ABSTRACT: Third-Order Vertical Variations in Parasequence Character of the Lower Gray Craggy Member, Katakturuk Dolomite (Proterozoic), Northeastern Brooks Range, Alaska

CLOUGH, JAMES G., Alaska Division of Geological and Geophysical Surveys, Fairbanks, AK, and ROBERT K. GOLDHAMMER, Exxon Production Research Company, Houston, TX

The Katakturuk Dolomite is a very thick succession (> 2400 m) of subtidal to peritidal cyclical carbonates deposited on a late Proterozoic passive margin ramp. The Katakturuk Dolomite, exposed in the anticlinal cores of the Sadlerochit and Shublik Mountains (northeastern Brooks Range, Alaska), is subdivided into second-order supersequences (approximately 500 m thick; 25 m.y. duration). Each supersequence contains several third-order depositional sequences (100-150 m thick; 5 m.y. duration), which roughly correspond to 15 informal formation members.

Vertical stacking patterns of meter-scale, high-frequency (fifth/fourth-order) parasequences within a third-order package have been analyzed to characterize systems tract development within the 100 m thick Lower Gray Craggy member, deposited in the middle of the Katakturuk Dolomite succession. We recognize two main types of parasequences: (1) subtidal parasequences--characterized by thick cross-bedded, amalgamated, intercalated grainstone and laterally-linked-to-discrete columnar stromatolites, and (2) peritidal parasequences--consisting of a basal subtidal cross-bedded oolitic grainstone capped by peritidal laminite. Evidence of high-energy, storm-related deposition and erosion indicate that some cycle caps may have been eroded. Peritidal laminite caps commonly consist of algal tufa freshwater?). Several intercalated small-scale (<1-2 m thick) karst horizons are noted in the vicinity of third-order boundaries.

Within a third-order sequence, parasequences systematically stack from thin peritidal cycles with karst horizons (late highstand to lowstand), to upward-thickening subtidal cycles (transgressive systems tract), to upward-thinning subtidal to peritidal cycles (highstand systems tract). In addition, stromatolite morphology and grainstone-interval thickness vary systematically within a sequence. These features and parasequence variation reflect progressive changes in third-order accommodation-space. The majority of sequences appear to be symmetrical, suggesting third-order sequence development on a thermally subsiding passive margin.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)