bonavista peninsulas of Newfoundland, Canada. Sedimentary analyses indicate that the pustular circular fossil Ivesheadia , previously regarded as a cnidarian or a degradational product, instead represents the remains of microbial colonies that occupied the sediment–water interface and resulted in distinct sedimentary structures. A second series of peculiar sedimentary structures colloquially known as “bubble trains” are believed to represent additional evidence of MISS from the Ediacaran of Newfoundland.

Abstract Biological soil crusts (BSCs) are ubiquitous and crucial components of modern dryland ecosystems and probably were the first community type to colonize the Precambrian land surface. BSCs are complex symbioses of eubacteria, cyanobacteria, green algae, mosses, lichens, and fungi. BSCs, having adapted to intense ultraviolet radiation and drastic variations in precipitation and temperature, have likely been prevalent in terrestrial environments since the Precambrian and are undoubtedly under-reported in the rock record. This is probably due to the crusts’ inconspicuous appearance and preservational taphonomy. In order to improve understanding of the diverse appearances of BSCs in sedimentary strata, this study reviews the biology, biologically produced structures, and morphological variation of modern BSCs using examples from Colorado Plateau BSC of southern Utah (Grand Staircase–Escalante National Monument). Sediment coring into modern BSCs identified a variety of pedogenic features. Simple compaction experiments on the cores illustrate the taphonomic destruction of pedogenic features. In addition, a comparison of the modern BSC features to those preserved in a Cretaceous BSC found in Utah demonstrates the utility of understanding the nature of the various stages of development of modern BSCs. These descriptions of potentially preserved expressions of BSCs should facilitate identification and separation of fossilized BSCs from other physical sedimentary structures.

Abstract Microbial earths are communities of microscopic organisms living in well-drained soil. Unlike aquatic microbial mats and stromatolites, microbial earths are sheltered from ultraviolet radiation, desiccation, and other surficial hazards within soil cracks and grain interstices. Currently, such ecosystems are best known in small areas of unusually cold, hot, or saline soils unfavorable to multicellular plants and animals. During the Precambrian, microbial earths may have been more widespread, but few examples have been reported. This review outlines a variety of features of modern microbial earths that can be used to distinguish them from aquatic microbial mats and stromatolites in the fossil record. Microbial earths have vertically oriented organisms intimately admixed with minerals of the soil, whereas microbial mats are laminated and detachable from their mineral substrate as flakes, skeins, and rollups. Microbial earths have irregular relief, healed desiccation cracks, and pressure ridges, whereas microbial mats have flexuous, striated domes, and tufts. Microbial earths form deep soil profiles with downward variations in oxidation, clay abundance, and replacive nodular subsurface horizons, whereas microbial mats form as caps to unweathered, chemically reduced sedimentary layers. Microbial earths develop increasingly differentiated soil profiles through time, whereas microbial mats build upward in laminar to domed increments. Microbial earths are found in nonmarine sedimentary facies, whereas microbial mats form in lacustrine, floodplain, and marine sedimentary facies. Microbial mats and stromatolites are known back to the oldest suitably preserved sedimentary rocks in the 3458 Ma Apex Chert and 3430 Ma Strelley Pool Formation (respectively) of the Pilbara region of Western Australia. The geological antiquity of microbial earths extends back to 2760 Ma in the Mount Roe paleosol of the Hamersley Group near Whim Creek, Western Australia.

Abstract Microbially induced sedimentary structures (MISS) are primarily known from transitional marine (tidal) and shallow-marine settings. New results from the ca. 1.1 Ga Midcontinent Rift System of North America extend their record into wholly terrestrial depositional settings, including capping paleosols, paludal environments, and alluvial sedimentary units. The MISS are present on both sides of present-day Lake Superior, at Good Harbor Bay, Minnesota, and at Copper Harbor, Michigan, indicating a regionally widespread biosphere. MISS in the Midcontinent Rift System include abraded Kinneyia, pustulose mound structures, multidirectional wave ripples, textured bedding plane surfaces, and stromatolites. Organic carbon is also preserved in MISS from both sides of the rift, and in finely laminated sedimentary strata, indicating that the biosphere extended also into the hinterland. Independent climatic evidence indicates a temperate setting; thus, the new MISS extend both the depositional and environmental niches where MISS are preserved.

Abstract The Ruyang (Mesoproterozoic; Pt 2 ) and Luoyu (Neoproterozoic; Pt 3 ) groups in the southern North China Platform are dominated by peritidal siliciclastic rocks and contain abundant microbially induced sedimentary structures (MISS), including multidirected and mat-protected ripple marks, mat chips, mat-smoothed ripple marks, various sand cracks, and irregular growth ridges. These MISS can be grouped into four MISS associations, each of which has distinctive MISS morphology resulting from changes in depositional environments. The upper subtidal to lower intertidal zone commonly lacks in situ MISS but contains redeposited mat chips. The upper intertidal zone is characterized by mat-protected ripple marks and mat chips. The lower supratidal zone is rich in various MISS, especially the large and morphologically complex sand cracks indicative of growth of thick mats in microenvironments with low hydrodynamics, sufficient moisture level, and frequently exposed substrates. The upper supratidal zone abounds with small sand cracks formed from relatively thin microbial mats. The morphological associations from tidal-flat deposits of the Ruyang and Luoyu groups are similar to those found in modern siliciclastic coastal environments. Similarities in MISS between modern and Proterozoic tidal flats suggest that the MISS morphological associations can be used for paleogeographical and paleoenvironmental reconstructions, especially in Precambrian siliciclastic successions.