Geochemical indicators (Collinson of source lithologies and … · 2011. 5. 6. · Geochemical...
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Geochemical indicators of source lithologies and weathering intensities in fine-grained Permian clastics, central Transantarctic Mountains LAWRENCE A. KmSSEK and TIM0rilY C. HoINEI Byrd Polar Research Center and Department of Geology and Mineralogy Ohio State University Columbus, Ohio 43210 During austral summer 1985-1986, we collected approxi- mately 310 samples of fine-grained clastics from 24 measured sections in the Permian sequence of the central Transantarctic Mountains (figure 1, table). Our fieldwork and preliminary results, as well as collaborative efforts by other sedimentolo- gists from Ohio State University and Vanderbilt University, MM MM MM MM MP,MPU R 84S W M A 4, M A 5 M A 6 eo ^4 1 km 50 - qMB 9 165E Figure 1. Geographic distribution of measured sections, Beard- more Glacier area, central Transantarctic Mountains. (See table for definitions of the section designations.) have been summarized in Krissek and Homer (1986, 1987, 1988, in press) and contributions to Antarctic Journal (Collinson and Isbell 1986, 1987, 1988; Isbell and Collinson 1988; Miller and Waugh 1986, 1987; Miller and Frisch 1986, 1987). Our ultimate objective is to extract provenance and paleoclimatic information from the mineral and chemical compositions of the fine-grained sediments in the Permian section, which rec- ords the transition from a glacial regime (Pagoda Formation), through a subaqueous fan/delta setting (Mackellar Formation, to fluvial environments (Fairchild Formation) with coals (Buck- ley Formation). In the past year, our efforts have concentrated on using the geochemistry of approximately 100 samples of fine-grained sediments to evaluate lithologies and weathering regimes of their source regions during the Permian. The aluminum oxide/ titanium oxide (Al 2 0 3/TiO 2 ) ratio has been used as a prelimi- nary indicator of source rock composition (Nesbitt 1979), while the chemical index of alteration (CIA) of Nesbitt and Young (1982; CIA = [Al 20 3 ± (Al 2 0 3 + CaO + Na 2 O + K2 0)] X 100)* has been used to estimate the intensity of weathering. The aluminum oxide/titanium oxide ratio increases from ap- proximately 10 for basalts and gabbros to approximately 47 for rhyolites and granites (Le Maitre 1976), while the chemical index of alteration increases from approximately 50 for unal- tered feldspar to approximately 75 for illite and 100 for kaolin- ite. Neither of these geochemical parameters covary with vitrinite reflectance values for this data set, confirming the relative sta- bility of the major element geochemistry of fine-grained sed- iments during diagenesis. Stratigraphic sections have been grouped geographically (ta- ble), and the geochemical parameters have been examined for geographic and stratigraphic variations within these groups. Average aluminum oxide/titanium oxide ratios range from 17 to 27 for all stratigraphic intervals in all areas (figure 2) indi- cating intermediate average compositions (equivalent to diorite to granodiorite) for the sediment sources. The ratios remain relatively uniform within each region through the Pagoda and Mackellar formations, with lowest values in the southeastern and southern portions of the study area. In all regions except the southernmost, ratios decrease above the Mackellar. The geographic distribution of values suggests that the southern parts of the study area consistently received a larger contri- bution of basic material, supporting the conclusion of Collin- son (in press) that a volcanic influence migrated from the southeast into the Beardmore Glacier region through this time. The general stratigraphic pattern is consistent with the obser- vation that a significant volcaniclastic contribution to the entire Beardmore Glacier region began during Buckley deposition (Barrett, Elliot, and Lindsay 1986; Collinson, Kemp, and Eggert 1987; Collinson in press). The southern portion of the study area does not follow the general stratigraphic trend, and con- tains anomalous Mackellar paleocurrents and mineralogic in- dicators of weathering conditions, suggesting that it records a transition from the Beardmore portion of the Permian depo- sitional basin to the remainder of the basin to the south and southeast. Chemical index of alteration values (figure 3) increase up- section in most geographic regions, recording the transition * CaO denotes calcium oxide; Na 2 0 denotes sodium oxide; K2 0 de- notes potassium oxide. zf, I tOE 83,S 84S 1989 REVIEW 13
Transcript of Geochemical indicators (Collinson of source lithologies and … · 2011. 5. 6. · Geochemical...
Geochemical indicatorsof source lithologies and
weathering intensitiesin fine-grained Permian clastics,central Transantarctic Mountains
LAWRENCE A. KmSSEK and TIM0rilY C. HoINEI
Byrd Polar Research Centerand
Department of Geology and Mineralogy
Ohio State UniversityColumbus, Ohio 43210
During austral summer 1985-1986, we collected approxi-mately 310 samples of fine-grained clastics from 24 measuredsections in the Permian sequence of the central TransantarcticMountains (figure 1, table). Our fieldwork and preliminaryresults, as well as collaborative efforts by other sedimentolo-gists from Ohio State University and Vanderbilt University,
MM
MMMMMM
MP,MPU
R84S W
M A 4, M A 5M A 6
eo ^4 1
km 50 -
qMB
9 165E
Figure 1. Geographic distribution of measured sections, Beard-more Glacier area, central Transantarctic Mountains. (See table fordefinitions of the section designations.)
have been summarized in Krissek and Homer (1986, 1987,1988, in press) and contributions to Antarctic Journal (Collinsonand Isbell 1986, 1987, 1988; Isbell and Collinson 1988; Millerand Waugh 1986, 1987; Miller and Frisch 1986, 1987). Ourultimate objective is to extract provenance and paleoclimaticinformation from the mineral and chemical compositions ofthe fine-grained sediments in the Permian section, which rec-ords the transition from a glacial regime (Pagoda Formation),through a subaqueous fan/delta setting (Mackellar Formation,to fluvial environments (Fairchild Formation) with coals (Buck-ley Formation).
In the past year, our efforts have concentrated on using thegeochemistry of approximately 100 samples of fine-grainedsediments to evaluate lithologies and weathering regimes oftheir source regions during the Permian. The aluminum oxide/titanium oxide (Al 203/TiO2) ratio has been used as a prelimi-nary indicator of source rock composition (Nesbitt 1979), whilethe chemical index of alteration (CIA) of Nesbitt and Young(1982; CIA = [Al203 ± (Al203 + CaO + Na2O + K20)] X
100)* has been used to estimate the intensity of weathering.The aluminum oxide/titanium oxide ratio increases from ap-proximately 10 for basalts and gabbros to approximately 47 forrhyolites and granites (Le Maitre 1976), while the chemicalindex of alteration increases from approximately 50 for unal-tered feldspar to approximately 75 for illite and 100 for kaolin-ite. Neither of these geochemical parameters covary with vitrinitereflectance values for this data set, confirming the relative sta-bility of the major element geochemistry of fine-grained sed-iments during diagenesis.
Stratigraphic sections have been grouped geographically (ta-ble), and the geochemical parameters have been examined forgeographic and stratigraphic variations within these groups.Average aluminum oxide/titanium oxide ratios range from 17to 27 for all stratigraphic intervals in all areas (figure 2) indi-cating intermediate average compositions (equivalent to dioriteto granodiorite) for the sediment sources. The ratios remainrelatively uniform within each region through the Pagoda andMackellar formations, with lowest values in the southeasternand southern portions of the study area. In all regions exceptthe southernmost, ratios decrease above the Mackellar. Thegeographic distribution of values suggests that the southernparts of the study area consistently received a larger contri-bution of basic material, supporting the conclusion of Collin-son (in press) that a volcanic influence migrated from thesoutheast into the Beardmore Glacier region through this time.The general stratigraphic pattern is consistent with the obser-vation that a significant volcaniclastic contribution to the entireBeardmore Glacier region began during Buckley deposition(Barrett, Elliot, and Lindsay 1986; Collinson, Kemp, and Eggert1987; Collinson in press). The southern portion of the studyarea does not follow the general stratigraphic trend, and con-tains anomalous Mackellar paleocurrents and mineralogic in-dicators of weathering conditions, suggesting that it records atransition from the Beardmore portion of the Permian depo-sitional basin to the remainder of the basin to the south andsoutheast.
Chemical index of alteration values (figure 3) increase up-section in most geographic regions, recording the transition
* CaO denotes calcium oxide; Na 20 denotes sodium oxide; K20 de-notes potassium oxide.
zf,
I tOE
83,S
84S
1989 REVIEW 13
Guide to measured sections in the Beardmore Glacier area, units at each section, and their geographic regions.
Section Locality Units Region
MK Mount Korsch Mackellar, Fairchild NMMC Mount Weeks Pagoda, Mackellar NWMMQ Moore Mountains Mackellar NWMMP Moore Mountains Pagoda NWMMA Moore Mountains Mackellar, Fairchild NWMMD Helm Glacier Mackellar, Fairchild, Buckley NETR Turnabout Ridge Pagoda, Mackellar, Fairchild NECP Clarkson Peak Pagoda, Mackellar NEMMZ Mount Miller Pagoda, Mackellar NEWG Wahl Glacier Buckley CentralTGA Tillite Glacier Pagoda, Mackellar CentralTGF Tillite Glacier Mackellar, Fairchild CentralMP Mount Picciotto Buckley CentralMA4 Mount Archernar Upper Buckley SWMA5 Mount Archernar Middle Buckley SWMA6 Mount Archernar Lower Buckley SWLP Lamping Peak Buckley SWMR Mount Ropar Buckley SWMD Mount Deakin Mackellar, Fairchild SECl Cherry Icefall Pagoda, Mackellar SEMB Mount Bowers Mackellar, Fairchild, Buckley S
AVERAGE Al 20 3 /Ti 02 RATIOSNNWNECENTRALSWSES
15
25 1525 1525 1525 1525 1525 1525II1IIIIIIIILlitIItII
UpperBuckley
LowerBuckley
Fairchild
UpperMackellar
MiddleMackellar
LowerMackellar
UpperPagoda
LowerPagoda
IFigure 2. Stratigraphic profiles of aluminum oxide/titanium oxide ratio by geographic region in the Beardmore Glacier area. Geographicgroupings are outlined in the table. Lower ratios reflect the increasing importance of basic source rocks.
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AVERAGE CIA VALUES
NNWNECENTRALSWSES
70UpperBuckley _]
LowerBuckley
Fairchild
UpperMackellar IMiddleMackellar
LowerMackellarUpperPagoda
LowerPagoda
80708070807080708070807080_....Jt1ItIIIIIIIIIIIII_I
/
Figure 3. Stratigraphic profiles of the chemical index of alteration (Nesbitt and Young 1982) by geographic region in the Beardmore Glacierarea. Geographic groupings are outlined in the table. Increasing chemical index of alteration values record the increasing importance ofchemical weathering.
from glacially dominated weathering in the pagoda sources tochemically dominated weathering during Buckley deposition.Variations in the style and stratigraphic position of the increasebetween regions are interpreted to reflect local variability inweathering regime, and have been recognized previously inthe clay mineralogy of these samples (Krissek and Homer 1987).The upsection decrease in chemical index of alteration valuesin the northern and northwestern portions of the study areaprobably reflects the general coarsening of Mackellar and Fair-child lithologies in that direction (Barrett, Elliot, and Lindsay1986; Homer and Krissek 1987), so that the chemical index ofalteration is controlled more by grain size-dependent changesin mineralogy than by weathering influence on equivalentgrainsizes.
These preliminary results indicate that the mineral andchemical compositions of fine-grained sediments can providevaluable provenance information in the study of a depositionalbasin. More detailed examinations of the geochemical data arepresently underway (Homer in preparation). This project wasfunded by National Science Foundation grant DPP 84-18354.
References
Barrett, P.J., D.H. Elliot, and J.F. Lindsay. 1986. The Beacon Super-group (Devonian-Triassic) and Ferrar Group (Triassic) in the Beard-more Glacier area, Antarctica. In M.D. Turner and J. F. Splettstoesser(Eds.), Geology of the Central Transantarctic Mountains. (Antarctic Re-search Series, Vol. 36.) Washington, D.C.: American GeophysicalUnion.
Collinson, J.W. In press. The paleo-Pacific margin as seen from EastAntarctica. In M.R.A. Thomson, J.A. Crame, and J.W. Thomson(Eds.), Geologic evolution of Antarctica. Cambridge: Cambridge Uni-versity Press. Collinson, J. W., and J. L. Isbell. 1986. Permian-Triassicsedimentology of the Beardmore Glacier region. Antarctic Journal ofthe U.S., 21(5), 29-30.
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Collinson, J.W., and J.L. Isbell. 1988. Sedimentologic comparison ofPermian post-glacial black shale sequences in the Ellsworth Moun-tains and Beardmore Glacier region. Antarctic Journal of the U.S.,23(5), 5-7.
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Collinson, J.W., N.R. Kemp, and J.T. Eggert. 1987. Comparison of theTriassic Gondwana Sequences in the Transantarctic Mountains andTasmania. In G.D. McKenzie (Ed.), Gondwana Six: Stratigraphy, Se-dimentology, and Paleontology. (Geophysical Monograph Series, Vol.41.) Washington, D.C.: American Geophysical Union.
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Krissek, L.A., and T.C. Homer. 1986. Sedimentology of fine-grainedPermain clastics, central Transantarctic Mountains. Antarctic Journalof the U.S., 21(5), 30-32.
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Miller, 1MG., and B.J. Waugh. 1986. Sedimentology of the PagodaFormation (Permian), Beardmore Glacier area. Antarctic Journal of theU. S., 21(5), 45-46.
Miller, 1MG., and B.J. Waugh. 1987. Paleotectonic implications ofthe Permo-Carboniferous Pagoda Formation, Beardmore Glacier area.
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mian) Mackellar Formation, Beardmore Glacier area. Antarctic Journalof the U.S., 21(5), 37-38.
Miller, M.F., and R.S. Frisch. 1987. Early Permian paleogeographyand tectonics of the central Transantarctic Mountains: Inferencesfrom the Mackellar Formation. Antarctic Journal of the U.S., 22(5), 24-25.
Nesbitt, H.W. 1979. Mobility and fractionation of rare earth elementsduring weathering of a granodiorite. Nature, 279, 206-210.
Nesbitt, H.W., and G.M. Young. 1982. Early Proterozoic climate andplate motions inferred from major element chemistry of lutites. Na-ture, 299, 715-717.
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