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.
{"title":"Flood Events that Helped Shape Colorado's Castlewood Canyon Carved into the Eocene Castle Rock Conglomerate: An Introduction to this Focused Issue","authors":"M. Longman, Joan Burleson, J. Hagadorn","doi":"10.31582/rmag.mg.61.1.5","DOIUrl":"https://doi.org/10.31582/rmag.mg.61.1.5","url":null,"abstract":"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.","PeriodicalId":506089,"journal":{"name":"Mountain Geologist","volume":"172 3-4","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140516614","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 : 2024-01-01DOI: 10.31582/rmag.mg.61.1.71
Nik Svihlik, M. Longman
Hydrous amorphous silica (aka opal) is a common cement in the Upper Eocene Castle Rock Conglomerate (CRC) of the southwestern Denver Basin. Petrographic study of standard thin sections indicates that this opal forms from 5% to as much as 40% of any given sample. It also commonly occurs as a precursor to fibrous length-fast chalcedony, a crystalline form of quartz cement. Similar opal cement apparently derived from shards of volcanic glass is even more common in the subjacent Wall Mountain Tuff, a welded deposit of volcanic ash that was the most likely source of silica in the opal cement in the CRC. This paper provides a first-of-its-kind attempt to quantify the amount of opal cement in selected samples of the CRC based on X-ray diffraction (XRD) analysis. Because opal is amorphous, its abundance cannot be quantified using standard XRD techniques, but experimenting with heating indicates that it is possible to convert powdered amorphous opal to a crystalline form of silica in less than 48 hours at a temperature of 550°C. Comparison of pre- and post-heat treatment XRD diffractograms thus provides a potential tool for quantifying opal content in the CRC and other opal-cemented samples. This new analytical technique and its results are described in this paper.
{"title":"A New Technique for Quantifying Opal Content in Colorado's Upper Eocene Castle Rock Conglomerate","authors":"Nik Svihlik, M. Longman","doi":"10.31582/rmag.mg.61.1.71","DOIUrl":"https://doi.org/10.31582/rmag.mg.61.1.71","url":null,"abstract":"Hydrous amorphous silica (aka opal) is a common cement in the Upper Eocene Castle Rock Conglomerate (CRC) of the southwestern Denver Basin. Petrographic study of standard thin sections indicates that this opal forms from 5% to as much as 40% of any given sample. It also commonly occurs as a precursor to fibrous length-fast chalcedony, a crystalline form of quartz cement. Similar opal cement apparently derived from shards of volcanic glass is even more common in the subjacent Wall Mountain Tuff, a welded deposit of volcanic ash that was the most likely source of silica in the opal cement in the CRC. This paper provides a first-of-its-kind attempt to quantify the amount of opal cement in selected samples of the CRC based on X-ray diffraction (XRD) analysis. Because opal is amorphous, its abundance cannot be quantified using standard XRD techniques, but experimenting with heating indicates that it is possible to convert powdered amorphous opal to a crystalline form of silica in less than 48 hours at a temperature of 550°C. Comparison of pre- and post-heat treatment XRD diffractograms thus provides a potential tool for quantifying opal content in the CRC and other opal-cemented samples. This new analytical technique and its results are described in this paper.","PeriodicalId":506089,"journal":{"name":"Mountain Geologist","volume":"36 6","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140517845","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 : 2024-01-01DOI: 10.31582/rmag.mg.61.1.17
S. Keller, Matthew L. Morgan
The late Eocene Castle Rock Conglomerate occurs mostly in Douglas and Elbert counties, Colorado. It is the uppermost and youngest Cenozoic unit in the southern Denver Basin and its outcrops occur in a swath trending from Sedalia southeast to Calhan. The unit is well exposed and topographically prominent, forming flat mesas, steep cliffs, and narrow canyons. The conglomerate is a fluvial unit deposited by a 3- to 10-km-wide braided stream system. Large-scale crossbedding, massive bedding, angular blocks of welded tuff, a variety of other clast lithologies, cut-and-fill structures, fining-upward sequences, fossil logs, and occasional fossilized bones are readily observable. Because the conglomerate is both geologically and scenically striking, it has interested geologists since the late 1860s. Because of improved access to the unit over the last 60 years (in Castlewood Canyon State Park and in county and municipal open spaces) it has increasingly attracted educators, students, and the public. The purpose of this two-part article is to be a source document for future investigators of the formation. Part 1 of the article (the present publication) is a chronology of the description, nomenclature, and mapping of the formation as presented by various investigators over the decades. Part 2 (for future publication) will cover the formation’s geologic history, depositional environment, age, and diagenesis and will present several suggestions for future research.
{"title":"Investigations in the Late Eocene Castle Rock Conglomerate of Colorado from 1869 to the present - Part 1: Description, Nomenclature, and Mapping","authors":"S. Keller, Matthew L. Morgan","doi":"10.31582/rmag.mg.61.1.17","DOIUrl":"https://doi.org/10.31582/rmag.mg.61.1.17","url":null,"abstract":"The late Eocene Castle Rock Conglomerate occurs mostly in Douglas and Elbert counties, Colorado. It is the uppermost and youngest Cenozoic unit in the southern Denver Basin and its outcrops occur in a swath trending from Sedalia southeast to Calhan. The unit is well exposed and topographically prominent, forming flat mesas, steep cliffs, and narrow canyons. The conglomerate is a fluvial unit deposited by a 3- to 10-km-wide braided stream system. Large-scale crossbedding, massive bedding, angular blocks of welded tuff, a variety of other clast lithologies, cut-and-fill structures, fining-upward sequences, fossil logs, and occasional fossilized bones are readily observable. Because the conglomerate is both geologically and scenically striking, it has interested geologists since the late 1860s. Because of improved access to the unit over the last 60 years (in Castlewood Canyon State Park and in county and municipal open spaces) it has increasingly attracted educators, students, and the public. The purpose of this two-part article is to be a source document for future investigators of the formation. Part 1 of the article (the present publication) is a chronology of the description, nomenclature, and mapping of the formation as presented by various investigators over the decades. Part 2 (for future publication) will cover the formation’s geologic history, depositional environment, age, and diagenesis and will present several suggestions for future research.","PeriodicalId":506089,"journal":{"name":"Mountain Geologist","volume":"26 8","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140524062","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 : 2024-01-01DOI: 10.31582/rmag.mg.61.1.49
M. Longman, Nik Svihlik, Joan Burleson, J. Hagadorn
The Castle Rock Conglomerate is one of Colorado’s most iconic, youngest, and coarsest grained rock units. It is also one of the hardest sedimentary rocks in Colorado and forms prominent buttes in the southwestern Denver Basin. Yet the reasons for its induration and resistance to weathering have not previously been investigated. Sedimentologic observations paired with sedimentary petrology indicate that much of the unit is comprised of a planar-bedded to cross-bedded, mostly poorly sorted, angular to subrounded assemblage of quartz, K-feldspar, quartzite, and unusually large volcanic rock fragments along with some plagioclase and mica flakes. The largest volcanic rock fragments are up to ∼2 m in size and composed of the immediately subjacent Wall Mountain Tuff of late Eocene age. Sedimentary rock fragments and well-rounded quartz grains are rare. Together these features suggest a diverse and relatively proximal provenance for the unit. Pervasive opaline cement coats most grains, and locally exhibits pendant features typical of vadose precipitation. These opal cements formed prior to any grain compaction and indicate early silica precipitation at shallow burial depths. Where the primary pores were not completely cemented by the opal, most were later filled with length-fast chalcedony cement. We hypothesize that cementation of the conglomerate began soon after deposition as weathering of the Wall Mountain Tuff and weathering of clasts of the tuff within the conglomerate, yielded ground water super-saturated with silica. These fluids initially catalyzed precipitation of common opal (hydrous amorphous silica) and later fostered precipitation of length-fast chalcedony. Together, these cements created a silica-cemented “concrete” much more resistant to weathering than any carbonate-cemented sandstone, and much harder than man-made calcite-cemented concrete found in many sidewalks and roadways.
{"title":"Opal Cement in the Eocene Castle Rock Conglomerate, Central Colorado","authors":"M. Longman, Nik Svihlik, Joan Burleson, J. Hagadorn","doi":"10.31582/rmag.mg.61.1.49","DOIUrl":"https://doi.org/10.31582/rmag.mg.61.1.49","url":null,"abstract":"The Castle Rock Conglomerate is one of Colorado’s most iconic, youngest, and coarsest grained rock units. It is also one of the hardest sedimentary rocks in Colorado and forms prominent buttes in the southwestern Denver Basin. Yet the reasons for its induration and resistance to weathering have not previously been investigated. Sedimentologic observations paired with sedimentary petrology indicate that much of the unit is comprised of a planar-bedded to cross-bedded, mostly poorly sorted, angular to subrounded assemblage of quartz, K-feldspar, quartzite, and unusually large volcanic rock fragments along with some plagioclase and mica flakes. The largest volcanic rock fragments are up to ∼2 m in size and composed of the immediately subjacent Wall Mountain Tuff of late Eocene age. Sedimentary rock fragments and well-rounded quartz grains are rare. Together these features suggest a diverse and relatively proximal provenance for the unit. Pervasive opaline cement coats most grains, and locally exhibits pendant features typical of vadose precipitation. These opal cements formed prior to any grain compaction and indicate early silica precipitation at shallow burial depths. Where the primary pores were not completely cemented by the opal, most were later filled with length-fast chalcedony cement. We hypothesize that cementation of the conglomerate began soon after deposition as weathering of the Wall Mountain Tuff and weathering of clasts of the tuff within the conglomerate, yielded ground water super-saturated with silica. These fluids initially catalyzed precipitation of common opal (hydrous amorphous silica) and later fostered precipitation of length-fast chalcedony. Together, these cements created a silica-cemented “concrete” much more resistant to weathering than any carbonate-cemented sandstone, and much harder than man-made calcite-cemented concrete found in many sidewalks and roadways.","PeriodicalId":506089,"journal":{"name":"Mountain Geologist","volume":"35 12","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140523061","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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