HR: 16:45h
AN: GP34A-04    [Abstracts]
TI: Paleomagnetism of ~635 Ma "Elatina Rhythmites," Reconsidered
AU: * Raub, T D
EM: timraub@caltech.edu
AF: Caltech, 1200 E. California Blvd. Division of Geological and Planetary Science, Pasadena, CA 91125,
AU: Raub, T M
EM: tmdraub@caltech.edu
AF: Caltech, 1200 E. California Blvd. Division of Geological and Planetary Science, Pasadena, CA 91125,
AB: Extraordinary laminated sand-siltstone of South Australia's synglacial/deglacial Elatina Formation-equivalent at Pichi Richi Pass is an iconic outcrop for paleomagnetists, paleoclimatologists, and astrophysicists. Cyclic- bedded stacks of coarse/fine-grained doublets have been interpreted as rhythmites of semi-diurnal tidal origin in a deep-water delta front environment. Cycle analysis has yielded estimates of Earth's paleorotation and revolution periods and lunar distance. The reported 10 m (interpreted 60 year) record disagrees somewhat with coeval and younger biological "sediment-clocks," and Williams (2000) argues that the sedimentological context and extraordinary preservation of Elatina rhythmites makes that record preferable. A paleomagnetic "synsedimentary fold" test on <8 cm amplitude antiforms from the Pichi Richi Pass rhythmites confirmed low-latitude Neoproterozoic glaciation (Sumner et al., 1987) and inspired the formal "Snowball Earth" hypothesis (Kirschvink, 1989). Repeated study of the Elatina rhythmites (Schmidt and Williams, 1995 and citations therein) has documented NRM essentially equal to ChRM carried by detrital hematite, "locked-in" synchronously with "deformation" of "antiforms" (i.e., within hours to days of deposition). "Synfolding" remanence has been ascribed to superposition of slump-shear and DRM. Anomalously shallow inclination of rhythmites (Sohl et al., 1999) could represent inadequate averaging of ca. 635 Ma geomagnetic secular variation. New measurement of ten stratigraphic sections through rhythmite-bearing facies outside of Pichi Richi Pass supports a distributary environment since rhythmites are expressed in multiple lithologies closely beneath ultimate deglacial, Nuccaleena cap dolostone. At Pichi Richi Pass, a full section of rhythmites approx. 18 meters thick is now documented, supporting a novel magnetostratigraphic collection of 130 samples. It is necessary to re-examine the DRM interpretation of Elatina rhythmite magnetization, however, in light of new observations: The structures previously considered "slump folds" are in fact sedimentary ripple crests. At Pichi Richi Pass, rhythmite facies grade upward from undisturbed, planar lamination into disorganized dunes, then low- wavelength, low-amplitude, linear-crested ripples, increasing steadily in wavelength and amplitude and climbing upward in >50 cm packages to culminate in unambiguous ladderback ripples. This shallowing-upward sequence reflects high sediment supply over 60 years and/or else significant sea level regression during (possibly longer) rhythmite deposition. The exceptional character of climbing-ripple rhythmites can be explained by pervasive microbial binding of sediment, in the anomalous mode characterizing many Ediacaran siliciclastic environments. Detrital magnetization seemingly should not around ripple crests. Either the interval of Snowball Earth deglaciation was characterized by unusually low geomagnetic field strength, biasing magnetization fidelity, or else Elatina rhythmites do not record DRM over a continuous 60-year timescale. This latter alternative may find support in the original studies, which document considerable dispersion, unexpected for a VGP. Kirschvink, 1992. in Schopf, J.W. and Klein, C., eds. The Proterozoic Biosphere, Cambridge University Press, Cambridge, pp. 51-52. Schmidt and Williams, 1995. Earth and Planetary Science Letters, v. 134, pp. 107-124. Sohl, Christie-Blick, and Kent, 1999. Geological Society of America Bulletin, v. 111, pp. 1120-1139. Sumner, Kirschvink, and Runnegar, 1987. EOS, v. 68, pg. 1251. Williams, 2000. Reviews of Geophysics, v. 38, pp. 37-59.
DE: 1520 Magnetostratigraphy
DE: 1521 Paleointensity
DE: 1522 Paleomagnetic secular variation
DE: 1533 Remagnetization
DE: 4926 Glacial
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting