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Provenance of the upper Eocene Castle Rock Conglomerate, south Denver Basin, Colorado, U.S.A.
Abstract
The Castle Rock Conglomerate contains distinctive clasts from the Colorado Front Range, and when combined with detrital zircon ages, the unit can be subdivided into two lithofacies. Precambrian quartzites and stretched-pebble conglomerates from Coal Creek Canyon (to the northwest of the Castle Rock Conglomerate outcrop belt) and detrital zircons from Precambrian and Tertiary igneous rocks identify a northern provenance with detritus derived from tens of kilometers northwest of Denver, Colorado. A second source, composed of mainly granite from the Pikes Peak batholith, lies in the southern Front Range west of the Castle Rock Conglomerate outcrop belt. Both the north and west lithofacies can be mapped in the Castle Rock Conglomerate outcrop belt by using the presence (north) and absence (west) of Coal Creek Canyon quartzite clasts. This distinction is confirmed by detrital zircon ages. The north lithofacies dominates the present-day, northernmost outcrops, but dilution and interbedding with west lithofacies increase as the southeast-flowing basin axial paleodrainage meets piedmont tributaries that carried Pikes Peak batholith detritus from the west and southwest. The basin axial drainage transported coarse conglomerate southward about 120 km during Castle Rock Conglomerate deposition (36.7-34.0 Ma). The Precambrian quartzite exposed in Coal Creek Canyon is interpreted to be an important point source that can be useful in provenance studies of sediments shed from the Colorado Front Range. Additionally, detrital zircons from Laramide-age igneous rocks show potential for improved stratigraphic resolution in Paleogene strata of the Denver Basin.
... The D2 sequence is composed of up to 100 m of mostly conglomerate and sandstone. The Castle Rock Conglomerate (Tcr) of late Eocene age is a very coarse fluvial deposit, 83 m thick, that cuts into the underlying D2 and trends diagonally (NW-SE) across the south end of the ranch (Koch et al., 2018). The Castle Rock Conglomerate is older than the end of the Eocene (33.9 ma) because it con-tains bones of titanotheres (Thorson, 2011). ...
... No palynological work or age dating has been done on the ranch prior to one zircon laser ablation sample taken in 2015 (Koch et al., 2018). The nearest age control is in the Castle Pines core hole 27995F. ...
... As discussed in the next section, our recent zircon laser ablation data does not agree with the Castle Pines core or conventional regional wisdom predicting older Paleocene as an age for our petrified wood horizons. Koch et al. (2018) utilized laser ablation dating of zircons to separate lithofacies within the Castle Rock Conglomerate. As part of this work, a sample was taken on Cherokee Ranch in D1 sandstone adjacent to the petrified wood 2 m below the base of the Denver Basin paleosol. ...
Fossil woods are common in the Late Cretaceous through early Eocene rocks of the Denver Basin, Colorado. The overwhelming majority of these woods are dicotyledonous angiosperms. A new locality for fossil woods, Cherokee Ranch, in the upper D1 stratigraphic sequence (Denver Formation) is described, and evidence for it being late Paleocene is reviewed. Most Cherokee Ranch woods resemble previously described Denver Basin angiosperm woods, but there is one new type of wood attributed to the family Lauraceae. A new genus, Ubiquitoxylon, is proposed for woods with the combination of features commonly seen in the Cherokee Ranch woods. Denver Basin Paleocene woods differ from Paleocene wood assemblages to the north (Wyoming and Montana), where conifer woods are common and angiosperms are rare. The width and spacing of the water-conducting vessels and the lack of distinct growth rings in almost all of the Cherokee Ranch woods suggest that these trees did not experience water stress, and there was no pronounced seasonality.
... The detrital zircon U-Pb age of the Ogallala Formation, presumably once overlying the conglomerate, is bracketed at 18-5.5 Ma in Kansas (Smith et al., 2016). Detrital zircon U-Pb ages from Ogallala Formation in eastern Colorado are ~28 Ma or younger (Fish Canyon Tuff) (Koch et al., 2018). One sample of the Ogallala Formation, collected by the CGS from a location near Cedar Point, Colorado, yielded zircons with a youngest age population of 29.3 ± 1.1 Ma (mean squared weighted deviation [MSWD] = 1.05; n [number of values] = 3) and a youngest single grain of 23 ± 1 Ma (Morgan et al., 2023). ...
... For the oldest age of the conglomerate, a sample presented by Sharman et al. (2018) has a prominent age peak of 37 Ma and a youngest maximum depositional age of 34.7 ± 0.4, which the authors note is in accord with the age of the Wall Mountain Tuff. Seven detrital zircon ages from conglomerate clasts (Koch et al., 2018) fell into groups with ages of ~37 Ma, about 70-42 Ma, ~1,100 Ma, ~1,400 Ma, and ~1,700 Ma, and there were no ages younger than 36.7 Ma (age of the Wall Mountain Tuff). The CGS collected two samples from the conglomerate in Castlewood Canyon State Park (field numbers TCR-1 and TCR-2) and these were analyzed for detrital zircons. ...
Mesozoic eolian oolitic carbonates are rarely documented in the Western Interior of North America, despite the ample presence of exposed carbonate strata spanning the Paleozoic through the Quaternary. This study reinterprets a distinct 15-meter-thick deposit in the Middle Jurassic lower Sundance Formation, located in Wyoming’s Bighorn Basin, traditionally interpreted as high-energy subtidal deposits. These large-scale cross-stratified oolitic strata are interpreted to be eolian deposits, attributed to the deflation of emergent oolitic shoals following structural uplift and sea level fall during the late Callovian. Evidence of an eolian origin is supported by the presence climbing trans-latent stratification produced by migrating wind ripples, composed of alternating laminations of ooids and silt-sized quartz grains. Additional evidence consists of coarsening-upward sequences of fragmented and abraded ooid grains, and evidence of vadose diagenesis. The cross-stratified oolitic bodies’ relationship with surrounding lithofacies also supports the eolian hypothesis, suggesting these oolitic limestones were deposited as isolated bedforms on an emergent deflation surface during a regression of the Jurassic Sundance Sea. The exceptional preservation of these eolian carbonates, was facilitated by low-energy conditions during a subsequent transgression.
... UCM 109045 was compared to specimens of Megacerops coloradensis housed in the UCM Fossil Vertebrate Collection. Most are from the Chadron Formation of the White River Group in Nebraska, although a partial mandible from the late Chadronian-aged Castle Rock Conglomerate in Colorado (Koch et al., 2018) was also used. Methods for measuring the teeth of UCM 109045 and M. coloradensis specimens in the UCM Fossil Vertebrate Collection follow Mihlbachler (2008). ...
... P. curryi is a relatively large brontothere with short unforked horns that retains the same plesiomorphic anterior lower dental characteristics observed above in UCM 109045. The second species is Megacerops kuwagatarhinus, which occurs in (1) Chadronian deposits from Capitol Rock, Custer National Forest, southeast Montana (Mader and Alexander, 1995); (2) the Castle Rock Conglomerate, Denver Basin, eastern Colorado (Koch et al., 2018); and (3) the early Chadronian Hunter quarry of the Cypress Hills Formation in southwest Saskatchewan (Russel, 1940;Mader and Alexander, 1995). M. kuwagatarhinus is rare compared to M. coloradensis, and has been identified only from skulls in which the anterior dentition is not preserved, and they have no associated mandibles or lower dental material. ...
Late Eocene brontotheres are documented most prevalently from formations in the Great Plains of North America. Here we describe UCM 109045, a mandible and lower dentition of a brontothere recovered from a latest Eocene (Chadronian) locality in the Antero Formation in South Park, Colorado. This is a high-altitude locality in which vertebrate fossils are rare. Lower incisor number and presence of a long postcanine diastema indicate that UCM 109045 does not belong to Megacerops coloradensisLeidy, 1870, by far the most abundant brontothere from the Chadronian North American Land Mammal Age. Instead, UCM 109045 is morphologically most similar to Protitanops curryiStock, 1936, from the early Chadronian of the southwestern United States, and nomen dubium Megacerops primitivusLambe, 1908, from the Chadronian of Saskatchewan, Canada. It is possible that Megacerops kuwagatarhinusMader and Alexander, 1995, is a junior synonym of M. primitivus. If UCM 109045 belongs to Megacerops primitivus (= M. kuwgatarhinus), it would support the hypothesis that only two species of brontothere—M. primitivus (= M. kuwgatarhinus) and M. coloradensis—survived into the latest Eocene. Regardless of its exact identification, the discovery of UCM 109045 in the Antero Formation provides insight into a poorly understood, high-altitude locality in North America from just before brontothere extinction at the Eocene–Oligocene boundary.
... Besides the spring, the surrounding area has a variety of quality lithic raw materials. The Dakota Hogback in the Front Range has many sources of Dakota Formation orthoquartzite, as well as chert, chalcedony, and silicified wood; the Denver Basin east of the Colorado Front Range has silicified wood in the Dawson (Parker petrified wood) and Denver Formations, and quartzite in the Castle Rock Conglomerate; and fans and channels at the foot of the Colorado Front Range and Denver Basin contain Dawson Arkose Formation chert, rhyolite, silicified wood and quartzite (Koch et al. 2018;Mustoe and Viney 2017;Nelson et al. 2008). Plum Creek is the closest source of Dawson Arkose chert, silicified wood and quartzite, though much of this lithic material and that in the surrounding area are too small and of less than ideal quality to create larger artifacts. ...
... Plum Creek is the closest source of Dawson Arkose chert, silicified wood and quartzite, though much of this lithic material and that in the surrounding area are too small and of less than ideal quality to create larger artifacts. Silicified wood and orthoquartzite are exceptions, with substantial outcrops of highly knappable, large size quartzite in the nearby Castle Rock Conglomerate (Koch et al. 2018) and silicified wood in the nearby Cherokee Ranch Petrified Forest (Mustoe and Viney 2017). Short grass prairie and herbaceous plants dominated the landscape around Lamb Spring before it was converted to farmland and housing tracts (Dixon et al. 1997:8). ...
The Late Paleoindian Cody complex component at the Lamb Spring site in Douglas County, Colorado, was reanalyzed to better document it and facilitate comparisons to other Cody kill sites. Cody hunters killed at least 27 bison near an active spring vent between the late fall/early winter and the middle of spring. Besides the nearly 2,000 recovered bison bones are seven heavily resharpened Eden points and fragments, a Cody knife fragment, and two small flakes. The absence of end scrapers, retouched flakes and paucity of flakes suggest on-site carcass processing was a minor activity. The composition of the assemblage is similar to other northern Great Plains Cody complex bison kill sites, but aspects of the projectile point technology are somewhat atypical of other sites. This heretofore little known cultural component has been overshadowed by the now possibly disproven Clovis or pre-Clovis component, leaving the Cody component as the primary cultural manifestation at Lamb Spring.
... For example, the foundation upon which the castle is built is a sand, pebble, and cobble conglomerate that formed from a river system with more energy than the one that deposited the sand and pebble conglomerate used for the base of the walls (Fig. 5 Right). The direction and flow of these paleochannels can be understood by mapping the conglomerate deposits and by determining the origin of igneous rock formations that served as a source for the pebbles and cobblestones (Keller and Morgan, 2013;Koch, Coleman, and Sutter, 2018). ...
Cherokee Castle in Sedalia, Colorado is nestled in the foothills of the Rocky Mountains 30 km south of Denver. This location places the castle in a depressed area geologists refer to as the Denver Basin. The provenance of Cherokee Castle includes two private ownerships and a land trust. Carl A. Johnson and Alice Conyngham Gifford Johnson built the residence as a hunting lodge in the 1920s, naming it Charlford after their sons Charles and Gifford. The residence includes design elements inspired by mid-fifteenth century Scottish castles. In 1954 “Tweet” Kimball purchased Charlford and some adjoining property, renaming the residence Cherokee Ranch and Castle. Today the Cherokee Ranch & Castle Foundation (CRCF), established by Kimball, holds the deed to the property acting as a steward, preserving its cultural heritage and natural habitats, and serving the community through public programming. The material used to construct the castle walls was mined from locally sourced Paleogene-aged deposits that include rhyolite, sandstone conglomerates, and petrified wood. The roof is made of Paleozoic-aged Vermont slate. The very structure of Cherokee Castle can be viewed as a collection of ancient paleoenvironments preserved as rocks and fossil wood through geologic time.
... Another unusual feature of this conglomerate is the large size and lithologic variability of its abundant clasts. These exceptionally coarse-grained rocks also form mesas, buttes, and cliffs throughout the area south of Denver described in considerable detail by Kettleman (1956), Morse (1979Morse ( , 1985, Morgan (2016, 2017) and Koch et al. (2018) The conglomerate attracted the attention of early settlers and geologists visiting the area as far back as 1869 when Ferdinand Hayden and his team conducted their surveys of the area (Hayden, 1869). Geologic maps and lithologic analyses from these early studies as well as work by Lee (1902) and Richardson (1912) led to the naming of what is now known as the Castle Rock Conglomerate (CRC), named after the towering butte near the city of Castle Rock along Interstate 25 in central Douglas County (Figure 1). ...
Castlewood Canyon is one of the most distinctive landforms on the Colorado plains—a geomorphology that developed as Cherry Creek and its precursors incised into the Eocene Wall Mountain Tuff and overlying Castle Rock Conglomerate (CRC). Outcrops of the CRC in Castlewood Canyon State Park (CCSP) contain boulders of the Wall Mountain Tuff that are up to two meters in diameter, and the conglomerate itself is composed of large (up to 0.5 m), diverse clasts of Precambrian granite, gneiss, quartzite, and other lithologies eroded from the Colorado Front Range that is 25 km to the west and as much as 100 kilometers to the northwest. These clasts and other evidence suggest transport and deposition by a sequence of flood events. Such flooding events, albeit smaller in scale, continue to occur in modern times, including a catastrophic flood caused by the failure of the Castlewood Dam in 1933, and a canyon-scouring event in 2023. These events and the geologic history of this canyon are described in this paper, illustrating that nature, mild though it may be for millennia, is still shaping the Castlewood Canyon system.
... Wallace (1995) noted a hiatus in magmatism between 60 and 45 Ma in the Leadville area. This interpretation is supported by detrital zircon data from the lowest members of the upper Eocene Castle Rock conglomerates, located south of Denver, that reveals distinct populations with peaks at approximately 45 and 60 Ma (Koch et al., 2018). Thus, the time period between approximately 55 and 45 Ma may have experienced relatively little magmatism, as evident from the migration of weighed means to the northeast and overall low abundance of samples in those bins (Fig. 5B). ...
Magmatism in northern Colorado beginning in the late Eocene is associated with the formation of Pb-Zn-Ag carbonate-replacement and polymetallic vein deposits, the onset of caldera-forming magmatism, and eventually, the formation of rift-related, F-rich Mo porphyries (“Climax-type” intrusions). We use high-precision U/Pb zircon geochronology to better evaluate the temporal framework of magmatism and mineralization in the region. Our results demonstrate that mineralization in the Leadville area occurred between 43.5 and 39.7 Ma and was followed by mesothermal mineralization in the Montezuma area at approximately 38.7 Ma. Mineralization is associated with a suite of approximately 43 to 39 Ma intermediate magmatic centers that extended from Twin Lakes through Montezuma. The oldest porphyries associated with F-rich Mo prospects and deposits (Middle Mountain; 36.45 Ma) intruded 900 kyr after the start of the ignimbrite flare-up in the region. Spatiotemporal analyses reveal that the pattern of magmatism shifted in orientation between 40 and 35 Ma. We propose a model wherein magmatism before 39 Ma was the result of fluids evolved from the subducted Farallon slab being focused through weak zones in the lithospheric mantle and into the lower crust. This was followed by a more diffuse and higher power melting event that corresponds to a distinct change in the spatial patterns of magmatism. Our data suggest that low-grade Mo porphyry deposits can form close in time to calderas. We hypothesize that the transition from subduction to extensional tectonics in the region was responsible for this more widespread melting and a distinct shift in the style of magmatic-hydrothermal mineralization.
The Southern Rocky Mountains first rose during the Laramide Orogeny (ca. 75–45 Ma), but today’s mountains and adjacent Great Plains owe their current height to later epeirogenic surface uplift. When and why epeirogeny affected the region are controversial. Sedimentation histories in two central Colorado basins, the South Park–High Park and Denver basins, shifted at 56–54 Ma from an orogenic to an epeirogenic pattern, suggesting central Colorado experienced epeirogeny at that time. To interrogate that hypothesis, we analyzed thermal histories for seven samples from central Colorado’s Arkansas Hills and High Park using thermochronometers with closure temperatures below ~180 °C, enabling us to track sample exhumation from ~5–7 km depth.
Three samples are from the Cretaceous Whitehorn pluton, and four are Precambrian granitoids. All zircon and titanite (U-Th)/He dates (ZHe and THe) and one apatite fission-track (AFT) date are similar to the 67 Ma pluton emplacement age. Whitehorn dates using the lower-temperature apatite (U-Th)/He (AHe) thermochronometer are 55–41 Ma. These data require two exhumation episodes, one ca. 67–60 Ma, the second beginning at 54–46 Ma. The pluton reached the surface by 37 Ma, based on the age of volcanic tuff filling a pluton-cutting paleovalley. The Precambrian samples do not further refine this thermal history owing to the comparatively higher He closure temperature of their more radiation-damaged apatite.
Laramide crustal shortening caused 67–60 Ma exhumation. Arkansas Hills shortening ended before 67 Ma, so shortening could not have caused the exhumation event that began 54–46 Ma; thermochronology supports the Eocene epeirogeny hypothesis. Epeirogeny affected >2.0 × 104 km2, from the Sawatch Range to the Denver Basin. We attribute epeirogeny to an Eocene mantle drip that likely triggered subsequent drips, causing younger exhumation events in adjacent areas.
The history of erosion of southwestern North America and its relationship to surface uplift is a long-standing topic of debate. We use geologic and thermochronometric data to reconstruct the erosion history of southwestern North America. We infer that erosion events occurred mostly in response to surface uplift by contemporaneous tectonism, and were not long-delayed responses to surface uplift caused by later climate change or drainage reorganization. Rock uplift in response to isostatic compensation of exhumation occurred during each erosion event, but has been quantified only for parts of the late Miocene-Holocene erosion episode. We recognize four episodes of erosion and associated tectonic uplift: (1) the Laramide orogeny (ca. 75-45 Ma), during which individual uplifts were deeply eroded as a result of uplift by thrust faults, but Laramide basins and the Great Plains region remained near sea level, as shown by the lack of significant Laramide exhumation in these areas; (2) late middle Eocene erosion (ca. 42-37 Ma) in Wyoming, Montana, and Colorado, which probably occurred in response to epeirogenic uplift from lithospheric rebound that followed the cessation of Laramide dynamic subsidence; (3) late Oligocene-early Miocene deep erosion (ca. 27-15 Ma) in a broad region of the southern Cordillera (including the southern Colorado Plateau, southern Great Plains, trans-Pecos Texas, and northeastern Mexico), which was uplifted in response to increased mantle buoyancy associated with major concurrent volcanism in the Sierra Madre Occidental of Mexico and in the Southern Rocky Moun-tains; (4) Late Miocene-Holocene erosion (ca. 6-0 Ma) in a broad area of southwestern North America, with loci of deep erosion in the western Colorado-eastern Utah region and in the western Sierra Madre Occidental. Erosion in western Colorado-eastern Utah reflects mantle-related rock uplift as well as an important isostatic component caused by compensation of deep fluvial erosion in the upper Colorado River drainage following its integration to the Gulf of California. Erosion in the western Sierra Madre Occidental occurred in response to rift-shoulder uplift and the proximity of oceanic base level following the late Miocene opening of the Gulf of California. We cannot estimate the amount of rock or surface uplift associated with each erosion episode, but the maximum depths of exhumation for each were broadly similar (typically similar to 1-3 km). Only the most recent erosion episode is temporally correlated with climate change. Paleoaltimetric studies, except for those based on leaf physiognomy, are generally compatible with the uplift chronology we propose here. Physiognomy-based paleoelevation data suggest that near-modern elevations were attained during the Paleogene, but are the only data that uniquely support such interpretations. High Paleogene elevations require a complex late Paleogene-Neogene uplift and subsidence history for the Front Range and western Great Plains of Colorado that is not compatible with the regional sedimentation and erosion events we describe here. Our results suggest that near-modern surface elevations in southwestern North America were generally not attained until the Neogene, and that these high elevations are the cumulative result of four major episodes of Cenozoic rock uplift of diverse origin, geographic distribution, and timing.
The late Eocene Castle Rock Conglomerate adjoins the east side of the Colorado Front Range and lies within the Colorado Piedmont. It may reach 40 m or more in thickness, is nearly flat-lying, and is discontinuous, capping both high-relief mesas and gentle hills. The unit occurs in a south-southeast-trending swath 65 km in length and 4 km to 10 km in width, extending from north of the town of Castle Rock to southeast of the town of Elbert. The conglomerate has an arkosic coarse sand and granule matrix with abundant pebble to boulder-sized clasts that decrease in frequency upward. Large trough cross beds are common in the fairly well exposed upper portion of the unit, and in this interval a comprehensive survey of paleocurrent directions from trough cross beds yielded 2,897 measurements from flat ledge and pavement exposures. At each trough we measured the axis azimuth (paleocurrent direction) and several other features. Length-azimuth rose diagrams and large-scale digital topographic maps allowed the paleocurrent data to be areally and stratigraphically discretized into 411 local paleocurrent directions. The result is a paleocurrent map of the south-southeast-flowing main paleochannel, and the recognition of two hitherto not described, east to northeast-flowing tributary paleochannnels that originated west and southwest of the main paleochannel. Both are approximately 12 km in length and 1 km to 2 km in width, and both display inverted topography along their upper reaches. In the area of Castlewood Canyon State Park the northern tributary widened northeastward to become an alluvial fan, which now is partially overlain by deposits of the main paleochannel. The southern tributary joined the main paleochannel in an area east of Running Creek and west of Elbert. Twenty-four clast surveys (10,804 clasts) of the Castle Rock Conglomerate were performed and each survey corresponded to a single trough. We recorded lithology, maximum dimension, and roundness of all clasts > 2 cm. Consolidated histograms of the 15 clast surveys in the main paleochannel versus the nine in the tributaries indicate some marked differences between the two populations. Histograms of local pairs and groups of main paleochannel surveys versus tributary surveys indicate significant differences, especially in clast lithology and size distribution.
Recent inference that Mesozoic Cordilleran plutons grew incrementally during >106 yr intervals, without the presence of voluminous eruptible magma at any stage, minimizes close associations with large ignimbrite calderas. Alternatively, Tertiary ignimbrites in the Rocky Mountains and elsewhere, with volumes of 1-5 × 103 km3, record multistage histories of magma accumulation, fractionation, and solidifi cation in upper parts of large subvolcanic plutons that were suffi -ciently liquid to erupt. Individual calderas, up to 75 km across with 2-5 km subsidence, are direct evidence for shallow magma bodies comparable to the largest granitic plutons. As exemplifi ed by the composite Southern Rocky Mountain volcanic fi eld (here summarized comprehensively for the fi rst time), which is comparable in areal extent, magma composition, eruptive volume, and duration to continental-margin volcanism of the central Andes, nested calderas that erupted compositionally diverse tuffs document deep composite subsidence and rapid evolution in subvolcanic magma bodies. Spacing of Tertiary calderas at distances of tens to hundreds of kilometers is comparable to Mesozoic Cordilleran pluton spacing. Downwind ash in eastern Cordilleran sediments records large-scale explosive volcanism concurrent with Mesozoic batholith growth. Mineral fabrics and gradients indicate unifi ed fl owage of many pluton interiors before complete solidifi cation, and some plutons contain ring dikes or other textural evidence for roof subsidence. Geophysical data show that low-density upper-crustal rocks, inferred to be plutons, are 10 km or more thick beneath many calderas. Most ignimbrites are more evolved than associated plutons; evidence that the subcaldera chambers retained voluminous residua from fractionation. Initial incremental pluton growth in the upper crust was likely recorded by modest eruptions from central volcanoes; preparation for calderascale ignimbrite eruption involved recurrent magma input and homogenization high in the chamber. Some eroded calderas expose shallow granites of similar age and composition to tuffs, recording sustained postcaldera magmatism. Plutons thus provide an integrated record of prolonged magmatic evolution, while volcanism offers snapshots of conditions at early stages. Growth of subvolcanic batholiths involved sustained multistage opensystem processes. These commonly involved ignimbrite eruptions at times of peak power input, but assembly and consolidation processes continued at diminishing rates long after peak volcanism. Some evidence cited for early incremental pluton assembly more likely records late events during or after volcanism. Contrasts between relatively primitive arc systems dominated by andesitic compositions and small upper-crustal plutons versus more silicic volcanic fi elds and associated batholiths probably refl ect intertwined contrasts in crustal thickness and magmatic power input. Lower power input would lead to a Cascade- or Aleutian-type arc system, where intermediate-composition magma erupts directly from middle- and lowercrustal storage without development of large shallow plutons. Andean and southern Rocky Mountain-type systems begin similarly with intermediate-composition volcanism, but increasing magma production, perhaps triggered by abrupt changes in plate boundaries, leads to development of larger upper-crustal reservoirs, more silicic compositions, large ignimbrites, and batholiths. Lack of geophysical evidence for voluminous eruptible magma beneath young calderas suggests that near-solidus plutons can be rejuvenated rapidly by high-temperature mafi c recharge, potentially causing large explosive eruptions with only brief precursors.
Elemental fractionation effects during analysis are the most significant impediment to obtaining precise and accurate U-Pb ages by laser ablation ICPMS. Several methods have been proposed to minimize the degree of downhole fractionation, typically by rastering or limiting acquisition to relatively short intervals of time, but these compromise minimum target size or the temporal resolution of data. Alternatively, other methods have been developed which attempt to correct for the effects of downhole elemental fractionation. A common feature of all these techniques, however, is that they impose an expected model of elemental fractionation behavior; thus, any variance in actual fractionation response between laboratories, mineral types, or matrix types cannot be easily accommodated. Here we investigate an alternate approach that aims to reverse the problem by first observing the elemental fractionation response and then applying an appropriate (and often unique) model to the data. This approach has the ver
The Castle Rock Conglomerate (CRC) is a late Eocene fluvial deposit flanking the east side of the Colorado Front Range and lying within the Colorado Piedmont. It may reach about 70 m (230 ft) in thickness, is nearly flatlying, and is discontinuous, capping both high-relief mesas and gentle hills. The unit occurs in a southsoutheast- to-southeast-trending swath 63 km (39 mi) in length and about 3 to 10 km (2 to 6 mi) in width, extending from north of the city of Castle Rock to southeast of the town of Elbert. The conglomerate has an arkosic, coarse sand and granule matrix with abundant pebble to boulder-sized clasts that decrease in abundance upward. Large trough cross-beds are common in the exposed upper portion of the unit. In this interval, a comprehensive survey of paleocurrent directions from trough cross-beds yielded 2,897 measurements from outcrop exposures. At each trough, the axis azimuth (paleocurrent direction) and several other parameters were measured. Lengthazimuth rose diagrams and large-scale digital topographic maps allowed the paleocurrent data to be a really and stratigraphically grouped into 411 consolidated local paleocurrent directions. The result is a new and detailed paleocurrent map of the upper portion of the unit. The map shows the paleocurrent pattern within the main paleochannel belt, which previously has been recognized as occupying a south-southeast to southeast-trending paleovalley; and two, newly documented, northeast to east-flowing tributary paleochannnels that originated southwest of the main paleochannel belt. In the area of Castlewood Canyon State Park, the JA Ranch paleochannel widened northeastward to become an alluvial fan, now partially obscured by deposits of the main paleochannel belt. The Bucks Mountain trend joined the main paleochannel belt in an area east of Running Creek and west of Elbert. A possible third tributary system may have existed north of Castle Rock, and is suggested by a small number of measurements in three isolated areas: Castle Rock Butte, Cherokee Mountain, and the mesa north of Newlin Gulch. Clast surveys at 24 locations recorded the lithology, maximum dimension, and roundness of all clasts >2 cm (0.8 in) in maximum dimension (nearly 11,000 clasts). In order of decreasing lithologic abundance, the gravel fraction of the conglomerate consists of granitics, Wall Mountain Tuff, quartz, blue-gray quartzite, other quartzites, and probable Lower Paleozoic sedimentary rocks (including possibly the Fountain Formation). Consolidated histograms of the 15 clast surveys in the main paleochannel belt versus nine surveys in the tributaries indicate some marked differences between the two populations: notably a larger proportion of granitics and a lower proportion of tuff exist in the main paleochannel belt than in the tributaries. Histograms of local sets of main paleochannel belt clast surveys versus tributary surveys indicate other notable differences, especially in clast lithology and size distribution. Blue-gray quartzite, hypothesized by previous workers to originate from Coal Creek Canyon south of Boulder, is common in the main paleochannel belt. Well-rounded volcanic clasts of probable dacitic composition were collected from the base of the main paleochannel belt in Castlewood Canyon State Park. Sensitive High Resolution Ion Microprobe (SHRIMP)-RG U-Pb zircon age dates of these clasts range from 46 to 55 million years ago (Ma). Potential source areas for these volcanic clasts lie along a northeast trend between Leadville and Boulder. All of the clast lithologies found in the conglomerate are found along the Front Range. Absent from the clast surveys is the suite of Mesozoic sedimentary rocks now exposed along the mountain front, suggesting that today’s prominent hogbacks of these rocks were not exposed, or not within the CRC source areas during the late Eocene. The present study indicates that large quantities of granitic and volcanic material existed along the Front Range in the late Eocene. This material likely buried the Mesozoic section at the range front and left part of the Paleozoic section (mainly the Fountain Formation) exposed. Exhumation of the Mesozoic rocks occurred sometime after deposition of the CRC.
The Castle Rock Conglomerate (Tcr) is a late Eocene fluvial deposit flanking the east side of the Colorado Front Range and lying within the Colorado Piedmont. It occurs as a northwest-southeast–trending swath ~63 km in length and between 3 and 10 km in width, and is ~70 m in thickness. The conglomerate consists of a matrix of arkosic coarse sand and granules along with pebble- to boulder-sized clasts that vary in abundance. Locally, the upper portion of the Tcr is well exposed in cliffs and ledges and also in flat outcrops along drainages. Large to very large-scale cross-bedding, of both the planar and trough types, is characteristic of the unit. Clasts are dominantly Front Range granitic rocks and Wall Mountain Tuff, and boulders of the latter can exceed 0.5 m in diameter. Minor quantities of quartzite and vein quartz, and rare sedimentary clasts, also are present. The large sizes of the bedforms and clasts indicate deep water and high velocity during deposition. A recent, extensive paleocurrent study of the upper Tcr has produced a new map of the fluvial system, indicating a main, southeast-trending paleochannel with two major northeast-trending tributaries. This field trip will present an overview of the Tcr lithology, bedforms, and associated geologic units; ideas about its deposition; and its bearing on uplift along the Front Range and in the southwest Denver Basin. Planned stops will be in Douglas and Elbert Counties, south of Denver, and include Castlewood Canyon State Park, Prairie Canyon Ranch open space, private farm and ranch properties, and an inactive quarry. Several of the stops provide excellent vistas of the Front Range, majestic Pikes Peak, and also of the Colorado Piedmont.
Laser-ablation split-stream (LASS) analysis—high-speed, high spatial-resolution, simultaneous isotopic and elemental analysis—enables petrochronology at a new level, through the interpretation of isotopic dates combined with elemental abundances and/or isotopic tracers. This contribution begins with an introduction to petrochronology, presents a new LASS technique using dual multi-collector–single-collector inductively-coupled plasma mass spectrometry, and offers examples of how this technique is used to decipher the evolution of rocks with complex geologic histories.
Since the development of SIMS and LA-ICP-MS technologies in the 1980s and 1990s, single grain U–Pb dating of detrital zircon has quickly become the most popular technique for sedimentary provenance studies. Currently by far the most widespread method for visualising detrital age distributions is the so-called Probability Density Plot (PDP), which is calculated by summing a number of Gaussian distributions whose means and standard deviations correspond to the individual ages and their respective analytical uncertainties. Unfortunately, the PDP lacks a firm theoretical basis and can produce counter-intuitive results when data quantity (number of analyses) and/or quality (precision) is high. As a more robust alternative to the PDP, this paper proposes a standard statistical technique called Kernel Density Estimation (KDE), which also involves summing a set of Gaussian distributions, but does not explicitly take into account the analytical uncertainties. The Java-based DensityPlotter program (http://densityplotter.london-geochron.com) was developed with the aim to facilitate the adoption of KDE plots in the context of detrital geochronology.
A large data set of single and multi-grain zircon and titanite analyses from a sample of the Oligocene Fish Canyon Tuff (FCT), a voluminous ash flow from the San Juan Mountains of Colorado and widely used 40Ar/39Ar geochronological standard, has been used to evaluate the influence of various sources of analytical and geological uncertainty on the calculated age of this tuff by the isotope dilution U-Pb zircon method. Twenty-three single zircon grains and seven small multi-grain fractions of the FCT yield an inverse-variance weighted mean 206Pb/238U date of 28.402 ± 0.023 Ma (2σ; MSWD 0.93) and a slightly older weighted mean 207Pb/235U date of 28.529 ± 0.030 Ma (MSWD 0.74), which are insensitive to common Pb corrections. Initial 230Th disequilibria calculated from a newly measured Th/U = 2.2 for FCT pumice shards indicate Th-deficiency for these zircons; its correction brings the weighted mean 206Pb/238U date to 28.478 ± 0.024 (MSWD 0.97), minimizing the discordance of the FCT zircon analyses. These calculations culminate in Concordia ages (Ludwig, 1998) for the crystallization of the FCT zircons of 28. 476 ± 0.029 Ma (MSWD 1.50) without propagating decay constant errors, or 28.498 ± 0.035 Ma (MSWD 1.03) with propagated decay constant errors.
Matrix-matched calibration by natural zircon standards and analysis of natural materials as a reference are the principle methods for achieving accurate results in microbeam U–Pb dating and Hf isotopic analysis. We describe a new potential zircon reference material for laser ablation ICP-MS that was extracted from a potassic granulite facies rock collected in the southern part of the Bohemian Massif (Plešovice, Czech Republic).Data from different techniques (ID-TIMS, SIMS and LA ICP-MS) and several laboratories suggest that this zircon has a concordant U–Pb age with a weighted mean 206Pb/238U date of 337.13 ± 0.37 Ma (ID-TIMS, 95% confidence limits, including tracer calibration uncertainty) and U–Pb age homogeneity on the scale used in LA ICP-MS dating. Inhomogeneities in trace element composition due to primary growth zoning prevent its use as a calibration standard for trace element analysis. The content of U varies from 465 ppm in pristine parts of the grains to ~ 3000 ppm in actinide-rich sectors that correspond to pyramidal faces with a high degree of metamictization (present in ca. 30% of the grains). These domains are easily recognized from high intensities on BSE images and should be avoided during the analysis. Hf isotopic composition of the Plešovice zircon (> 0.9 wt.% Hf) is homogenous within and between the grains with a mean 176Hf/177Hf value of 0.282482 ± 0.000013 (2SD). The age and Hf isotopic homogeneity of the Plešovice zircon together with its relatively high U and Pb contents make it an ideal calibration and reference material for laser ablation ICP-MS measurements, especially when using low laser energies and/or small diameters of laser beam required for improved spatial resolution.
New field studies combined with U-Pb zircon geochronology constrain the ages of deposition and sedimentary provenance of Paleoproterozoic quartzite successions exposed in the southwestern United States. Orthoquartzites were deposited in short-lived basins at two times (ca. 1.70 and 1.65 Ga) during crustal assembly of southern Laurentia. The more voluminous ca. 1.70 Ga successions occur in southern Colorado, northern New Mexico, and central Arizona and are interpreted here to-be time correlative, though not necessarily deposited in the same basins. Detrital zircon from quartzites and metaconglomerates exposed in southern Colorado and northern New Mexico is characterized by a single population with a relatively narrow range of ages (1.80-1.70 Ga) and minimal Archean input (< 5% of grains analyzed). Peak detrital zircon ages (1.76-1.70 Ga) vary slightly from location to location and mimic the age of underlying basement. Unimodal detrital populations suggest local sources and a first-cycle origin of the orthoquartzites within a short time interval (1.70-1.68 Ga) during unroofing of local underlying basement. The maximum age of quartzite exposed at Blue Ridge, Colorado, is constrained by the 1705-1698 Ma coarse-grained granitoid basement on which quartzite was deposited unconformably. The minimum age of Ortega Formation quartzite (New Mexico) is constrained by ca. 1680-1670 Ma metamorphic monazite overgrowths. These dates agree with direct ages on the lower Mazatzal Group, Arizona, and suggest that orthoquartzite deposition occurred over a wide region during and soon after the ca. 1.70 Ga Yavapai orogeny. Regional structural arguments and the thrust style of quartzite deformation suggest that the metasedimentary successions were deformed during the ca. 1.66-1.60 Ga Mazatzal orogeny, thus making them important, time markers separating the Yavapai and Mazatzal orogenic events. Our model for syntectonic deposition involves extensional basin development followed by thrust closure, possibly due to opening and closing of slab rollback basins related to outboard suhduction. The first-cycle origin of orthoquartzites near the end of the arc collisions of the Yavapai orogeny seems to contrast sharply with their extreme compositional maturity. This can be explained in terms of protracted, extreme diagenesis and/or special environmental influences that enhanced chemical weathering but were unique to the transitional atmosphere and ocean chemistry of the Proterozoic. Similarities among quartzites exposed throughout the southwestern United States and along the Laurentian margin suggest that they represent a widespread regional, and perhaps global, episode of sedimentation involving a distinctive syntectonic setting and unique climatic conditions, a combination that might make these units a signature lithology for Paleoproterozoic time.
This paper reports the results from a second characterisation of the 91500 zircon, including data from electron probe microanalysis, laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS), secondary ion mass spectrometry (SIMS) and laser fluorination analyses. The focus of this initiative was to establish the suitability of this large single zircon crystal for calibrating in situ analyses of the rare earth elements and oxygen isotopes, as well as to provide working values for key geochemical systems. In addition to extensive testing of the chemical and structural homogeneity of this sample, the occurrence of banding in 91500 in both backscattered electron and cathodoluminescence images is described in detail. Blind intercomparison data reported by both LA-ICP-MS and SIMS laboratories indicate that only small systematic differences exist between the data sets provided by these two techniques. Furthermore, the use of NIST SRM 610 glass as the calibrant for SIMS analyses was found to introduce little or no systematic error into the results for zircon. Based on both laser fluorination and SIMS data, zircon 91500 seems to be very well suited for calibrating in situ oxygen isotopic analyses.
Thesis--Johns Hopkins University. Vita. Includes bibliographical references (leaves 331-343). Microfilm of typescript.
Geologic map of the Estes Park 30ʹ × 60ʹ quadrangle, north-central Colorado
- Cole
Geology underfoot along Colorado’s Front Range
- Abbott
Notes on the Denver Basin geologic maps: Bedrock geology, structure, and isopach maps of the Upper Cretaceous to Paleogene strata between Greeley and Colorado Springs, Colorado
- Dechesne
The trail walker’s guide to the Dinosaur Ridge, Red Rocks, and Green Mountain area
- Drewes
The Castle Rock Conglomerate
- Evanoff
The Castle Rock Conglomerate and associated placer gold deposits
- V G Gabriel
Latest Cretaceous and Paleogene magmatism in the Front Range
- E E Larson
- Incoates Colorado
- M-M Evanoff
Cenozoic paleogeography of the west-central United States, Rocky Mountain paleogeography, symposium 3: Cenozoic paleogeography of the west-central United States
- D G Morse
- R M Inflores
- S S Kaplan
Geology of the Eldorado Springs quadrangle, Boulder and Jefferson counties, Colorado
- Wells
Geology of Upper Cretaceous, Paleocene, and Eocene strata in the southwestern Denver
- J P Thorson
Geologic atlas of the United States, Castle Rock folio, Colorado
- Richardson
Map showing late Eocene erosion surface, Oligocene–Miocene paleovalleys, and Tertiary deposits in the Pueblo, Denver, and Greeley 1° × 2° quadrangles, Colorado
- Scott