TY - JOUR
T1 - Recognition of primary and diagenetic magnetizations to determine the magnetic polarity record and timing of deposition of the moat-fill rocks of the Oligocene Creede caldera, Colorado
AU - Reynolds, Richard L.
AU - Rosenbaum, Joseph G.
AU - Sweetkind, Donald S.
AU - Lanphere, Marvin A.
AU - Roberts, Andrew P.
AU - Verosub, Kenneth L.
PY - 2000
Y1 - 2000
N2 - Sedimentary and volcaniclastic rocks of the Oligocene Creede Formation fill the moat of the Creede caldera, which formed at about 26.9 Ma during the eruption of the Snowshoe Mountain Tuff. Paleomagnetic and rock magnetic studies of two cores (418 and 703 m long) that penetrated the lower half of the Creede Formation, in addition to paleomagnetic and isotopic dating studies of stratigraphically bracketing volcanic units, provide information on the age and the time span of sedimentation of the caldera fill. Normal polarity magnetizations are found in Snowshoe Mountain Tuff beneath the moat sediments; in detrital-magnetite-bearing graded tuffs near the bottom of the moat fill; in an ash-fall deposit about 200 m stratigraphically above the top of core 2; and in postcaldera lava flows of the Fisher Dacite that overlie the Creede Formation. Normal polarity also characterizes detrital-magnetite-bearing tuff and sandstone units within the caldera moat rocks that did not undergo severe sulfidic alteration. The combination of initially low magnitude of remanent magnetization and the destructive effects of subsequent diagenetic sulfidization on detrital iron oxides results in a poor paleomagnetic record for the fine-grained sedimentary rocks of the Creede Formation. These finegrained rocks have either normal or reversed polarity magnetizations that are carried by magnetite and/or maghemite. Many more apparent reversals are found than can be accommodated by any geomagnetic polarity time scale over the interval spanned by the ages of the bracketing extrusive rocks. Moreover, opposite polarity magnetizations are found in specimens separated by only a few centimeters, without intervening hiatuses, and by specimens in several tuff beds, each of which represents a single depositional event. These polarity changes cannot, therefore, be attributed to detrital remanent mag- netization. Many polarity changes are apparently related to chemical remanent magnetizations carried by postdepositional magnetite and maghemite that formed in rocks in which most or all detrital magnetic iron oxide was destroyed. Incipient oxidation of early diagenetic pyrite may have produced the secondary magnetic iron oxides. The 40Ar/39Ar dates on the normal polarity Snowshoe Mountain Tuff (26.89 ± 0.05 Ma, 1 σ ) and on the normal polarity postcaldera Fisher lava flows (as young as 26.23 ± 0.05 Ma, 1 σ ) indicate that deposition of the Creede Formation spanned about 340-660 k.y. The intermittently defined normal polarity magnetization for the caldera-fill sequence, compared with the different versions of the geomagnetic polarity time scale, is consistent with the shorter time span.
AB - Sedimentary and volcaniclastic rocks of the Oligocene Creede Formation fill the moat of the Creede caldera, which formed at about 26.9 Ma during the eruption of the Snowshoe Mountain Tuff. Paleomagnetic and rock magnetic studies of two cores (418 and 703 m long) that penetrated the lower half of the Creede Formation, in addition to paleomagnetic and isotopic dating studies of stratigraphically bracketing volcanic units, provide information on the age and the time span of sedimentation of the caldera fill. Normal polarity magnetizations are found in Snowshoe Mountain Tuff beneath the moat sediments; in detrital-magnetite-bearing graded tuffs near the bottom of the moat fill; in an ash-fall deposit about 200 m stratigraphically above the top of core 2; and in postcaldera lava flows of the Fisher Dacite that overlie the Creede Formation. Normal polarity also characterizes detrital-magnetite-bearing tuff and sandstone units within the caldera moat rocks that did not undergo severe sulfidic alteration. The combination of initially low magnitude of remanent magnetization and the destructive effects of subsequent diagenetic sulfidization on detrital iron oxides results in a poor paleomagnetic record for the fine-grained sedimentary rocks of the Creede Formation. These finegrained rocks have either normal or reversed polarity magnetizations that are carried by magnetite and/or maghemite. Many more apparent reversals are found than can be accommodated by any geomagnetic polarity time scale over the interval spanned by the ages of the bracketing extrusive rocks. Moreover, opposite polarity magnetizations are found in specimens separated by only a few centimeters, without intervening hiatuses, and by specimens in several tuff beds, each of which represents a single depositional event. These polarity changes cannot, therefore, be attributed to detrital remanent mag- netization. Many polarity changes are apparently related to chemical remanent magnetizations carried by postdepositional magnetite and maghemite that formed in rocks in which most or all detrital magnetic iron oxide was destroyed. Incipient oxidation of early diagenetic pyrite may have produced the secondary magnetic iron oxides. The 40Ar/39Ar dates on the normal polarity Snowshoe Mountain Tuff (26.89 ± 0.05 Ma, 1 σ ) and on the normal polarity postcaldera Fisher lava flows (as young as 26.23 ± 0.05 Ma, 1 σ ) indicate that deposition of the Creede Formation spanned about 340-660 k.y. The intermittently defined normal polarity magnetization for the caldera-fill sequence, compared with the different versions of the geomagnetic polarity time scale, is consistent with the shorter time span.
UR - http://www.scopus.com/inward/record.url?scp=84870956125&partnerID=8YFLogxK
U2 - 10.1130/0-8137-2346-9.77
DO - 10.1130/0-8137-2346-9.77
M3 - Article
SN - 0072-1077
VL - 346
SP - 77
EP - 93
JO - Special Paper of the Geological Society of America
JF - Special Paper of the Geological Society of America
ER -