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St. John's 2001
Technical Programme

GS8:  Sedimentology 
Organizers / Organisateurs:  Programme Committee
Room / Salle:  SN2018

Date:  29/05/2001
Time:  8:00 AM
Presenter:  Thomas T. HadlariI

Sedimentology of ca. 1.83 Ga alluvial fan, braided stream, and lacustrine deposits of the Baker Lake Group, Baker Lake Basin, Nunavut

HADLARI, T., Dept. of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, ON, K1S 5B6, thadlari@chat.carleton.ca, and RAINBIRD, R.H., Geological Survey of Canada, 601 Booth St, Ottawa, ON, K1A 0E8, Rrainbir@nrcan.gc.ca

The ca. 1.83-1.79 Ga Baker Lake Group (Dubawnt Supergroup) is an unconformity-bounded volcano-sedimentary succession that was deposited in Baker Lake Basin, a group of generally northeast-trending continental rifts in crystalline basement of the Western Churchill Province of Laurentia.  Lithofacies assemblages representing alluvial fan, braided stream, and lacustrine deposits are well preserved along the southeastern margin of one of these basins (Baker Lake sub-basin).  Alluvial fan deposits comprise cobble conglomerate with locally derived clasts (lithofacies: Gm; Gcd; Gco; Gt; and Stx), and were deposited by transverse drainage systems.  Braided stream deposits include sandstone and conglomerate (lithofacies: Gcm; Gt; and Stx) deposited by transverse and axial drainage systems.  Lacustrine  deposits of sandstone, siltstone, and mudstone (Stx; Sr; FSw; FSl; and Fl) were deposited in local depocentres along the axis, and at the main depocentre, of the Baker Lake Sub-Basin half-graben.

Bedsets from the alluvial fan facies assemblage have erosional bases, and comprise weakly stratified clast-supported conglomerate passing upward into trough-filling conglomerate or trough cross-stratified sandstone.  Bedsets stack in self-similar fining-upward units on 5-10 m scale and are bounded by larger-scale erosional surfaces, interpreted as fourth-order sequence boundaries.

Bedsets from the braided stream facies assemblage consist of massive conglomerate or trough cross-stratified sandstone overlain by thin mudstone or fine sandstone containing reverse-graded, wind-ripple lamination.  Bounding surfaces are erosional and are commonly marked by a pebble lag. Sets of bedsets fine upward over ~10 m and are interpreted as fourth-order sequences.

3-5 m thick bedsets from the lacustrine facies are characterized by an erosional base, usually with a thin pebble layer, overlain by laminated siltstone and mudstone, which passes upward into trough cross-stratified sandstone. The bedsets are interpreted as lacustrine to fluvial shoaling-upward packages with a flooding surface overlying a transgressive surface of erosion and a transgressive lag.  As such, they are analogous to marine shoaling-upward packages (parasequences) or fourth-order sequences.

Fourth-order sequences in the alluvial fan deposits are well organized in terms of a repetition of stratigraphic motif, as coarse detritus was mobilized intermittently during flash floods.  Braided streams were less ephemeral and hence the deposits are more homogenous.  Lacustrine deposits record flooding events followed by progradation of fluvial or eolian systems.  Identification of the sedimentary paleoenvironments enables a dynamic reconstruction of the Baker Lake sub-basin as the half-graben underwent pulses of subsidence, and the locus of sedimentation migrated from the basin margins to the basin centre.