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Teller County Colorado
ABSTRACT The late Eocene Florissant Formation in central Colorado is a rich and diverse continental Lagerstätte yielding well-preserved fossil assemblages from lacustrine and fluvial facies. This investigation focused on the lacustrine facies at Clare’s Quarry and used biotic and abiotic evidence to characterize aspects of the lake and processes that resulted in the accumulation and preservation of the host rock and its fossils. Autecology of modern analogs representing the fossil diatom taxa was used to augment sedimentary data in characterizing the lake, propose peripheral habitats within the catchment area, and suggest a terrestrial source for mudstone units. The sedimentary and stratigraphic record at the study site reveals a lake with sufficient depth to allow bottom waters to remain isolated and anoxic for long periods. Sediments that accumulated in the lake produced distinct lacustrine lithofacies that are interpreted as representing at least three modes of origin: stable lake, pyroclastic, and mud turbidite sedimentation. Slow, suspension settling of fine clays and volcanic ash into a moderately deep, stable lake resulted in laminated shales. These laminated shales contain frustules of diatoms from planktic and benthic lake habitats; diatoms transported into the lake from streams and wetlands; fish, mollusks, ostracods, and insects; and plants from marginal and upslope environments. Intermittent volcanic eruptions produced air-fall ash and granular tuff that accumulated as interbeds within the lake shales. Periods of stable lake sedimentation were frequently interrupted by rapid influxes of suspended fine clays, perhaps as mud-dominated turbidites that prograded into the lake at intervals of high runoff triggered by climatic, volcanic, or tectonic events.
Identifying pathfinder elements for gold in bulk-rock geochemical data from the Cripple Creek Au–Te deposit: a statistical approach
Abstract Cripple Creek is among the largest epithermal districts in the world, with more than 800 metric tons (t) Au (>26.4 Moz). The ores are associated spatially, temporally, and genetically with ~34 to 28 Ma alkaline igneous rocks that were emplaced into an 18-km 2 diatreme complex and surrounding Proterozoic rocks. Gold occurs in high-grade veins, as bulk tonnage relatively low-grade ores, and in hydrothermal breccias. Pervasive alteration in the form of potassic metasomatism is extensive and is intimately associated with gold mineralization. Based on dating of intrusions and molybdenite and gangue minerals (primarily using 40 Ar/ 39 Ar and Re-Os techniques), the region experienced a protracted but intermittent history of magmatism (over a period of at least 5 m.y.) and hydrothermal activity (intermittent over the final ~3 m.y. of magmatic activity). Key factors that likely played a role in the size and grade of the deposit were (1) the generation of alkaline magmas during a transition between subduction and extension that tapped a chemically enriched mantle source; (2) a long history of structural preparation, beginning in the Proterozoic, which created deep-seated structures to allow the magmas and ore fluids to reach shallow levels in the crust, and which produced a fracture network that increased permeability; and (3) an efficient hydrothermal system, including effective gold transport mechanisms, and multiple over-printed hydrothermal events.
Beryllium mineralization in pegmatites and quartz dikes of Mount Rosa Complex Area, Colorado Front Range, Colorado, USA
Digitization reveals and remediates challenges to research on dispersed museum collections from Florissant fossil beds, Colorado
ABSTRACT Florissant fossil beds ranks among the best documented Cenozoic fossil deposits in the world in number of scientific publications and named species. The history of geoscience research on the Upper Eocene Florissant Formation spans nearly one and a half centuries. New excavations and transfers of historic collections have spread Florissant fossils to nearly 30 natural history museums during that period. The history of acquisition, conservation, and taxonomic study of each museum’s collection is unique, so Florissant collections provide examples of how taxonomic diversity, physical conservation, and public exhibition of collections vary with provenance. Dispersal of fossils among museums, including separation of type specimen parts and counterparts, has led to a variety of challenges for research on Florissant fossils. First, an exploratory, quantitative analysis of taxonomic diversity in four collections of fossil insects from Florissant uncovers a pattern of identification bias. Some taxonomists preferentially identify common taxa or consistently misidentify rare taxa, for instance. In light of this result, it is recommended that researchers vet any set of identifications made by multiple researchers or, ideally, identify specimens anew. Second, observations of Florissant specimens at different museums show that a large number of fossils have been lost, damaged, or destroyed due to actions such as travel on loan, display in exhibits, or application of non-archival conservation techniques. Through the digitization process, including cataloging and imaging specimens, curatorial staffs have discovered the extent of uncatalogued or missing material. Digitization has mitigated some of the challenges associated with dispersion of specimens. Collaborative projects across museums have led to rediscovery of lost specimens or discovery for the first time of parts and counterparts that correspond to the same fossil but are housed at different institutions. Online databases that serve specimen images allow researchers to assign new taxonomic determinations, controlling for bias from earlier researchers, or to examine fossils remotely from photographs, reducing the need to handle and ship fragile material for loans. Moreover, providing public access to museum specimen records through collaborative digitization projects expands the opportunities to exhibit and develop specimen-based educational curricula.
PRESERVING FOSSILS IN THE NATIONAL PARKS: A HISTORY
FINE-GRAINED DEBRIS FLOWS IN COARSE-GRAINED ALLUVIAL SYSTEMS: PALEOENVIRONMENTAL IMPLICATIONS FOR THE LATE PALEOZOIC FOUNTAIN AND CUTLER FORMATIONS, COLORADO, U.S.A.
Freshwater molluscan fauna from the Florissant Formation, Colorado: paleohydrologic reconstruction of a latest Eocene lake
Basement-hosted sandstone injectites of Colorado: A vestige of the Neoproterozoic revealed through detrital zircon provenance analysis
DEPOSITIONAL SETTING AND FOSSIL INSECT PRESERVATION: A STUDY OF THE LATE EOCENE FLORISSANT FORMATION, COLORADO
THE CRYSTAL STRUCTURE AND GENESIS OF KRENNERITE, Au 3 AgTe 8
Paleontological studies at Florissant have been ongoing for more than 13 decades. As the focus of these studies has shifted through this time, the site has provided important insights into the evolution of paleontology as a science from its beginnings in the nineteenth century through its subsequent development. Early studies focused on the description of new taxa from collections that were being made by the early scientific surveys of the American West, particularly the Hayden Survey during the early 1870s and an expedition from Princeton in 1877. The first studies and descriptions of these fossils were by Leo Lesquereux on the fossil plants, S.H. Scudder on the fossil insects, and E.D. Cope on the fossil vertebrates. At the beginning of the twentieth century, T.D.A. Cockerell conducted field expeditions in 1906–1908, and subsequently published ∼130 papers on fossil plants, insects, and mollusks. Work by these early researchers was the first to consider the implications of the Florissant fossils for evolution, extinction, biogeography, and paleoclimate. Even greater emphasis on these broader implications began when H.D. MacGinitie made excavations during 1936–1937 and published a comprehensive monograph on the fossil flora in 1953, including numerous taxonomic revisions and detailed interpretations of stratigraphic context, paleoecology, paleoclimate, paleoelevation, biogeography, and taphonomy. Other workers during the late 1900s initiated the first studies on pollen, dicotyledonous woods, and multiple organ reconstructions of extinct plant genera, and developed more quantified methods for determining paleoelevation and paleoclimate. Current work emphasizes plant-insect interactions, the use of diatoms as fresh-water paleoen-vironmental indicators and as agents in macrofossil taphonomy, and the use of insects as terrestrial environmental indicators.
Scanning electron microscopy (SEM) of insect and plant fossils in the lacustrine shales of the Eocene Florissant Formation of Florissant, Colorado, was used to investigate the mechanisms of fossil preservation. The fossiliferous Florissant “paper shales” are composed of thin laminae of diatomite that form couplets with alternating smectitic clay laminae. The millimeter-scale sedimentary couplets may preserve an episodic record of sedimentation and are interbedded with less frequent, coarser volcaniclastic layers. The insect and plant fossils are associated with biofilms of extracellular polymeric substances (EPS) secreted by diatoms. The preserved organisms are entangled in the diatom aggregates coated with the EPS biofilm. We suggest that decomposition of the organisms was arrested during sedimentation and burial by the protective nature of the mucus covering, the properties of which limited the actions of bacteria and grazers and may have enhanced fossilization. A novel contribution of the study is a demonstration that this mechanism of exceptional preservation is also common at other similar lacustrine fossil sites, as supported by a further SEM analysis of insect and plant fossils from other Cenozoic lake deposits formed in environments comparable to the Floris-sant Formation. The deposits include the Oligocene shale at Canyon Ferry, Montana; the Miocene Savage Canyon Formation, Stewart Valley, Nevada; and the Miocene Shanwang Beds of Shandong Province in northeast China. In addition, cultures of diatomaceous biofilms, grown in the laboratory display morphological features identical to those of the fossil diatomaceous biofilms. Our contribution indicates the significance of biofilms in fossil preservation at Florissant and other deposits.
We used higher taxonomic composition of 241 modern forest plots from across the New World to identify the closest modern analog of the Florissant fossil flora and to infer late Eocene paleotemperature for Florissant. Nonmetric multidimensional scaling (NMS) based on both genus and family presence-absence placed Florissant in a no-analog taxonomic space surrounded by North American warm temperate broad-leaved forests, Mexican humid pine-oak forests, and subtropical moist forests from Florida, Mexico, and Argentina. The site most similar to Florissant, as indicated by the mean of Euclidean distances in genus and family NMS space, was a subtropical moist forest in southern Florida, followed by the humid pine-oak forests of central and northeastern Mexico, and the broad-leaved deciduous forests of eastern North America. Weighted-averaging partial least-squares regression (WAPLS) based on genus composition predicted a mean annual temperature (MAT) for Florissant of 14.7 ± 2.2 °C. WAPLS based on family composition predicted a MAT of 15.6 ± 2.5 °C. Our estimates fall between the relatively cool temperatures predicted by leaf physiognomy and the higher temperatures predicted by the nearest living relative method. Although this study demonstrates the feasibility of using higher taxa for paleoclimate reconstruction and analog analysis, its methods are subject to many of the same biases and assumptions as other biological proxy techniques. Furthermore, interpretation of differences between results obtained using different taxonomic levels remains unclear. Some of these limitations may be resolved by employing methods based on phylogenies rather than taxonomic ranks.
The biogeographic affinities of the Florissant flora are in need of reevaluation. We give a critical review, based on megafossil and pollen records representing genera whose affinities we accept as well founded. The Florissant assemblage includes taxa of diverse modern geographic distribution. The flora is composed mainly of Laurasian elements, some of which are now confined to Asia ( Ailanthus , Dipteronia , Eucommia , Platycarya , Pteroceltis ) and a wide number co-occurring in the eastern United States and Asia. Others are now confined to western North America ( Sequoia , Cercocarpus , Sarcobatus ) and many occur in Mexico. The major geographic affinities of the Florissant genera discussed here are broad and include the present-day warm temperate and subtropical floras of Mexico, central and southern China, and the southeastern United States. Many taxa appear to have been shared between North America and Asia by Eocene time. The Rocky Mountain flora was distinct from that of the southeastern United States, probably because of the barrier represented by the Cannonball epeiric sea that traversed the Midcontinent in the Paleocene. Similarity of Florissant taxa to the South American flora is low. The deterioration of climate after the time of Florissant deposition represents one of the most significant decreases in temperature of the entire Tertiary. Following the warm interval of the latest Eocene, a few Florissant genera were locally extirpated, a few became extinct, some were already at or dispersed to lower-elevation regions, and others persisted in the southern Rocky Mountains. Over longer geologic time spans, some taxa seem to have persisted on the West Coast of North America through the Miocene, and in a few cases even up to the present. Many deciduous taxa have persisted in the summer-wet climate area of the eastern United States.
A comparison of plant-insect associations in the middle Eocene Green River Formation and the Upper Eocene Florissant Formation and their climatic implications
The fossil plants found in the Eocene Florissant Formation and Green River Formation are preserved with a level of detail that allows one to closely examine traces of insect feeding damage. Levels (amounts) and patterns (abundance of various types) of fossilized insect feeding damage from Florissant and the middle Eocene Green River Formation were compared. This allowed for a detailed examination of feeding damage and provided an opportunity to examine long-term patterns of change in insect herbivory during a period of climate fluctuation. Samples including 624 fossil leaves from Florissant and 584 fossil leaves from the Green River Formation were examined to document overall damage levels, the presence/absence of specific feeding guilds (i.e., hole-feeding, skeletonization, leaf-mining), and host-specific damage types. Florissant insects show host specificity in their feeding preferences as evidenced through the distribution of feeding damage on plants and through the presence of identifiable host-specific interactions. Some of these interactions appear to be long lasting as they are also apparent on the same, or closely related, leaf species found in the Green River Formation. Insect damage levels declined from the middle to late Eocene. This decline is correlated with a cooling event during this time interval and is in concurrence with the findings of other authors who have examined fossilized herbivory and climate change patterns. There is also an increase in the abundance of galls during this same interval, which also may be related to climate change.
Using available shape characters we conducted an outline morphometric analysis to make family-level identifications of fossil spiders from the Florissant Formation in Colorado. In this analysis we used carapace shape because it is a character that can be observed on most fossil spiders, and we also used linear leg characters. All measurements were first made on 202 modern spiders from eight families found in localities similar to the fossil lake environment. A multiple discriminant analysis (MDA) of the eigenshape axes was used to predict family placement among the modern data set to test the accuracy of the predictions. The modern spider families that were predicted correctly most often were the Salticidae (91.2%), Linyphiidae (80%), Dictynidae (76.5%), Tetragnathidae (68.2%), Clubionidae (66.7%), and Araneidae (65.5%). Families that produced less successful results were the Agelenidae (46.7%) and the Lycosidae (39.1%). Forty-three fossil spiders from Florissant were then added to the model to determine their family placement. All fossils were placed into modern families with varying degrees of accuracy. Only 42% of our identifications agree with those made by previous authors, but it is likely that these specimens were originally misidentified. With the addition of more taxa and characters, we believe that an outline morphometric approach shows great promise for helping to identify fossil taxa that are lacking traditional taxonomic characters.
The Chadronian mammalian fauna of the Florissant Formation, Florissant Fossil Beds National Monument, Colorado
During the past five years, renewed prospecting and collecting of mammalian fossils in the Florissant Formation within Florissant Fossil Beds National Monument in central Colorado has nearly tripled the known diversity of fossil mammals from this rock unit. Taxa first recorded here from the Florissant Formation include the eomyid rodent Paradjidaumo trilophus , the lagomorph Palaeolagus , and the rare artiodactyl Pseudoprotoceras longinaris . We also describe an isolated deciduous premolar of a protoceratid. We update the mammalian faunal list of the Florissant Formation, which includes some 16 species in 13 families and 6 orders. The mammalian fauna corroborates the Chadronian (latest Eocene) age determined by others. Geographic ranges of Pelycomys , Palaeolagus , and Paradjidaumo trilophus are extended slightly southwest from northeastern Colorado, and the range of Pseudoprotoceras longinaris is extended southwest from Wyoming and Nebraska. Based upon comparison with nearest living relatives and plausible analogs, the mammalian taxa represented in the Florissant Formation seem to be consistent with the moist, warm temperate, relatively high elevation wetland and woodland habitats that have been inferred by others for the area in and around late Eocene Lake Florissant.
Mineralogy and geochemistry of late Eocene silicified wood from Florissant Fossil Beds National Monument, Colorado
Silicified stumps preserved within a late Eocene lahar deposit have diverse mineralogy, ranging from opal-CT to chalcedony. In specimens that contain both silica polymorphs, the minerals appear to have originated independently, rather than from diagenetic transformation of an opaline parent material. This petrifaction process is unlike the progressive transformation of opal-A→opal-CT→chalcedony that has long been accepted as a general model for wood silicification. At the Florissant fossil forest, petrifaction occurred in several stages, beginning with precipitation of amorphous silica on cell wall surfaces. Cell lumina later became filled with opal-CT and chalcedony. A final phase of silica deposition is evidenced by chalcedony-filled fractures that crosscut permineralized tissues in some specimens. Spaces between adjacent tracheids commonly remain unmineralized, causing the silicified wood to remain permeable to water, and to readily cleave radially and tangentially. To a lesser degree, the fossilized wood is subject to transverse fracturing. This combination of structural characteristics causes Florissant fossil stumps to be susceptible to damage from freeze-thaw weathering.
Silicified stumps at Florissant Fossil Beds National Monument are subject to degradation from a variety of causes, including freeze-thaw weathering. In the past, noninvasive measures have been taken to conserve selected fossils, including use of metal bands for reinforcement and construction of shelters. Our study had two goals. The first was to document the temperature and relative humidity to which petrified stumps are exposed at Florissant. The second was to conduct a preliminary examination of the feasibility of using consolidants and adhesives to reinforce fossil wood and reduce its susceptibility to weathering. Electronic data loggers were used to monitor temperature and relative humidity both internally and externally for one exposed and one sheltered fossil stump, and results indicated that over 289 days of the cold season, there were 119 freeze-thaw events for the surface of the exposed stump and 95 events for the surface of the stump protected by a roof. Temperature and humidity variations were markedly lower for the subsurface sensors. Seven organosilicate formulations were field tested as consolidants on samples of petrified wood. Adhesives used to repair fractures included two polyvinyl butyral (PVB) formulations, two casein adhesives, and one epoxy. After eight months of exposure, fractures repaired with epoxy remained well bonded. Specimens repaired with PVB remained intact, but outer regions were visibly damaged, presumably from UV radiation. Of the two casein products that were tested, one had poor bond strength and the other was difficult to apply because of its low viscosity. Evaluating the effectiveness of organosilicate emulsions as consolidants to reduce weathering rates will require more research. Preliminary results of our field tests have identified goals for future efforts and established performance criteria for adhesives and consolidants used for silicified wood.