Magnetic Susceptibility, Biostratigraphy, and Sequence Stratigraphy: Insights into Timing of Devonian Carbonate Platform Development and Basin Infilling, Western Alberta*
By
Michael T. Whalen 1 and Jed Day 2
Search and Discovery Article #30033 (2005)
Posted September 1, 2005
*Oral presentation at AAPG Annual Convention, Calgary, Alberta, June 19-22, 2005.
Click to view presentation in PDF format.
1University of Alaska-Fairbanks, Fairbanks, AK ([email protected])
2Illinois State University, Normal, IL
Abstract
High-resolution magnetic susceptibility (MS) data from slope and basin deposits near Miette and Ancient Wall provide insight into patterns of carbonate platform development and infilling of the Devonian Alberta basin. These MS data, combined with conodont-brachiopod biostratigraphic data and sequence stratigraphy, provides additional control on the relative timing of five major and fifteen higher frequency MS excursions and nine depositional sequences, seven of which correlate to Johnson et al. Devonian Transgressive-Regressive cycles IIa-2 to IIe. Spikes in the MS record coincide to events associated with lowstand or initial transgression. The first positive MS excursion (Thornton Cr. Member, Flume Formation, IIa-2) represents initial mid-Devonian (late Givetian) onlap at Ancient Wall. The remaining Flume consists of two carbonate ramp sequences with fluctuating MS corresponding to T-R cycle IIb-1 (norrisi Zone) and IIb-2. The second significant MS spike (Maligne Formation, Montagne Noir (MN) Zone 4) is associated with an early Frasnian flooding (IIb-3) that resulted in isolated prograding rimmed platforms (lower Cairn Formation) and lower MS values. An MS spike near the base of the Perdrix Formation coincides with the IIc transgression (MN 5) and upper Cairn backstepping. An MS increase in the upper Perdrix/lowermost Mount Hawk Formation signals lowstand/initial transgression of an intra-T-R cycle IIc sea-level event (MN 7?-10), that resulted in platform aggradation (Peechee Member, Southesk Formation). The lower-middle Mount Hawk Fm. (MN 11-12) displays increasing MS values related to clastic influx that continued until swamped by increased highstand carbonate input (Arcs Member, Southesk, IId-1). The uppermost Mount Hawk and overlying Simla/Ronde Members (Southesk) record fluctuating MS values (MN 13, IId-2). The early Famennian Sassenach Formation records the initial triangularis Zone flooding (T-R IIe) with somewhat higher MS values than seen in latest Frasnian carbonate ramp deposits of sequence 8.
Figures with Notes
Sequence 1 coincides with TR IIa-2 in upper disparilis zone. Base of sequence 2 coincides with IIb-1 and the norrisi zone, but top sequence 2/base sequence 3 cannot as yet be pinned down biostratigraphically but could potentially coincide with IIb-2. If so, Sequence 4 represents another intra-IIb sea level event (IIb-3). Sequences 5 and 6 correlate biostratigraphically with TR cycle IIc (MN zones 5-10). The boundary between 5/6 cannot be pinned down accurately but appears to fall in zone 7. The top of sequence 6 coincides with the base of IId-1 at the base of MN 11. Sequence 7 correlates with IId-1 and spans MN 11-12. Sequence 8 correlates with MN IId-2 and was deposited during MN 13. The lowermost portion of the middle triangularis zone is unrepresented, but sequence 9 correlates with the lower portion of IIe.
MS measurements courtesy Brooks Ellwood, LSU on sensitive Kappa bridge. MS signal is controlled by detrital magnetic and paramagnetic mineral content. Most of the detrital material in these rocks is paramagnetic and susceptibilities are very low. It has been postulated that the MS signal in deep marine rocks is generally controlled by global sea level and has been proposed as an oxygen isotope proxy in Tertiary deep marine sediments.
We have documented five long term and 16 short term MS events. Events defined by identifying either maximum or minimum MS values in any one portion of the curve.
Spline smoothed MS signal from Late Devonian condensed sections in Morroco (from Ellwood et al., 1999) compared to data from western Alberta. Morrocan sections are very thin condensed intervals that were measured at 5 cm intervals. Outcrop data, from much thicker sections, were sampled at 0.5 m intervals. Despite the difference in lithofacies and extreme distance, major MS events constrained by biostratigraphy can be correlated.
Conclusions
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Nine depositional sequenced deposited from late Givetian to early Famennian.
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Five major MS excursions, 16 higher frequency events.
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MS signal – detrital proxy, highest during late HST, LST, or early TST.
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Provides higher resolution subdivision than biostratigraphy but is dependent on biostratigraphy for temporal control.
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MS signal useful for long range correlation and may be a proxy for O isotopes or eolian input to the deep sea.