Calibration of a Sequence Stratigraphic Model Using Astronomically Tuned Magnetic Susceptibility Data: Upper Devonian, Western Canada
Cyclostratigraphy and astromonomical tuning of high-resolution magnetic susceptibility (MS) data through eight depositional sequences, in Upper Devonian rocks from western Canada, provides new insight into the pattern and timing of sequence deposition. Identification of Milankovitch periodicities within the MS data permitted documentation of sixteen 400 kyr long eccentricity cycles within the Frasnian stage of the Upper Devonian. Our prior sequence stratigraphic analysis, along the southeast margins of the isolated Miette and Ancient Wall platforms, identified eight depositional sequences spanning the uppermost Givetian up to the Frasnian-Famennian boundary. These sequences were defined based on geometries, facies stacking patterns, and important surfaces indicating maximum flooding and sequence bounding unconformities. Sequences 1-3 record a prograding and aggrading, regionally extensive carbonate ramp and are much thinner than the five overlying sequences. Sequence 4 records initiation of isolated platform development while sequences 5 and 6 display backstepping and then aggrading platform margins. Sequences 7 and 8 record a second episode of carbonate ramp progradation and both are capped by subaerial exposure surfaces. Based on prior, poorly constrained biochronology, and the overall duration of the Frasnian stage, we interpreted the sequences to be on the order of 1-2 Myr duration, i.e. third order sensu Vail. This duration is partially supported by the cyclostratigraphic analysis but our work indicates that sequences range in duration from about 400-1600 kyr. Based on biostratigraphy it was unclear whether the three older, thinner sequences indicated similar durations to younger thicker sequences but this analysis indicates that the thinner sequences, spanning the uppermost Givetian to lower Middle Frasnian, are shorter duration (∼400 – 600 kyr) and thus higher frequency events compared to the upper Middle and Upper Frasnian sequences. These younger Frasnian sequences range in duration from about 800-1600 kyr and are interpreted as third order. The three older sequences, being significantly less than 1 Myr in duration, are interpreted to be fourth order and are nested within a single third order transgressive-regressive cycle. Based on the duration of these third order sequences and the periodicity of Earth's orbital parameters, sequences may in part be controlled by the long-term modulation of obliquity that displays a 1.2 Myr cycle.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014