Regional Geology, Sioux Lookout Orogenic Belt - Geology Ontario

Pillows are locally abundant and form laterally continuous units. Physical volcanological details
include: 1) concentric layering within pillows (or pillow tubes?); 2) pillow budding (lateral sequential
development of pillows via breached margins); 3) downward pointing keels; 4) thin selvages; 5) vesicles
(recording exsolution of gas in a shallow water environment); 6) large (1 m) pillows; 7) pillow breccia
(flow top breccia, plus some flow margin or flow snout rubble facies); 8) one example of a flow with
pillow imbrication (pillows were rolled forward, flow to the south); 9) spalled--off hyaloclastic pillow rims;
10) rare concentric zonation of varioles within pillows; and 11) rare gabbroic areas (co--magmatic dikes?).
Pillow structures are characteristic of subaqueous flows, and are usually assumed to be marine
deposits. For parts of this western andesitic succession, the possibility of a lacustrine paleoenvironment for
the pillowed flows merits consideration. It is well known that volcanic eruptions often disturb fluvial
drainage and create lakes (e.g., Cas and Wright 1987). This situation would allow for a minor amount of
subaqueous units within a predominantly subaerial succession. Alternatively, this predominantly
volcaniclastic succession may have been submerged by marine transgressions during some periods of
mafic--intermediate (basaltic andesite) flow deposition.
OTHER LITHOLOGIES
Quartz porphyry dikes with coarse (0.5–1 cm) phenocrysts, and some finer--grained felsic to granitic dikes,
are rare.
The absolute age of these felsic dikes is unknown, but the quartz porphyry dikes appear to cross--cut
the youngest pyroclastic unit in the Northeast Bay area. It would be of interest to see whether these dikes
correspond to a younger stage of more felsic volcanism (e.g., Stage 4), or are related to postvolcanic
granitic intrusions (e.g., Stage 6; for stages, se below). In the latter case, the dikes might record the
“stitching together” of components of the orogenic belt by granitic intrusions.
Rocks identified during the present study as coarse--grained mafic--intermedite volcanic rocks and a
very different looking coarse intrusive trondjhemite--diorite were all classed as “diorite” by Johnston
(1972). A brief description of the dioritic intrusive body in the Northeast Bay area is given below.
Johnston (1972) grouped mafic pillow breccia with intermediate tuff--breccia as “mafic--intermediate
agglomerate”. Distinction of these lithologies by Devaney et al. (1995b) improves stratigraphic resolution
of this portion of the map area.
Interpretation of Paleoenvironments and Stratigraphic Trends
As also noted in previous preliminary reports (Devaney and Babin 1996, Devaney 1998, 1999a), the
present study interprets the upper andesitic, predominantly pyroclastic succession as proximal to medial,
subaqueous to subaerial deposits with minor sedimentary reworking. Evidence of hot deposition is rare, but
the unreworked nature (e.g., angular clasts, poor sorting, poor stratification) of much of the strata suggests
that they are indeed pyroclastic (or slightly reworked, more equivocal “volcaniclastic” facies), rather than
more fully reworked sedimentary facies. Thin pillowed units are thought to be subaqueous deposits, versus
the thicker units of coarse fragmental facies, most of which are interpreted to be subaerial deposits. The
latter coarse, supposedly subaerial units do not contain features such as mudstone beds or units, exhalite
layers, and common quenched lava fabrics which might indicate a subaqueous paleoenvironment, and do
not contain deposits indicative of shallow water (above wave base) conditions (e.g., wave--formed
structures such as wave ripples, hummocky cross--stratification, and crossbedded shoreface sequences).
Primary structures potentially diagnostic of deposition in a subaerial environment (interpreted above)
include: antidune laminae in pyroclastic surge beds, heat welded rims on blocks, airfall bomb clasts,
subaerial vapour phase crystallization in lapillistone, aa flows lacking quench fabrics, a colluvial talus
wedge adjacent to a mega--block, asymmetric current ripples with paleocurrents approximately to the north
(suggestive of fluvial paleoflow away from the inferred proximal volcanic vent area), and well stratified
fluvial beds with laminated conglomerate (open framework--filling laminae) and notably rounded clasts.
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