Evolution of early Proterozoic passive-margin carbonate platform, Rocknest Formation, Wopmay Orogen, N.W.T., Canada
Abstract
The superbly exposed Rocknest Formation, Wopmay Oroqen,
N.W.T., Canada, is an early Proterozoic (l.B9 Ga) passivemargin
carbonate platform. It is an eastward-thinning
prism, 0 to 1,100 m thick, extending for over 220 km
parallel to strike, and over 200 k.m perpendicular to strike.
It contains from west to east, slope, outer-shelf, shoaloomplex,
and inner-shelf facies. Slope facies are
rhythmites, megabreccias and rhythmite breccias. Outershelf
faciea include stromatolitic reefal boundstone and
backreef ooid/intraclast grainstore. Shoal-complex facies
are shallowing-upward cycles of tufa, cryptalgalaminite,
laminated dolosiltite/lutite, overlain by tepees, breccias
and pisolite; these formed a paleotopographic high,
restricting an eastern "lagoon" from western ocean. Innershelf
facies are dominantly asymmetric, shallowing-upward
cycles of "lagoonal" mixed carbonates and siliciclastics,
that grade up into intertidal stromatolitic dolomites;
cyclic facies pass eastward into non—cyclic siliclastics of
central and eastern "lagoon". The shelf was terminally
drovmed during attempted subduction of the margin.
Individual cycles are correlatable for over 200 km
parallel to strike and over 120 Ion perpendicular to strike.
Cycles formed in response to small, asymmetric (rapid
rise/slow fall) eustatic oscillaticns in sea-level (= 10
m). An alpine glacio-eustatic model accounts for the origin of
cycles best. During sea-level rise, rapid transgression and
submergence of tidal flats occurred along eastern margin of
the shoal-complex, followed by eastward expansion of the
shoal—complex, and slow progradation of tidal flats over
lagoonal facies during sea-level fall.
Computer modelling of Rocknest cycle types across the
platform provides important information on absolute l
sedimentation rates and lag times.
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- Doctoral Dissertations [13611]