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 

Today I’d like to present some results from ongoing research on the Devonian of western Canada. My colleague Jed Day and I are building upon earlier sequence stratigraphic analysis that I, along with colleagues Gregor Eberli, Frans van Buchem, and Peter Homewood, conducted during the late 1990’s. We have further documented additional depositional sequences, enhanced our understanding of the biostratigrpahy that is crucial for correlation regionally and globally and are now collecting data on magnetic susceptibility of slope and basin sediments to further improve correlations.

During the Late Devonian North America was at near equatorial latitudes and a system of attached and isolated carbonate platforms developed in western Canada. Our research has centered on the SE margins of two isolated platforms in western Alberta. Miette was approximately 165 km² while Ancient Wall was substantially larger at about 1200 km². Slope and basin successions are approximately 400 m thick. The red lines indicate the location of stratigraphic cross sections that I’ll illustrate and the red dots indicate the locations of slope and basin successions sampled for magnetic susceptibility.

The stratigraphic nomenclature for the Devonian differs both between this surface and subsurface and platform and basin. Outcrop platform units include The Flume, Cairn, and Southesk Formations. The Southesk is subidivided into the Peechee, Arcs, and Ronde/Simla members. Basinal facies include the Maligne, Perdrix, Mount Hawk, and Sassenach formations. The Flume Formation unconformably overlies Cambrian rocks that were transgressed during the late Givetian.

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.

Mineralogy of the Devonian basinal section at Marmot Cirque and MS data from Poachers Creek. Sections are directly correlative but exposed in two different thrust sheets along the SE margin of the Miette Platform.  Total % carbonate and detrital sediment determined with a carbonate bombe. TOC by LECO carbon analyzer. Note that the major MS excursions are directly related to decreased carbonate and increased detrital input.

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.

The Thornton Creek Member represents a new member of the Flume Formation identified along the SE margin of Ancient Wall. It represents the earliest marine transgression of the west Alberta Ridge in western Alberta. It records three carbonate-siliciclastic parasequences, each of which is capped by a more siliciclastic rich facies.

Base of section exposes sequence 1 with slightly elevated MS signal. Other major MS events are associated with lowstand or early transgressive intervals.

All five long-term MS events can be correlated between the area east of Miette to Ancient Wall. Due to cover, not all 16 short-term MS events can be recognized in all sections. Dashed lines are sequence boundaries.

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.

Recent O isotopic data collected from brachiopods in Iowa, Missouri, and Manitoba provide data for comparison with our MS trends. Two small positive excursions in the Upper disparilis zone and MN zone 2 appear to coincide with Sequences 1 and 2 in the Flume Formation. A broad minor positive excursion from MN 9 through 11 corresponds to the major MS increase in the uppermost Perdrix and Mount Hawk (Sequences 6 and 7). A small positive ¹⁸O excursion in zones 12-13 appears to coincide with upper Sequence 7 and 8. Positive excursion near the F-F boundary coincides with MS increase in uppermost Ronde/Simla and lower Sassenach.

Conclusions 

• Nine depositional sequenced deposited from late Givetian to early Famennian.

• Five major MS excursions, 16 higher frequency events.

• MS signal – detrital proxy, highest during late HST, LST, or early TST.

• Provides higher resolution subdivision than biostratigraphy but is dependent on biostratigraphy for temporal control.

• MS signal useful for long range correlation and may be a proxy for O isotopes or eolian input to the deep sea.