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39 mineral ages
CURRENT LITERATURE SURVEY GEOCHRONOLOGY Chronology of nappe assemblage in the Pan-African Dahomeyide orogen, West Africa: evidence from 40Sr/39 mineral ages Attoh K., Dallmeyer R.D. & Affaton P., Precambrian research, 1997, 82/l -2 ( 153- 17 l).The Pan-African Dahomeyide orogen represents the SW segments of the E tectonic zone along which the West African Craton was incorporated into’ Gondwana. Orogenic contraction produced nappe complexes comprised of passive margin sediments and accreted exotic magmatic rocks in Ghana and Togo. External nappes include the Atacora, composed of lower amphibolite facies quartzite and mica schist which were structurally imbricated with mylonitic gneiss derived horn c.2.0 Ga foreland basement. The suture zone is represented by high-pressure granulite facies garnet hornblende gneiss. Muscovite and hornblende concentrates form external and suture-zone nappes were analyzed in incremental heating 40Ar/39Ar experiments. The age data are consistent with coeval forward- and hindward-propagating thrusting and nappe imbrication, and constrain the amalgamation in northwest Gondwana to have occurred in the latest Proterozoic rather than Cambrian.
SEDIMENTARY
GEOLOGY
Sequence stratigraphic analysis of pre-Tertiary succession in Ganga Basin Singh A.K. & Singh J.N., Journal - Geological Societyoflndia, 1997, 49/6 (629-646).The paper defones the Pre-Tertiary stratigraphy of the Ganga Basin which is concealed beneath the thick cover of Cenozoic sediments. The sub-surface structural configuration is characterised by development of four major depressions, separated by transverse uplifts. Ganga Basin represents two main stages of geological evolution: apre-collisional (Pre-Tertiary) stage consisting mainly of Proterozoic and Gondwana sediments, and a post-collisional (Tertiary) stage representing the Siwalik molasse. The Pre-Tertiary succession is classified into four mega sequences, three of these representing Precambrian sediments, comprising of Aravalli, Bijawar, Delhi and Vindhyan systems and one megasequence corresponding to Gondwana system in the Purriea Depression.
STRATIGRAPHY Gondwanan floristic and sedimentological trends during the Permian-Triassic transition: New evidence from the Amery Group, northern Prince Charles Mountains, East Antarctica McLaughlin S., Lindstrom S. & Drinnan A.N., Antacrtic Science, 1997, 9/3 (281-298).The Permian-Triassic boundary within the Amery Group of the Lambert Graben is placed at the contact between the Bainmedart Coal Measures and overlying Flagstone Bench Formation, based on the first regular occurrence of Lumatisporites pellucidus and the first appearance of Aratrisporites and Lepidopteris species. The Permian-Triassic boundary is marked by the extinction of glossopterid and cordaitalean gymnosperms, and by the disappearance or extreme decline of a range of gymnospermous and pteridophytic palynomorph groups. Earliest Triassic macrofloras and palynofloras of the Flagstone Bench Formation are dominated y peltasperms and lycophytes, corystosperms, conifers, and ferns become increasingly common elements of assemblages through the Lower Triassic part of the formation and dominate floras of the Upper Triassic strata. The sedimentary transition across this boundary is conformable but marked by a termination of coal deposits; overlying lowermost Triassic sediments contain only carbonaceous siltstones. Typical red-bed facies are not developed
until at least 100 m above the base of the Flagstone Bench Formation, in strata containing ?Middle Triassic palynofloras. Across Gondwana the diachronous disappearance of coal deposits and appearance of red-beds is suggestive of a response to shifting climatic belts, resulting in progressively drier seasonal conditions at successively higher palaeolatitudes during the Late Permian to Middle Triassic. The abrupt and approximately synchronous replacement of plant groups at the Permian-Triassic boundary suggests that factors independent of, or additional to, climate change were responsible for the turnover in terrestrial floras. Stratigraphic record of the early Mesozoic breakup of Pangea in the Laurasia-Gondwana rift system Olsen P.E., Annual Review of Earth and Planetary Sciences, 1997, 25/- (337-401).Rift basins ofthe Central Atlantic Margins (CAM) of North America and Morocco preserve largely continental sequences of sedimentary strata and less important minor basalt flows spanning much of the early Mesozoic. The best known is the Newark basin of New Jersey, New York, and Pennsylvania where an astronomically cahbratedmagnetic polarity time scaIe is developed. Lacustrine cycles of Milankovitch origin are commonly present in CAM basins, with the period changing from 10 ky (paleoequator with coals), to 20 ky (4”-lOoN), to perhaps 40 ky northward with evaporites. Four mostly unconformity-bounded tectonostratigraphic sequences are present. The North American plate’s slow northward drift resulted in a relative shift of climate, although the rapid humidification during the latest Triassic and Early Jurassic is associated with a sea-level rise. The Triassic-Jurassic mass extinction is of independent origin, plausibly impact related. Nonmarine sedimentation in an early Cretaceous extensional continental-margin arc, Byers Peninsula, Livingston Island, South Shetland Islands Hathway B., Journal of Sedimentary Research B: Stratigraphy and Global Studies, 1997, 6714 (686-697).Upper Jurassic-Lower Cretaceous rocks of the Byers Group, exposed on Byers Peninsula, Livingston Island, record the expansion of Gondwana-margin continental-arc facies nito a marine intr-arc basin. At least 1.3 km of marine elastic rocks are overlain by about 1.4 km of Lower Cretaceous nonmarine volcani-elastic strata assigned to the Cerro Negro Formation . The base of the nonmarine sucession is marked by a low-angle unconformity. The lower 200-240 m is largely pale green- and gray-weathering, and consists mainly of welded and nonwelded silicic ignimbrites, with subordinate reworked silicic tuffs and ignimbritic conglomerates. A change in color to dark re-puple at the top ofthis interval broadly coincides with a change to a largely basaltic-intermediate provenance. The rest of the succession consists mainly of poorly sorted lithic lapilli-tuffs and tuffaceous breccias largely interpreted as debris-flow and flood-flow deposits. It also includes two welded silicic ignimbrite units rich in basaltic clasts, and is considered to represent a volcaniclastic apron flanking one or more basaltic andesite stratovolcanoes. Though it is dominated by syneruption deposits, this upper division also includes laterally impersistent, subsidence-driven inter-eruption facies, including basaltic conglomerates deposited in incidsed fluvial channels, monr lacustrine intervals, and rare paleosols. A thicker (loom), peninsulawide, mud-stone/sandstone-dominated horizon represents a more extended period of inter-eruption deposition, during which the area was the site of a substantial lake. Throughout the Cerro negro Formation, thickness and facies changes provide evidence of synsedimentary displacement across a series of ENE-trending normal faults, most with downthrow to the south. In the upper part of the formation, resulting differential subsidence led to southward thickening accompanied by increased preservation of inter-eruption facies, and on a smaller scale trapping of a fluvial channel against the footwall of a synsedimentary fault. This tectonism appears to form part of an Early Cretaceous episode of arc-perpendicular extension well documented elsewhere in the Antarctic Peninsula region.
SEDIMENTARY
GEOLOGY
Sequence stratigraphic analysis of pre-Tertiary succession in Ganga Basin Singh A.K. & Singh J.N., Journal - Geological Societyoflndia, 1997, 49/6 (629-646).The paper defones the Pre-Tertiary stratigraphy of the Ganga Basin which is concealed beneath the thick cover of Cenozoic sediments. The sub-surface structural configuration is characterised by development of four major depressions, separated by transverse uplifts. Ganga Basin represents two main stages of geological evolution: apre-collisional (Pre-Tertiary) stage consisting mainly of Proterozoic and Gondwana sediments, and a post-collisional (Tertiary) stage representing the Siwalik molasse. The Pre-Tertiary succession is classified into four mega sequences, three of these representing Precambrian sediments, comprising of Aravalli, Bijawar, Delhi and Vindhyan systems and one megasequence corresponding to Gondwana system in the Purriea Depression.
STRATIGRAPHY Gondwanan floristic and sedimentological trends during the Permian-Triassic transition: New evidence from the Amery Group, northern Prince Charles Mountains, East Antarctica McLaughlin S., Lindstrom S. & Drinnan A.N., Antacrtic Science, 1997, 9/3 (281-298).The Permian-Triassic boundary within the Amery Group of the Lambert Graben is placed at the contact between the Bainmedart Coal Measures and overlying Flagstone Bench Formation, based on the first regular occurrence of Lumatisporites pellucidus and the first appearance of Aratrisporites and Lepidopteris species. The Permian-Triassic boundary is marked by the extinction of glossopterid and cordaitalean gymnosperms, and by the disappearance or extreme decline of a range of gymnospermous and pteridophytic palynomorph groups. Earliest Triassic macrofloras and palynofloras of the Flagstone Bench Formation are dominated y peltasperms and lycophytes, corystosperms, conifers, and ferns become increasingly common elements of assemblages through the Lower Triassic part of the formation and dominate floras of the Upper Triassic strata. The sedimentary transition across this boundary is conformable but marked by a termination of coal deposits; overlying lowermost Triassic sediments contain only carbonaceous siltstones. Typical red-bed facies are not developed
until at least 100 m above the base of the Flagstone Bench Formation, in strata containing ?Middle Triassic palynofloras. Across Gondwana the diachronous disappearance of coal deposits and appearance of red-beds is suggestive of a response to shifting climatic belts, resulting in progressively drier seasonal conditions at successively higher palaeolatitudes during the Late Permian to Middle Triassic. The abrupt and approximately synchronous replacement of plant groups at the Permian-Triassic boundary suggests that factors independent of, or additional to, climate change were responsible for the turnover in terrestrial floras. Stratigraphic record of the early Mesozoic breakup of Pangea in the Laurasia-Gondwana rift system Olsen P.E., Annual Review of Earth and Planetary Sciences, 1997, 25/- (337-401).Rift basins ofthe Central Atlantic Margins (CAM) of North America and Morocco preserve largely continental sequences of sedimentary strata and less important minor basalt flows spanning much of the early Mesozoic. The best known is the Newark basin of New Jersey, New York, and Pennsylvania where an astronomically cahbratedmagnetic polarity time scaIe is developed. Lacustrine cycles of Milankovitch origin are commonly present in CAM basins, with the period changing from 10 ky (paleoequator with coals), to 20 ky (4”-lOoN), to perhaps 40 ky northward with evaporites. Four mostly unconformity-bounded tectonostratigraphic sequences are present. The North American plate’s slow northward drift resulted in a relative shift of climate, although the rapid humidification during the latest Triassic and Early Jurassic is associated with a sea-level rise. The Triassic-Jurassic mass extinction is of independent origin, plausibly impact related. Nonmarine sedimentation in an early Cretaceous extensional continental-margin arc, Byers Peninsula, Livingston Island, South Shetland Islands Hathway B., Journal of Sedimentary Research B: Stratigraphy and Global Studies, 1997, 6714 (686-697).Upper Jurassic-Lower Cretaceous rocks of the Byers Group, exposed on Byers Peninsula, Livingston Island, record the expansion of Gondwana-margin continental-arc facies nito a marine intr-arc basin. At least 1.3 km of marine elastic rocks are overlain by about 1.4 km of Lower Cretaceous nonmarine volcani-elastic strata assigned to the Cerro Negro Formation . The base of the nonmarine sucession is marked by a low-angle unconformity. The lower 200-240 m is largely pale green- and gray-weathering, and consists mainly of welded and nonwelded silicic ignimbrites, with subordinate reworked silicic tuffs and ignimbritic conglomerates. A change in color to dark re-puple at the top ofthis interval broadly coincides with a change to a largely basaltic-intermediate provenance. The rest of the succession consists mainly of poorly sorted lithic lapilli-tuffs and tuffaceous breccias largely interpreted as debris-flow and flood-flow deposits. It also includes two welded silicic ignimbrite units rich in basaltic clasts, and is considered to represent a volcaniclastic apron flanking one or more basaltic andesite stratovolcanoes. Though it is dominated by syneruption deposits, this upper division also includes laterally impersistent, subsidence-driven inter-eruption facies, including basaltic conglomerates deposited in incidsed fluvial channels, monr lacustrine intervals, and rare paleosols. A thicker (loom), peninsulawide, mud-stone/sandstone-dominated horizon represents a more extended period of inter-eruption deposition, during which the area was the site of a substantial lake. Throughout the Cerro negro Formation, thickness and facies changes provide evidence of synsedimentary displacement across a series of ENE-trending normal faults, most with downthrow to the south. In the upper part of the formation, resulting differential subsidence led to southward thickening accompanied by increased preservation of inter-eruption facies, and on a smaller scale trapping of a fluvial channel against the footwall of a synsedimentary fault. This tectonism appears to form part of an Early Cretaceous episode of arc-perpendicular extension well documented elsewhere in the Antarctic Peninsula region.