Regional Geology, Sioux Lookout Orogenic Belt - Geology Ontario

Interpretation
The conglomeratic assemblage displays features typical of deposition in a proximal braided river
environment (e.g., Eriksson 1978, Boothroyd and Nummedal 1978, Miall 1978, Rust 1978, Devaney
1987). The thick sandy horizons (Turner and Walker 1973) should represent a more medial to distal braided
fluvial facies. (Note: older references on the sedimentology of braided river deposits have been used
intentionally; the generally poor quality of the small exposures does not provide the level of resolution
necessary for the more sophisticated and contemporary “fluvial architecture” approach.) Some
conglomerates deposited in deep marine settings can resemble some coarse fluvial facies (Hein 1984), but
the features of a resedimented or turbiditic facies assemblage (Hein 1984, Ojakangas 1985) are not present
in the Ament Bay Formation
Turner and Walker (1973) considered the Ament Bay Formation to consist of alluvial fan rather than
fluvial deposits, but their interpretation was based on criteria which now, as a result of subsequent research
on fluvial facies models (see references above), would be considered antiquated and incorrect. Alluvial fan
sequences usually contain sediment gravity flow and hyperconcentrated flow deposits (Blair and
McPherson 1994), and such facies do not appear to be present in the Ament Bay Formation. However, it
should be remembered that, based on observations of modern fan settings, alluvial fans can be constructed
partly or wholly by braided streams (Schumm 1977).
Sedimentological details, mostly from the best exposures, at Ament Bay, include: 1) cobble bands,
with no small pebbles present, along scour surfaces (suggestive of high--energy winnowed lag surfaces);
2) a fining-- and thinning-- upward sequence 2.8 m thick (typical of thin braided river channel fill
sequences); 3) an unusual example of clast imbrication; in this case, disc--shaped clasts lie flat along foreset
surfaces and thus dip toward the paleoflow direction, or downflow, opposite to the typical upflow--dipping
clast imbrication fabric produced via traction of clasts in gravelly braided rivers; 4) subtle top indicators;
5) wedging of beds (produced by thinning near channel margins and/or erosional truncation by overlying
beds); 6) an intra--outcrop horizon of thin conglomerate beds shingled obliquely to the main strike of
bedding (low--angle cross--stratification or lateral accretion surfaces); 7) beds in which the larger clasts are
well rounded (in both two-- and three--dimensional views) versus smaller, more angular clasts (suggesting
that smaller clasts were less abraded during transport and primary clast shapes are well preserved); and
8) the largest clast observed, a 1.40 by 1.05 m granitoid boulder (on an island at central Abram Lake).
Sedimentological details in the western sandstone--dominant unit include: 1) rare sigmoidal foresets;
2) overturned foresets (tractional drag--induced soft-- sediment deformation); 3) downclimbing foresets
(deposition on downstream accretion surfaces); 4) argillite--clast breccia (intraclastic channel floor lag);
5) load structures and convoluted laminae (balls, loads and flames); 6) mudstone laminae (slack water or
waning stage mud drapes); 7) a tiny dewatering pipe; 8) “ripple trains” (i.e., beds composed of a regularly
spaced series of asymmetric ripples); 9) odd nodules (concretions?); 10) desiccation cracks; 11) a 1 m thick
bed composed mostly of mafic clasts (sourced from a highly local (“short--headed”) stream or weathered
mafic boulder?); and 12) possible fining--upward sequences. The various well preserved sedimentary
structures top to the south.
In this western area, ripple paleocurrents are to the west, versus crossbed foresets indicating paleoflow
to the east. The small number of observations makes any paleocurrent analysis difficult and unreliable. Two
speculative scenarios are that the ripples could be: 1) the result of eolian reworking on the subaerially
exposed tops of sandbars; or 2) tidal deposits (e.g., tidal flood stage--oriented ripples versus fluvial
crossbeds?). No evidence diagnostic of eolian or tidal environments was noted.
There may be a proximal (conglomeratic) to the east, distal (sandy) to the west pattern in the Ament
Bay Formation in the study area, but stratigraphic resolution is severely limited by the paucity of exposure
and potential stratigraphic and structural complexities.
The highly polymict suite of clast lithologies implies a fairly complete or diverse erosional sampling of
greenstone belt--type lithologies, such as would result from synorogenic erosion of a greenstone belt (which
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