Pub Date : 1985-07-01DOI: 10.31582/rmag.mg.22.3.128
D. Madden
{"title":"Description and Origin of the Lower Part of the Mesaverde Group in Rifle Gap, Garfield County, Colorado","authors":"D. Madden","doi":"10.31582/rmag.mg.22.3.128","DOIUrl":"https://doi.org/10.31582/rmag.mg.22.3.128","url":null,"abstract":"","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1985-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115080126","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 : 1985-07-01DOI: 10.31582/rmag.mg.22.3.110
S. J. Luft
Bedded airfall tuffs, mainly rhyolitic in composition and locally very thick, occur throughout the Browns Park Formation (upper Oligocene to upper Miocene) in northwestern Colorado and northeastern most Utah. First mentioned in the geologic literature by Bradley (1935), they have received only cursory attention other than for the purpose of radiometric dating. The present writer began study of the tuffs in 1980, hoping to use them as time-stratigraphic marker beds within the formation. Several tuff-rich stratigraphy sections were measured and numerous samples were collected. The results of petrographic and petrochemical studies of these samples are presented. Refractive indices of vitric shards from 52 samples increase slightly and irregularly with apparently decreasing age of the samples. Seemingly, the trend points toward less felsic compositions with time. Quartz, feldspar, and especially heavy-mineral suites from these from these 52 samples were studied petrographically. Because airfall components (phenocrystic minerals) are not generally readily distinguishable from contaminating detrital grains, time-composition trends in airfall assemblages are problematical and difficult to establish. Clinopyroxene of likely airfall origin may increase slightly in abundance with decreasing age. The ratio of apatite (mainly of airfall origin) to zircon (mixed origins) appears to decrease with time. Recalculated analyses of major oxides for the 24 whole-rock samples that appear to be least contaminated by detritus show the following variations with decreasing age: increased silica, potash, total alkalis, and titania, and decreased alumina, total iron oxides, magnesia, lime, and soda, and soda: potash ratio. In general, petrochemical results indicate that Browns Park tuffs became increasingly more felsic with time, albeit irregularly. This contradicts the conclusions drawn from refractive-index determinations on the same samples. Preliminary correlations of airfall-tuff beds by petrochemistry, refractive indices of glass, and mineralogy and population of heavy minerals, have been partly successful Minor-element distribution provides useful information, but data are incomplete. Prevailing westerly winds in mid- to late-Tertiary time brought ash into the Browns Park Formation and correlative units elsewhere in the Rocky Mountain region from distant sources m numerous volcanically active areas in the western United States. Bulk mineralogy and major-oxide compositions and variations from selected eruptive centers in the Basin and Range province commonly resemble those of Brown Park tuff. Source areas of silicic eruptive material were in, but not necessarily limited to, Utah, Nevada, eastern California, southwestern Idaho, and southeastern Oregon.
{"title":"Airfall Tuff in the Browns Park Formation, Northwestern Colorado and Northeastern Utah","authors":"S. J. Luft","doi":"10.31582/rmag.mg.22.3.110","DOIUrl":"https://doi.org/10.31582/rmag.mg.22.3.110","url":null,"abstract":"Bedded airfall tuffs, mainly rhyolitic in composition and locally very thick, occur throughout the Browns Park Formation (upper Oligocene to upper Miocene) in northwestern Colorado and northeastern most Utah. First mentioned in the geologic literature by Bradley (1935), they have received only cursory attention other than for the purpose of radiometric dating. The present writer began study of the tuffs in 1980, hoping to use them as time-stratigraphic marker beds within the formation. Several tuff-rich stratigraphy sections were measured and numerous samples were collected. The results of petrographic and petrochemical studies of these samples are presented. Refractive indices of vitric shards from 52 samples increase slightly and irregularly with apparently decreasing age of the samples. Seemingly, the trend points toward less felsic compositions with time. Quartz, feldspar, and especially heavy-mineral suites from these from these 52 samples were studied petrographically. Because airfall components (phenocrystic minerals) are not generally readily distinguishable from contaminating detrital grains, time-composition trends in airfall assemblages are problematical and difficult to establish. Clinopyroxene of likely airfall origin may increase slightly in abundance with decreasing age. The ratio of apatite (mainly of airfall origin) to zircon (mixed origins) appears to decrease with time. Recalculated analyses of major oxides for the 24 whole-rock samples that appear to be least contaminated by detritus show the following variations with decreasing age: increased silica, potash, total alkalis, and titania, and decreased alumina, total iron oxides, magnesia, lime, and soda, and soda: potash ratio. In general, petrochemical results indicate that Browns Park tuffs became increasingly more felsic with time, albeit irregularly. This contradicts the conclusions drawn from refractive-index determinations on the same samples. Preliminary correlations of airfall-tuff beds by petrochemistry, refractive indices of glass, and mineralogy and population of heavy minerals, have been partly successful Minor-element distribution provides useful information, but data are incomplete. Prevailing westerly winds in mid- to late-Tertiary time brought ash into the Browns Park Formation and correlative units elsewhere in the Rocky Mountain region from distant sources m numerous volcanically active areas in the western United States. Bulk mineralogy and major-oxide compositions and variations from selected eruptive centers in the Basin and Range province commonly resemble those of Brown Park tuff. Source areas of silicic eruptive material were in, but not necessarily limited to, Utah, Nevada, eastern California, southwestern Idaho, and southeastern Oregon.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1985-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"124430931","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 : 1985-04-01DOI: 10.31582/rmag.mg.22.2.78
L. Pickering, G. Indelicato
Abnormal formation pressure is defined as any pressure that deviates from the normal hydrostatic gradient. In order for these pressures to form and be preserved, a near-seal is required. The seal is not necessarily impermeable but may be a low permeability formation, such as shale, or a fault-related barrier. The possible causes of abnormal formation pressure are: 1) compaction of sediments, 2) tectonic activities, 3) temperature changes, 4) osmosis, 5) diagenesis, 6) methane generation, and 7) buoyancy. Temperature changes due to decay, diagenesis, and changes in burial depth of the formation are one of the two most important causes of abnormal formation pressure. The other important cause is compaction pressure related to the weight of overburden with increased depth of burial. If the pore fluids cannot escape, they will support a greater proportion of the total overburden stress and become abnormally highly pressured. Tectonic activities such as faulting as well as removal of overburden by erosion also play an important role in either raising or lowering pore fluid pressure. Osmosis, diagenesis, methane generation, and buoyancy are all additive to the overall effects of pressure and temperature.
{"title":"Abnormal Formation Pressure: A Review","authors":"L. Pickering, G. Indelicato","doi":"10.31582/rmag.mg.22.2.78","DOIUrl":"https://doi.org/10.31582/rmag.mg.22.2.78","url":null,"abstract":"Abnormal formation pressure is defined as any pressure that deviates from the normal hydrostatic gradient. In order for these pressures to form and be preserved, a near-seal is required. The seal is not necessarily impermeable but may be a low permeability formation, such as shale, or a fault-related barrier. The possible causes of abnormal formation pressure are: 1) compaction of sediments, 2) tectonic activities, 3) temperature changes, 4) osmosis, 5) diagenesis, 6) methane generation, and 7) buoyancy. Temperature changes due to decay, diagenesis, and changes in burial depth of the formation are one of the two most important causes of abnormal formation pressure. The other important cause is compaction pressure related to the weight of overburden with increased depth of burial. If the pore fluids cannot escape, they will support a greater proportion of the total overburden stress and become abnormally highly pressured. Tectonic activities such as faulting as well as removal of overburden by erosion also play an important role in either raising or lowering pore fluid pressure. Osmosis, diagenesis, methane generation, and buoyancy are all additive to the overall effects of pressure and temperature.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1985-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"114284278","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 : 1985-04-01DOI: 10.31582/rmag.mg.22.2.41
J. Spang, James P. Evans, R. Berg
Balanced, restorable cross sections for the Precambrian-cored Sage Creek anticline in the Wind River Basin are combined with constraints on the mechanical behavior of various types of basement rocks and sedimentary rocks to give a new model of the apparent folding of the upper basement surface and the early stages of a fold-thrust structure. Vertical relief on the upper basement surface develops as a result of motion along a zone of discrete, parallel faults, giving an upper basement surface which appears to be folded on some scale of observation. The sedimentary rocks above the uplift are folded into a tight anticline-syncline pair. Progressive restoration of the cross sections using the proposed geometry of the early stages of uplift shows that folding and overturning above the fault zone is the major component of shortening in the cover rocks. The late "mountain flank" thrusts, which cut the fold increase the vertical relief of the upper basement surface and extend the cover rocks which increases the fold amplitude in the cover rocks. Cross sections depicting the present geometry must balance throughout the development of the structure and satisfy the predicted mechanical behavior of the rocks involved at the conditions under which the structure formed.
{"title":"Balanced Cross Sections of Small Fold-thrust Structures","authors":"J. Spang, James P. Evans, R. Berg","doi":"10.31582/rmag.mg.22.2.41","DOIUrl":"https://doi.org/10.31582/rmag.mg.22.2.41","url":null,"abstract":"Balanced, restorable cross sections for the Precambrian-cored Sage Creek anticline in the Wind River Basin are combined with constraints on the mechanical behavior of various types of basement rocks and sedimentary rocks to give a new model of the apparent folding of the upper basement surface and the early stages of a fold-thrust structure. Vertical relief on the upper basement surface develops as a result of motion along a zone of discrete, parallel faults, giving an upper basement surface which appears to be folded on some scale of observation. The sedimentary rocks above the uplift are folded into a tight anticline-syncline pair. Progressive restoration of the cross sections using the proposed geometry of the early stages of uplift shows that folding and overturning above the fault zone is the major component of shortening in the cover rocks. The late \"mountain flank\" thrusts, which cut the fold increase the vertical relief of the upper basement surface and extend the cover rocks which increases the fold amplitude in the cover rocks. Cross sections depicting the present geometry must balance throughout the development of the structure and satisfy the predicted mechanical behavior of the rocks involved at the conditions under which the structure formed.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1985-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"126684176","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 : 1985-04-01DOI: 10.31582/rmag.mg.22.2.47
T. Davis
Reflection seismic studies in the Denver basin near Greeley, Colorado, illustrate an association between listric normal faults (which sole out in detachment or decollement zones) and basement-controlled faults. Recurrent movement on basement-controlled faults triggered development of listric normal faults in tectonically sensitive stratigraphic intervals of the Cretaceous. Stratigraphic intervals exhibiting listric normal faults include the Cretaceous Laramie-Fox Hills-Upper Pierre, Middle Pierre Hygiene zone, and the Niobrara-Carlile Greenhorn. Listric normal faults are prevalent on the flank of a fault-bounded basement-controlled paleostructural block termed the Wattenberg block by Weimer and Sonnenberg (1982). Listric normal faults influence Cretaceous reservoir systems in the Hambert field area on the north flank of the Wattenberg-Greeley Lineament Zone.
{"title":"Seismic Evidence of Tectonic Influence on Development of Cretaceous Listric Normal Faults, Boulder-Wattenberg-Greeley Area, Denver Basin, Colorado","authors":"T. Davis","doi":"10.31582/rmag.mg.22.2.47","DOIUrl":"https://doi.org/10.31582/rmag.mg.22.2.47","url":null,"abstract":"Reflection seismic studies in the Denver basin near Greeley, Colorado, illustrate an association between listric normal faults (which sole out in detachment or decollement zones) and basement-controlled faults. Recurrent movement on basement-controlled faults triggered development of listric normal faults in tectonically sensitive stratigraphic intervals of the Cretaceous. Stratigraphic intervals exhibiting listric normal faults include the Cretaceous Laramie-Fox Hills-Upper Pierre, Middle Pierre Hygiene zone, and the Niobrara-Carlile Greenhorn. Listric normal faults are prevalent on the flank of a fault-bounded basement-controlled paleostructural block termed the Wattenberg block by Weimer and Sonnenberg (1982). Listric normal faults influence Cretaceous reservoir systems in the Hambert field area on the north flank of the Wattenberg-Greeley Lineament Zone.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1985-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"134173991","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 : 1985-04-01DOI: 10.31582/rmag.mg.22.2.55
R. M. Pollastro, C. J. Martinez
Indigenous hydrocarbons are produced from organic-rich chalk beds of the Upper Cretaceous Niobrara Formation in the Denver basin, eastern Colorado. The types of hydrocarbons produced from these chalks are determined by the level of thermal maturity associated with present-day burial or paleoburial conditions. Detailed analyses of deeply-buried chalk from core of the Smoky Hill Chalk Member of the Niobrara Formation in the Champlin Petroleum #2 Boxelder Farms well combined with core data from other Niobrara wells have helped identify many depositional and diagenetic relationships. Porosity of the chalk is proportional to maximum burial depth and inversely proportional to the amount of non-carbonate material (acid-insoluble residue content) in the chalk. Total organic carbon content in the chalk is proportional to the amount of acid insoluble residue and relative abundance of pyrite in the acid-insoluble fraction. Quartz is inversely proportional to the amount of insoluble material, and the amount of clay tends to increase as insolubles increase, suggesting that detritus in these chalks is greatly influenced by reworked, altered, volcanic products rather than siliceous clastics. Mixed-layer illite/smectite clay of bentonite beds in the Niobrara Formation appears to be a good geothermometer, and its composition provides an indication of thermal maturity of the indigenous hydrocarbons. Scanning electron microscopy of the chalk fabric shows progressive cementation with increasing burial depth. Oxygen isotopes of the carbonate become progressively more negative with increasing burial. Oxygen Isotopes, therefore, record the effect of progressive cementation in these chalks and support the idea that pressure solution and reprecipitation of the carbonate is the primary process for porosity reduction in chalk reservoirs of the Niobrara Formation in the Denver basin and adjacent areas.
{"title":"Mineral, Chemical, and Textural Relationships in Rhythmic-bedded, Hydrocarbon-productive Chalk of the Niobrara Formation, Denver Basin, Colorado","authors":"R. M. Pollastro, C. J. Martinez","doi":"10.31582/rmag.mg.22.2.55","DOIUrl":"https://doi.org/10.31582/rmag.mg.22.2.55","url":null,"abstract":"Indigenous hydrocarbons are produced from organic-rich chalk beds of the Upper Cretaceous Niobrara Formation in the Denver basin, eastern Colorado. The types of hydrocarbons produced from these chalks are determined by the level of thermal maturity associated with present-day burial or paleoburial conditions. Detailed analyses of deeply-buried chalk from core of the Smoky Hill Chalk Member of the Niobrara Formation in the Champlin Petroleum #2 Boxelder Farms well combined with core data from other Niobrara wells have helped identify many depositional and diagenetic relationships. Porosity of the chalk is proportional to maximum burial depth and inversely proportional to the amount of non-carbonate material (acid-insoluble residue content) in the chalk. Total organic carbon content in the chalk is proportional to the amount of acid insoluble residue and relative abundance of pyrite in the acid-insoluble fraction. Quartz is inversely proportional to the amount of insoluble material, and the amount of clay tends to increase as insolubles increase, suggesting that detritus in these chalks is greatly influenced by reworked, altered, volcanic products rather than siliceous clastics. Mixed-layer illite/smectite clay of bentonite beds in the Niobrara Formation appears to be a good geothermometer, and its composition provides an indication of thermal maturity of the indigenous hydrocarbons. Scanning electron microscopy of the chalk fabric shows progressive cementation with increasing burial depth. Oxygen isotopes of the carbonate become progressively more negative with increasing burial. Oxygen Isotopes, therefore, record the effect of progressive cementation in these chalks and support the idea that pressure solution and reprecipitation of the carbonate is the primary process for porosity reduction in chalk reservoirs of the Niobrara Formation in the Denver basin and adjacent areas.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1985-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"131593320","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 : 1985-01-01DOI: 10.31582/rmag.mg.22.1.28
C. D. Harrington
Timbered Island, a ridge of till in the Jackson Hole lowland, was originally interpreted as an end moraine constructed by a piedmont glacier extending eastward from the Teton Range. Instead, a study of till fabric suggests that Timbered Island was deposited along the western margin of a large ice lobe extending south into the lowland. The marked similarity in the till fabric of Timbered Island and other moraines deposited by the intermontane glacier in the northern part of the lowland suggests that all were formed in a similar manner. Timbered island has survived because its trend was parallel to the direction of meltwater flow during subsequent glacial advances, whereas segments of the moraine normal to meltwater flow would more likely have been buried or removed.
{"title":"A Revision in the Glacial History of Jackson Hole, Wyoming","authors":"C. D. Harrington","doi":"10.31582/rmag.mg.22.1.28","DOIUrl":"https://doi.org/10.31582/rmag.mg.22.1.28","url":null,"abstract":"Timbered Island, a ridge of till in the Jackson Hole lowland, was originally interpreted as an end moraine constructed by a piedmont glacier extending eastward from the Teton Range. Instead, a study of till fabric suggests that Timbered Island was deposited along the western margin of a large ice lobe extending south into the lowland. The marked similarity in the till fabric of Timbered Island and other moraines deposited by the intermontane glacier in the northern part of the lowland suggests that all were formed in a similar manner. Timbered island has survived because its trend was parallel to the direction of meltwater flow during subsequent glacial advances, whereas segments of the moraine normal to meltwater flow would more likely have been buried or removed.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1985-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"116642166","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 Dry Hollow Member of the Upper Cretaceous (Turonian) Frontier Formation in the Wyoming-Idaho-Utah thrust belt is comprised predominantly of fluvial deposits. In northeastern Utah, these strata include massive cobble conglomerates, horizontally-stratified very coarse sandstones, planar cross-stratified pebbly sandstones, and rare siltstones. Farther eastward in southwestern Wyoming, the Dry Hollow Member contains sandstone lenses which possess basal conglomeratic (pebble) lenses and fine upward to medium-grained, trough cross stratified sandstones interbedded shales, coals, and thin, fine-grained, ripple-drift cross-laminated sandstones account for much of the Dry Hollow Member. Regional variations in stratification styles, grain size, and geometry of the sandstone units are interpreted as a consequence of downslope variations in channel pattern. These variations represent a change from near-source braided streams in northeastern Utah to distal meandering streams in southwestern Wyoming.
{"title":"Synorogenic Sedimentation of Upper Cretaceous Frontier Formation Conglomerates and Associated Strata, Wyoming-Idaho-Utah Thrust Belt","authors":"J. Schmitt","doi":"10.31582/rmag.mg.22.1.5","DOIUrl":"https://doi.org/10.31582/rmag.mg.22.1.5","url":null,"abstract":"The Dry Hollow Member of the Upper Cretaceous (Turonian) Frontier Formation in the Wyoming-Idaho-Utah thrust belt is comprised predominantly of fluvial deposits. In northeastern Utah, these strata include massive cobble conglomerates, horizontally-stratified very coarse sandstones, planar cross-stratified pebbly sandstones, and rare siltstones. Farther eastward in southwestern Wyoming, the Dry Hollow Member contains sandstone lenses which possess basal conglomeratic (pebble) lenses and fine upward to medium-grained, trough cross stratified sandstones interbedded shales, coals, and thin, fine-grained, ripple-drift cross-laminated sandstones account for much of the Dry Hollow Member. Regional variations in stratification styles, grain size, and geometry of the sandstone units are interpreted as a consequence of downslope variations in channel pattern. These variations represent a change from near-source braided streams in northeastern Utah to distal meandering streams in southwestern Wyoming.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1985-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"123921911","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 : 1985-01-01DOI: 10.31582/rmag.mg.22.1.17
D. Carr
Field analysis of terrigenous elastic units lying within the upper Pennsylvanian-lower Permian cyclic elastic/ carbonate sequence (Holder and Labore/ta Formations) of the northern Sacramento Mountains, New Mexico, illustrates a marine to non-marine environmental progression as infilling of the Orogrande basin occurred. The oldest units studied, flat-based sandstone bodies A, B and C (Virgilian), coarsen upward texturally and dis play large-scale (up to 5m thick) foresets, hummocky stratification, bimodal or polymodal cross-stratification trends, and abundant burrows. These characteristics indicate that siliciclastic shelf bars were present on a storm dominated and tide-influenced shelf. Higher in the section, units D (upper Virgilian) and E (lower Wolfcampian) exhibit fluvially influenced depositional features such as concave-up erosional bases, upward-fining textural sequences and unimodal or asymmetric, bimodal paleocurrent distributions. Units D and E are interpreted as estuarine and fan deltaic deposits, respectively Several upward-coarsening fan-deltaic sequences were recognized in the study area suggesting that delta shifting , rather than sea level fluctuations, was a significant local cause of cyclicity. The sequence of units A-E may represent the relatively continuous progradation of terrigenous elastic sediments from the Pedernal uplift into the Orogrande basin during late Paleozoic time.
{"title":"Depositional Trends In Upper Paleozoic Terrigenous Clastic Rocks, Sacramento Mountains, New Mexico","authors":"D. Carr","doi":"10.31582/rmag.mg.22.1.17","DOIUrl":"https://doi.org/10.31582/rmag.mg.22.1.17","url":null,"abstract":"Field analysis of terrigenous elastic units lying within the upper Pennsylvanian-lower Permian cyclic elastic/ carbonate sequence (Holder and Labore/ta Formations) of the northern Sacramento Mountains, New Mexico, illustrates a marine to non-marine environmental progression as infilling of the Orogrande basin occurred. The oldest units studied, flat-based sandstone bodies A, B and C (Virgilian), coarsen upward texturally and dis play large-scale (up to 5m thick) foresets, hummocky stratification, bimodal or polymodal cross-stratification trends, and abundant burrows. These characteristics indicate that siliciclastic shelf bars were present on a storm dominated and tide-influenced shelf. Higher in the section, units D (upper Virgilian) and E (lower Wolfcampian) exhibit fluvially influenced depositional features such as concave-up erosional bases, upward-fining textural sequences and unimodal or asymmetric, bimodal paleocurrent distributions. Units D and E are interpreted as estuarine and fan deltaic deposits, respectively Several upward-coarsening fan-deltaic sequences were recognized in the study area suggesting that delta shifting , rather than sea level fluctuations, was a significant local cause of cyclicity. The sequence of units A-E may represent the relatively continuous progradation of terrigenous elastic sediments from the Pedernal uplift into the Orogrande basin during late Paleozoic time.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1985-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"126130233","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 : 1985-01-01DOI: 10.31582/rmag.mg.22.1.33
Gabrielle Schavran
Laramide deformation along the east flank of the Sangre de Cristo Range, Colorado, has produced an imbricate thrust system with associated major folds in the Middle Pennsylvanian Minturn Formation, west of the town of Gardner. Thrusts dip 5 to 15 degrees to the west and are offset along strike by small tear faults. Major folds are inclined to overturned near the leading edges of the thrusts and become open and diminish in amplitude to the west, farther from the leading edges. Fold axes trend between N 10 Wand N 60 Wand plunge gently to the northwest or southeast. Tectonic transport was from west-southwest to east-northeast as interpreted from ma1or thrust and fold trends. Detailed analyses of minor structures such as bedding-plane thrusts, minor folds, and angle faults substantiate the style of deformation and the interpreted direction of transport Pennsylvanian sedimentary rocks were detached and thrusted, probably above a major decollement surface. Folds, bedding thrust reverse faults, and tear faults developed during thrusting and imbrication. Regionally, Precambrian rocks to the west in the Sangre de Cristo Range are interpreted to be allochthonous suggesting that the fold and thrust belt represents a zone of Laramide crustal shortening.
{"title":"Structural Features in the Huerfano Park Area, East Flank, Sangre de Cristo Range, Colorado","authors":"Gabrielle Schavran","doi":"10.31582/rmag.mg.22.1.33","DOIUrl":"https://doi.org/10.31582/rmag.mg.22.1.33","url":null,"abstract":"Laramide deformation along the east flank of the Sangre de Cristo Range, Colorado, has produced an imbricate thrust system with associated major folds in the Middle Pennsylvanian Minturn Formation, west of the town of Gardner. Thrusts dip 5 to 15 degrees to the west and are offset along strike by small tear faults. Major folds are inclined to overturned near the leading edges of the thrusts and become open and diminish in amplitude to the west, farther from the leading edges. Fold axes trend between N 10 Wand N 60 Wand plunge gently to the northwest or southeast. Tectonic transport was from west-southwest to east-northeast as interpreted from ma1or thrust and fold trends. Detailed analyses of minor structures such as bedding-plane thrusts, minor folds, and angle faults substantiate the style of deformation and the interpreted direction of transport Pennsylvanian sedimentary rocks were detached and thrusted, probably above a major decollement surface. Folds, bedding thrust reverse faults, and tear faults developed during thrusting and imbrication. Regionally, Precambrian rocks to the west in the Sangre de Cristo Range are interpreted to be allochthonous suggesting that the fold and thrust belt represents a zone of Laramide crustal shortening.","PeriodicalId":101513,"journal":{"name":"Mountain Geologist","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1985-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"131219867","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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