Pub Date : 2018-11-01DOI: 10.24872/RMGJOURNAL.53.2.113
Jeffrey G. Eaton, W. Korth, D. Brinkman, D. Brinkman
{"title":"Vertebrate fossils from the Claron Formation, Sweetwater Creek area, Garfield County, Utah, U.S.A.","authors":"Jeffrey G. Eaton, W. Korth, D. Brinkman, D. Brinkman","doi":"10.24872/RMGJOURNAL.53.2.113","DOIUrl":"https://doi.org/10.24872/RMGJOURNAL.53.2.113","url":null,"abstract":"","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2018-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.24872/RMGJOURNAL.53.2.113","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42455699","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2018-11-01DOI: 10.24872/RMGJOURNAL.53.2.75
Emily K. Halverson, J. Eberle
{"title":"A new middle Paleocene (early Tiffanian) mammalian fauna from the Overland Member of the Fort Union Formation, Great Divide Basin, Wyoming, U.S.A.","authors":"Emily K. Halverson, J. Eberle","doi":"10.24872/RMGJOURNAL.53.2.75","DOIUrl":"https://doi.org/10.24872/RMGJOURNAL.53.2.75","url":null,"abstract":"","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2018-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.24872/RMGJOURNAL.53.2.75","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49556061","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2018-06-01DOI: 10.24872/rmgjournal.53.1.29
Allan J. Koch, D. Coleman, Amanda M. Sutter
&NA; 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.
{"title":"Provenance of the upper Eocene Castle Rock Conglomerate, south Denver Basin, Colorado, U.S.A.","authors":"Allan J. Koch, D. Coleman, Amanda M. Sutter","doi":"10.24872/rmgjournal.53.1.29","DOIUrl":"https://doi.org/10.24872/rmgjournal.53.1.29","url":null,"abstract":"&NA; 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.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"53 1","pages":"29–43"},"PeriodicalIF":0.0,"publicationDate":"2018-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46809308","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2018-06-01DOI: 10.24872/rmgjournal.53.1.1
Alexie E. G. Millikin, L. Morgan, J. Noblett
&NA; The Upper Cretaceous and Lower Paleogene Table Mountain Shoshonite lava flows and their proposed source, the Ralston Buttes intrusions, provide insight into the volcanic history of the Colorado Front Range. This study affirms the long‐held hypothesis linking the extrusive Table Mountain lava flows and their intrusive equivalents at Ralston Buttes through major‐ and trace‐ element geochemistry. Systematic 40Ar/39Ar geochronology from all flows and intrusive units refines the eruptive history, improves precision on previously reported ages, and provides tighter constraints on the position of the K‐Pg boundary in this location. Four flows are recognized on North and South Table mountains outside of Golden, Colorado. Flow 1 (66.5 ± 0.3 Ma, all ages reported with 2&sgr; uncertainty) is the oldest, most compositionally distinct flow and is separated from younger flows by approximately 35 m of sedimentary deposits of the Denver Formation. Stratigraphically adjacent flows 2 (65.8 ± 0.2 Ma), 3 (65.5 ± 0.3 Ma), and 4 (65.9 ± 0.3 Ma) are compositionally indistinguishable. Lavas (referred to here as unit 5) that form three cone‐shaped structures (shown by this study to be volcanic vents of a new unit 5) on top of North Table Mountain are compositionally similar to other units, but yield an age almost 20 m.y. younger (46.94 ± 0.15 Ma). Geochemistry and geochronology suggest that the rim phase of the Ralston plug (65.4 ± 0.2 Ma) is a reasonable source for flows 2, 3, and 4. All units are shoshonites—potassic basalts containing plagioclase, augite, olivine, and magnetite phenocrysts—and plot in the continental‐arc field in tectonic discrimination diagrams. A continental‐arc setting coupled with Late Cretaceous to early Paleogene ages suggest the high‐K magmatism is associated with Laramide tectonism.
{"title":"40Ar/39Ar geochronology and petrogenesis of the Table Mountain Shoshonite, Golden, Colorado, U.S.A.","authors":"Alexie E. G. Millikin, L. Morgan, J. Noblett","doi":"10.24872/rmgjournal.53.1.1","DOIUrl":"https://doi.org/10.24872/rmgjournal.53.1.1","url":null,"abstract":"&NA; The Upper Cretaceous and Lower Paleogene Table Mountain Shoshonite lava flows and their proposed source, the Ralston Buttes intrusions, provide insight into the volcanic history of the Colorado Front Range. This study affirms the long‐held hypothesis linking the extrusive Table Mountain lava flows and their intrusive equivalents at Ralston Buttes through major‐ and trace‐ element geochemistry. Systematic 40Ar/39Ar geochronology from all flows and intrusive units refines the eruptive history, improves precision on previously reported ages, and provides tighter constraints on the position of the K‐Pg boundary in this location. Four flows are recognized on North and South Table mountains outside of Golden, Colorado. Flow 1 (66.5 ± 0.3 Ma, all ages reported with 2&sgr; uncertainty) is the oldest, most compositionally distinct flow and is separated from younger flows by approximately 35 m of sedimentary deposits of the Denver Formation. Stratigraphically adjacent flows 2 (65.8 ± 0.2 Ma), 3 (65.5 ± 0.3 Ma), and 4 (65.9 ± 0.3 Ma) are compositionally indistinguishable. Lavas (referred to here as unit 5) that form three cone‐shaped structures (shown by this study to be volcanic vents of a new unit 5) on top of North Table Mountain are compositionally similar to other units, but yield an age almost 20 m.y. younger (46.94 ± 0.15 Ma). Geochemistry and geochronology suggest that the rim phase of the Ralston plug (65.4 ± 0.2 Ma) is a reasonable source for flows 2, 3, and 4. All units are shoshonites—potassic basalts containing plagioclase, augite, olivine, and magnetite phenocrysts—and plot in the continental‐arc field in tectonic discrimination diagrams. A continental‐arc setting coupled with Late Cretaceous to early Paleogene ages suggest the high‐K magmatism is associated with Laramide tectonism.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"53 1","pages":"1–28"},"PeriodicalIF":0.0,"publicationDate":"2018-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44936618","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2018-06-01DOI: 10.24872/rmgjournal.53.1.45
C. Newman
&NA; Rocky Mountain National Park (RMNP) in north‐central Colorado exhibits a number of spectacular geologic features that exemplify geologic timescales and the breadth and power of geologic processes. Combined with the extensive system of trails, these features allow excellent access to effective Earth‐science educational opportunities. In July 2015, to help celebrate the centennial of RMNP, the author guided a geology‐based educational hike for the public on two occasions. The hiking route is approximately 15 km (˜9.3 mi) round‐trip and traverses through primarily Proterozoic metamorphic and igneous rocks that comprise the majority of bedrock in RMNP. In addition to crystalline bedrock, the route also incorporated observation of numerous glacial features and discussion of area hydrology and anthropogenic impacts. A post‐instructional survey indicates that the route and features observed helped to increase public awareness of geologic processes and sense of connection with the landscape. The route, although considered strenuous by the National Park Service, could be utilized for public outreach and geologic education for a wide variety of user groups.
{"title":"Geoscience education and public outreach in Rocky Mountain National Park, Colorado, U.S.A.","authors":"C. Newman","doi":"10.24872/rmgjournal.53.1.45","DOIUrl":"https://doi.org/10.24872/rmgjournal.53.1.45","url":null,"abstract":"&NA; Rocky Mountain National Park (RMNP) in north‐central Colorado exhibits a number of spectacular geologic features that exemplify geologic timescales and the breadth and power of geologic processes. Combined with the extensive system of trails, these features allow excellent access to effective Earth‐science educational opportunities. In July 2015, to help celebrate the centennial of RMNP, the author guided a geology‐based educational hike for the public on two occasions. The hiking route is approximately 15 km (˜9.3 mi) round‐trip and traverses through primarily Proterozoic metamorphic and igneous rocks that comprise the majority of bedrock in RMNP. In addition to crystalline bedrock, the route also incorporated observation of numerous glacial features and discussion of area hydrology and anthropogenic impacts. A post‐instructional survey indicates that the route and features observed helped to increase public awareness of geologic processes and sense of connection with the landscape. The route, although considered strenuous by the National Park Service, could be utilized for public outreach and geologic education for a wide variety of user groups.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"53 1","pages":"45–55"},"PeriodicalIF":0.0,"publicationDate":"2018-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.24872/rmgjournal.53.1.45","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41869260","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2017-12-01DOI: 10.24872/RMGJOURNAL.52.2.107
Joshua K. Davis, M. Hudson, V. Grauch
{"title":"A paleomagnetic age estimate for the draining of ancient Lake Alamosa, San Luis Valley, south-central Colorado, U.S.A.","authors":"Joshua K. Davis, M. Hudson, V. Grauch","doi":"10.24872/RMGJOURNAL.52.2.107","DOIUrl":"https://doi.org/10.24872/RMGJOURNAL.52.2.107","url":null,"abstract":"","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"52 1","pages":"107-117"},"PeriodicalIF":0.0,"publicationDate":"2017-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.24872/RMGJOURNAL.52.2.107","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49357364","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2017-06-01DOI: 10.24872/RMGJOURNAL.52.1.17
R. Moscati, W. Premo, E. Dewitt, J. Wooden
A broad study of zircons from plutonic rocks of the Sawatch and Mosquito ranges of west-central Colorado (U.S.A.) was undertaken to significantly refine the magmatic chronology and chemistry of this under-studied region of the Colorado province. This region was chosen because it lies just to the north of the suspected arc-related Gunnison-Salida volcano-plutonic terrane, which has been the subject of many recent investigations—and whose origin is still debated. Our new results provide important insights into the processes active during Proterozoic crustal evolution in this region, and they have important ramifications for broader-scope crustal evolution models for southwestern North America. Twenty-four new U-Pb ages and sequentially acquired rare-earth element (REE), U, Th, and Hf contents of zircon have been determined using the sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG). These zircon geochemistry data, in conjunction with whole-rock major- and trace-element data, provide important insights into zircon crystallization and melt fractionation, and they help to further constrain the tectonic environment of magma generation. Our detailed zircon and whole-rock data support the following three interpretations: (1) The Roosevelt Granite in the southern Sawatch Range was the oldest rock dated at 1,766 ± 7 Ma, and it intruded various metavolcanic and metasedimentary rocks. Geochemistry of both whole-rock and zircon supports the contention that this granite was produced in a magmatic arc environment and, therefore, is likely an extension of the older Dubois Greenstone Belt of the Gunnison Igneous Complex (GIC) and the Needle Mountains (1,770–1,755 Ma). Rocks of the younger Cochetopa succession of the GIC, the Salida Greenstone Belt, and the Sangre de Cristo Mountains (1,740–1,725 Ma) were not found in the Sawatch and Mosquito ranges. This observation strongly suggests that the northern edge of the Gunnison-Salida arc terrane underlies the southern portion of the Sawatch and Mosquito ranges. (2) Calc-alkalic to alkali-calcic magmas intruded this region approximately 55 m.y. after the Roosevelt Granite with emplacement of pre-deformational plutons at ca. 1,710 Ma (e.g., Henry Mountain Granite and diorite of Denny Creek), and this continued for at least 30 m.y., ending with emplacement of post-deformational plutons at ca. 1,680 Ma (e.g., Kroenke Granodiorite, granite of Fairview Peak, and syenite of Mount Yale). The timing of deformation can be constrained to sometime after intrusion of the diorite of Denny Creek and likely before the emplacement of the undeformed granite of Fairview Peak. Geochemistry of both whole-rock and zircon indicates that the older group of ca. 1,710-Ma plutons formed at shallower depths, and then they intruded the younger group of more deeply generated, commonly peraluminous and sodic plutons. Although absent in the Sawatch and Mosquito ranges, Mazatzal-age (ca. 1,680–1,620 Ma) plutonic rocks are present
{"title":"U-Pb ages and geochemistry of zircon from Proterozoic plutons of the Sawatch and Mosquito ranges, Colorado, U.S.A.: Implications for crustal growth of the central Colorado province","authors":"R. Moscati, W. Premo, E. Dewitt, J. Wooden","doi":"10.24872/RMGJOURNAL.52.1.17","DOIUrl":"https://doi.org/10.24872/RMGJOURNAL.52.1.17","url":null,"abstract":"A broad study of zircons from plutonic rocks of the Sawatch and Mosquito ranges of west-central Colorado (U.S.A.) was undertaken to significantly refine the magmatic chronology and chemistry of this under-studied region of the Colorado province. This region was chosen because it lies just to the north of the suspected arc-related Gunnison-Salida volcano-plutonic terrane, which has been the subject of many recent investigations—and whose origin is still debated. Our new results provide important insights into the processes active during Proterozoic crustal evolution in this region, and they have important ramifications for broader-scope crustal evolution models for southwestern North America. Twenty-four new U-Pb ages and sequentially acquired rare-earth element (REE), U, Th, and Hf contents of zircon have been determined using the sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG). These zircon geochemistry data, in conjunction with whole-rock major- and trace-element data, provide important insights into zircon crystallization and melt fractionation, and they help to further constrain the tectonic environment of magma generation. Our detailed zircon and whole-rock data support the following three interpretations: (1) The Roosevelt Granite in the southern Sawatch Range was the oldest rock dated at 1,766 ± 7 Ma, and it intruded various metavolcanic and metasedimentary rocks. Geochemistry of both whole-rock and zircon supports the contention that this granite was produced in a magmatic arc environment and, therefore, is likely an extension of the older Dubois Greenstone Belt of the Gunnison Igneous Complex (GIC) and the Needle Mountains (1,770–1,755 Ma). Rocks of the younger Cochetopa succession of the GIC, the Salida Greenstone Belt, and the Sangre de Cristo Mountains (1,740–1,725 Ma) were not found in the Sawatch and Mosquito ranges. This observation strongly suggests that the northern edge of the Gunnison-Salida arc terrane underlies the southern portion of the Sawatch and Mosquito ranges. (2) Calc-alkalic to alkali-calcic magmas intruded this region approximately 55 m.y. after the Roosevelt Granite with emplacement of pre-deformational plutons at ca. 1,710 Ma (e.g., Henry Mountain Granite and diorite of Denny Creek), and this continued for at least 30 m.y., ending with emplacement of post-deformational plutons at ca. 1,680 Ma (e.g., Kroenke Granodiorite, granite of Fairview Peak, and syenite of Mount Yale). The timing of deformation can be constrained to sometime after intrusion of the diorite of Denny Creek and likely before the emplacement of the undeformed granite of Fairview Peak. Geochemistry of both whole-rock and zircon indicates that the older group of ca. 1,710-Ma plutons formed at shallower depths, and then they intruded the younger group of more deeply generated, commonly peraluminous and sodic plutons. Although absent in the Sawatch and Mosquito ranges, Mazatzal-age (ca. 1,680–1,620 Ma) plutonic rocks are present ","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"52 1","pages":"17-106"},"PeriodicalIF":0.0,"publicationDate":"2017-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.24872/RMGJOURNAL.52.1.17","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42431606","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2017-03-20DOI: 10.24872/RMGJOURNAL.52.1.1
K. Berry
There is considerable debate regarding the paleogeography of the Western Interior Seaway (WIS) near the close of the Cretaceous. To investigate this issue, the paleogeographic implications of recent advancements in the biostratigraphy of the lower to upper Maastrichtian, transitional-marine strata in the Raton Basin are explored. In southern Colorado, the western shoreline of the WIS should be shifted about 100 km farther west than current projections for the end of the early Maastrichtian. Strong marine connections among the WIS, the Gulf of Mexico, and the North Atlantic appear to have persisted at least until the end of the early Maastrichtian. A marine connection between the WIS and the Gulf of Mexico is projected to have lasted through the end of the Cretaceous. Closure of the WIS to the Arctic Ocean is projected to have occurred earlier and farther north than other models, which close the WIS to the Arctic Ocean through the formation of the Dakota Isthmus in the latest Maastrichtian. Closure of the WIS in Canada during the early late-Maastrichtian ( Hoploscaphites birkelundae ammonite biozone) appears to have permitted the dispersal of land plants, such as “Cissites” panduratus and Credneria protophylloides , among landmasses previously isolated by epeiric seaways covering much of North America and western Greenland during the Late Cretaceous and is consistent with preexisting biostratigraphic constraints on the paleogeography of the WIS.
{"title":"New paleontological constraints on the paleogeography of the Western Interior Seaway near the end of the Cretaceous (late Campanian–Maastrichtian) with a special emphasis on the paleogeography of southern Colorado, U.S.A.","authors":"K. Berry","doi":"10.24872/RMGJOURNAL.52.1.1","DOIUrl":"https://doi.org/10.24872/RMGJOURNAL.52.1.1","url":null,"abstract":"There is considerable debate regarding the paleogeography of the Western Interior Seaway (WIS) near the close of the Cretaceous. To investigate this issue, the paleogeographic implications of recent advancements in the biostratigraphy of the lower to upper Maastrichtian, transitional-marine strata in the Raton Basin are explored. In southern Colorado, the western shoreline of the WIS should be shifted about 100 km farther west than current projections for the end of the early Maastrichtian. Strong marine connections among the WIS, the Gulf of Mexico, and the North Atlantic appear to have persisted at least until the end of the early Maastrichtian. A marine connection between the WIS and the Gulf of Mexico is projected to have lasted through the end of the Cretaceous. Closure of the WIS to the Arctic Ocean is projected to have occurred earlier and farther north than other models, which close the WIS to the Arctic Ocean through the formation of the Dakota Isthmus in the latest Maastrichtian. Closure of the WIS in Canada during the early late-Maastrichtian ( Hoploscaphites birkelundae ammonite biozone) appears to have permitted the dispersal of land plants, such as “Cissites” panduratus and Credneria protophylloides , among landmasses previously isolated by epeiric seaways covering much of North America and western Greenland during the Late Cretaceous and is consistent with preexisting biostratigraphic constraints on the paleogeography of the WIS.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"52 1","pages":"1-16"},"PeriodicalIF":0.0,"publicationDate":"2017-03-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.24872/RMGJOURNAL.52.1.1","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48357597","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Few areas preserve the Cretaceous–Paleogene (K–Pg) boundary and earliest Paleocene (Puercan) mammalian faunas better than Colorado9s Denver Basin. Research conducted decades ago described a diverse Puercan fauna from the Denver Basin, but many would agree that the best-known fauna in the basin—the Littleton fauna—probably represents a late early-Puercan (i.e., late Pu1) assemblage. Renewed collecting of fossil mammals from Denver Museum of Nature & Science (DMNH) loc. 2560 in the Denver Formation on the eastern side of the Denver Basin—approximately nine meters stratigraphically above the K–Pg boundary—has produced a Puercan faunal assemblage containing isolated teeth of at least nine species of fossil mammal. Based upon its faunal composition, low diversity, absence of taxa characteristic of middle and late Puercan faunal assemblages, and close stratigraphic proximity to the K–Pg boundary, the fauna from DMNH loc. 2560 probably represents a Puercan fauna that is earlier than the Littleton fauna. It is most similar to early Puercan (Pu1) faunas in Wyoming and northeast Montana. We document the occurrence of three species of the multituberculate Mesodma ( M. ambigua, M. formosa , and M. hensleighi ), the cimolodont Cimexomys minor , the marsupial Thylacodon montanensis , and four species of archaic ungulate (or ‘condylarth’), including Protungulatum donnae , Oxyprimus sp., Baioconodon nordicus , and Maiorana sp. The presence of P. donnae defines the onset of the Puercan age, while Oxyprimus, Maiorana , and B. nordicus are restricted elsewhere to Pu1. C. minor is known from Lancian and early Puercan (Pu1) localities. The faunal assemblage from DMNH loc. 2560 indicates that the characteristic, low-diversity Pu1 fauna found in Montana and Wyoming extended as far south as Colorado. Additionally, we report the occurrence of the ‘condylarth’ Ampliconus browni from the South Table Mountain locality (DHMH loc. 2814) near Golden, Colorado. Its presence corroborates research by others that this locality is probably temporally correlative to the late early-Puercan (late Pu1) Alexander locality south of Denver, Colorado (UCM loc. 77267 [UCM = University of Colorado Museum of Natural History]). Our research provides a more complete picture of early Puercan mammalian diversity in the Denver Basin and a means to temporally correlate early Puercan faunas from Montana to Colorado.
{"title":"A new earliest Paleocene (Puercan) mammalian fauna from Colorado's Denver Basin, U.S.A.","authors":"E. L. Dahlberg, J. Eberle, J. Sertich, I. Miller","doi":"10.2113/GSROCKY.51.1.1","DOIUrl":"https://doi.org/10.2113/GSROCKY.51.1.1","url":null,"abstract":"Few areas preserve the Cretaceous–Paleogene (K–Pg) boundary and earliest Paleocene (Puercan) mammalian faunas better than Colorado9s Denver Basin. Research conducted decades ago described a diverse Puercan fauna from the Denver Basin, but many would agree that the best-known fauna in the basin—the Littleton fauna—probably represents a late early-Puercan (i.e., late Pu1) assemblage. Renewed collecting of fossil mammals from Denver Museum of Nature & Science (DMNH) loc. 2560 in the Denver Formation on the eastern side of the Denver Basin—approximately nine meters stratigraphically above the K–Pg boundary—has produced a Puercan faunal assemblage containing isolated teeth of at least nine species of fossil mammal. Based upon its faunal composition, low diversity, absence of taxa characteristic of middle and late Puercan faunal assemblages, and close stratigraphic proximity to the K–Pg boundary, the fauna from DMNH loc. 2560 probably represents a Puercan fauna that is earlier than the Littleton fauna. It is most similar to early Puercan (Pu1) faunas in Wyoming and northeast Montana. We document the occurrence of three species of the multituberculate Mesodma ( M. ambigua, M. formosa , and M. hensleighi ), the cimolodont Cimexomys minor , the marsupial Thylacodon montanensis , and four species of archaic ungulate (or ‘condylarth’), including Protungulatum donnae , Oxyprimus sp., Baioconodon nordicus , and Maiorana sp. The presence of P. donnae defines the onset of the Puercan age, while Oxyprimus, Maiorana , and B. nordicus are restricted elsewhere to Pu1. C. minor is known from Lancian and early Puercan (Pu1) localities. The faunal assemblage from DMNH loc. 2560 indicates that the characteristic, low-diversity Pu1 fauna found in Montana and Wyoming extended as far south as Colorado. Additionally, we report the occurrence of the ‘condylarth’ Ampliconus browni from the South Table Mountain locality (DHMH loc. 2814) near Golden, Colorado. Its presence corroborates research by others that this locality is probably temporally correlative to the late early-Puercan (late Pu1) Alexander locality south of Denver, Colorado (UCM loc. 77267 [UCM = University of Colorado Museum of Natural History]). Our research provides a more complete picture of early Puercan mammalian diversity in the Denver Basin and a means to temporally correlate early Puercan faunas from Montana to Colorado.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"51 1","pages":"1-22"},"PeriodicalIF":0.0,"publicationDate":"2016-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.51.1.1","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68313808","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Ignacio Quartzite—exposed in the San Juan Mountains of southwestern Colorado—is composed of red and brown arkose and subarkose sandstones and minor interbedded shales. The formation is newly divided here into the Tamarron Member (0–24 m) and the overlying Spud Hill Member (0–21 m). The Spud Hill Member has a greater abundance of sandstones with shale clasts, weakly fissile shale beds, and trace fossils than the Tamarron Member. The McCracken Sandstone Member of the Elbert Formation, which overlies the Ignacio, is chiefly white and off-white quartz-cemented quartzarenites. The McCracken is divided for the first time into the Mill Creek facies (0–12 m) to the south of Coal Bank Pass and the Sultan Creek facies (0–36 m) to the north of the pass. The Sultan Creek facies contains dolostone-sandstone parasequence tidal-flat cycles up to 70 cm thick with a composite thickness of 14 m. Eastward transgression across the western edge of the Transcontinental Arch permitted the accumulation of fluvial deposits of the lower Tamarron Member in the deepest channels incised into the craton. As sea level continued to rise, fluvial channels evolved into estuaries dominated by sandy tidal flats (upper Tamarron Member, Mill Creek facies, and Sultan Creek facies) and mixed sand and mud tidal flats (Spud Hill Member). Sandstone composition and ages of detrital zircons indicate that sand grains were derived from a complex terrain that included granitoid plutonic rocks (∼0.46 to >2.4 Ga), metamorphic rocks, and well-rounded quartz sand from eolian ergs. The area between the present Coal Bank and Molas passes was a boundary between a northern fluvial source with an abundance of superbly rounded quartz grains of eolian erg origin from a southern fluvial source with few such grains, but large amounts of K-feldspar. The Ignacio and McCracken units are, at least in part, coeval and of Late Devonian age as shown by the stratigraphic distribution of rocks resting on the basement, the presence of an Ordovician zircon in the Ignacio, and the presence in the same Ignacio sample of oboloid brachiopods of questionably late Cambrian age with well-dated Late Devonian placoderm fish plates.
{"title":"Stratigraphy, petrography, and depositional history of the Ignacio Quartzite and McCracken Sandstone Member of the Elbert Formation, southwestern Colorado, U.S.A.","authors":"E. McBride","doi":"10.2113/GSROCKY.51.2.23","DOIUrl":"https://doi.org/10.2113/GSROCKY.51.2.23","url":null,"abstract":"The Ignacio Quartzite—exposed in the San Juan Mountains of southwestern Colorado—is composed of red and brown arkose and subarkose sandstones and minor interbedded shales. The formation is newly divided here into the Tamarron Member (0–24 m) and the overlying Spud Hill Member (0–21 m). The Spud Hill Member has a greater abundance of sandstones with shale clasts, weakly fissile shale beds, and trace fossils than the Tamarron Member. The McCracken Sandstone Member of the Elbert Formation, which overlies the Ignacio, is chiefly white and off-white quartz-cemented quartzarenites. The McCracken is divided for the first time into the Mill Creek facies (0–12 m) to the south of Coal Bank Pass and the Sultan Creek facies (0–36 m) to the north of the pass. The Sultan Creek facies contains dolostone-sandstone parasequence tidal-flat cycles up to 70 cm thick with a composite thickness of 14 m. Eastward transgression across the western edge of the Transcontinental Arch permitted the accumulation of fluvial deposits of the lower Tamarron Member in the deepest channels incised into the craton. As sea level continued to rise, fluvial channels evolved into estuaries dominated by sandy tidal flats (upper Tamarron Member, Mill Creek facies, and Sultan Creek facies) and mixed sand and mud tidal flats (Spud Hill Member). Sandstone composition and ages of detrital zircons indicate that sand grains were derived from a complex terrain that included granitoid plutonic rocks (∼0.46 to >2.4 Ga), metamorphic rocks, and well-rounded quartz sand from eolian ergs. The area between the present Coal Bank and Molas passes was a boundary between a northern fluvial source with an abundance of superbly rounded quartz grains of eolian erg origin from a southern fluvial source with few such grains, but large amounts of K-feldspar. The Ignacio and McCracken units are, at least in part, coeval and of Late Devonian age as shown by the stratigraphic distribution of rocks resting on the basement, the presence of an Ordovician zircon in the Ignacio, and the presence in the same Ignacio sample of oboloid brachiopods of questionably late Cambrian age with well-dated Late Devonian placoderm fish plates.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"51 1","pages":"23-68"},"PeriodicalIF":0.0,"publicationDate":"2016-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.51.2.23","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68313979","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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