Geology Reference
In-Depth Information
Fig. 21.24
Stratigraphic columns (
a
-
c
) and the peritidal shale-
based cycle of the inner shelf cyclic facies (
d
) of the Rocknest
Formation in the Lower Proterozoic carbonate platform succession.
Numbers 1 through 5 denote lagoonal and tidal fl at facies or facies
groups within the cycle (Modifi ed from Hoffman
1975
and
Grotzinger
1986b
) .
1
: transgressive lag deposit;
2
: distal inner-shelf
lagoon facies;
3
: proximal inner-shelf lagoon facies;
4
: upper sub-
tidal-lower intertidal facies;
5
: upper intertidal-supratidal facies
submerged, preventing communication between the
open sea and the low energy inner shelf lagoon. The
tidal fl at and lagoonal deposits of the Rocknest
Formation are characterized by mixed carbonate and
siliciclastic facies that grade eastward to siliciclastic
deposits.
The Rocknest peritidal facies are arranged into
asymmetric shallowing-upward cycles (Fig.
21.24d
),
which display various basal lithofacies refl ecting
their paleogeographic location on the platform.
Consequently, the shale-based cycles of the distal
inner-shelf lagoon pass westward into laminated dolo-
stone-based cycles and stromatolite-based cycles of
the 'shoal-complex'. The cycles were initiated by
rapid transgression and fl ooding of tidal fl ats followed
by eastward progradation of the tidal fl at facies over
the lagoonal facies of the inner shelf during sea level
fall. Shale-based cycles up to 15 m thick contain the
most diverse facies, which from base to top include
(Fig.
21.24d
): (1) intraclast packstone to grainstone,
5-30 cm thick that covers erosional tops of cycles and
commonly consists of rounded stromatolitic frag-
ments derived from the upper part of the underlying
cycle (transgressive lag deposit); (2) mixed carbon-
ate-siliciclastic facies consisting of below- to above-
wave-base interstratifi ed argillaceous dolomudstone
and shale containing intercalations (up to 30 cm thick)
of massive, planar laminated or hummocky cross-strat-
ifi ed sheets and lenses of siltstone, sandstone and
sand-size dolostone (distal inner-shelf lagoon); (3)
laminated dolostone that consists of very thin-bedded
laminated or wave-rippled silt-size dolostone with
