Middle and Upper Keweenawan Siliciclastics of the Lake Superior Basin

Abstract

<p>Middle and Upper Keweenawan sediments of the Lake Superior Basin were deposited by braided streams on large, gently sloping, fluvially-dominated alluvial fans.</p> <p>The conglomeratic facies association (upper fan deposits) dominates the Keweenawan succession. The conglomerates are coarse and poorly sorted with fabrics between cIast-supported and (sandy) matrix-supported. Sorting is so poor that the clast-matrix division is arbitrary, so the term c-m supported (from clast and matrix supported) is used. The conglomerates were deposited on longitudinal bars by simultaneous deposition of sand, granules, pebbles, cobbles and boulders. This style of deposition reflects high sediment yields and flashy stream behaviour resulting from the Iack of terrestrial plants. Intercalated sandy lenses with low-angle cross-stratification were deposited in channels by antidunes or poorly developed dunes.</p> <p>Conglomerates of the conglomerate-sandstone facies association (mid-fan) are better sorted and imbricated and show some planar cross-stratification, indicating that longitudinal and transverse bars were present. Intercalated sandstones are trough cross-stratified and fIat-bedded.</p> <p>The sandy facies association (lower fan) comprises trough cross-stratified sandstones (channeI deposits) and flat bedded and rippled sandstones (bar-top deposits).</p> <p>The abundance of trough cross-stratified sandstones implies that slopes on the Keweenawan fans were lower than those indicated by alluvial fan facies models. More attention shouId be paid to large, gently sloping, fluviaIly-dominated alluvial fans.</p> <p>Abrupt transitions from proximal fan conglomerates to lacustrine sediments reflect volcanic damming. More gradual and poorly defined vertical facies transitions resulted from changes in tectonism.</p> <p>The Keweenawan streams were ephemeraI, and desiccation features are abundant. These conditions enabled the development of fields of small transverse aeolian dunes, giving rise to large-scale cross-stratified well sorted sandstones. Aeolian palaeocurrents may indicate topographically constrained palaeo-trade winds.</p>