Page 243 - Carbonate Facies in Geologic History

230 Permo-Triassic Buildups and Late Triassic Ecologic Reefs

margin through massive boulder talus to fossiliferous wackestone, offers compel-
ling proof of the discrete periods of carbonate production, independent of terrig-
enous accumulation.
The fine sandstones constituting the bulk of the basinal sediments generally
do not intergrade with the carbonates but lap out against them on the slope.
Petrographically the sands show derivation from the granitic terrain of the Peder-
nalland mass to the north. They relate mineralogically to the widespread sheets
of shelf sands such as Yates and Queen which intercalate with the lagoonal-tidal
flat sediments of the platform. They must in some way have crossed the massive
carbonate fringe at the shelf margin, now devoid of sandstone strata.
Figures 11-20 and 11-21 illustrate the process of deposition of these sands. They
were formed by disintegration of the northern granitic terrain, spread as sheets
across the shelf under marine-littoral conditions during marine regression, possi-
bly windblown and washed across the shelf edge into the basin during low sea-
level stands. Those that passed through the island barrier partly filled the cavities
in it. Much sand may have come into the basin from the wider threshold at its
northern end (Motts, 1972). Once in the basin the sands were spread by chan-
nelled distributaries downslope in relatively deep water.
Dunham (1972) advanced arguments for large vertical drops of sea level and
considerable duration of times of exposure on the shelf and slope:
1. Extensive sedimentary structures indicative of vadose diagenesis (pisolite,
tepee, giant polygons structure, druse veins, fenestral fabric, caliche-like cavity
linings) pervade the sands of the crest of the shelf and the" organic reef' -bioclas-
tic limemud-wackestone accumulation many tens of meters below the inter-
preted crest. Today these structures are commonly found in littoral environments
and alternate wetting and drying and carbonate precipitation under marine va-
dose as well as meteoric influence is probably responsible for them.
2. Kaolinitized weathered feldspar from the quartz sands occurs in shelf mar-
gin sands, in silt fillings of cavities in the "organic reef', and in breccia and talus
beds along the front of the buildup; unweathered feldspars occur in the sands of
the shelf and basin. This distribution of kaolinite indicates that extensive sub-
aerial exposure occurred from time to time at the margin of the shelf and down
its foreslope. I
3. Boulder beds in the conglomerates down the slope and into the basin
include abundant cemented rocks eroded from the lower part of the slope. Some
of these dark pebbles were soft when emplaced but many were lithified. The
presumption that subaerial exposure was necessary to harden this fine sediment,
and that wave action or cliff collapse was most probably the means by which
these lithified blocks were initiated, argues for extensive sea-level drops (of at least
300 m) down the front. Other writers considering these deposits to be spectacular
mud-boulder flows from channels in the front of the shelf, have believed that the
blocks initiated under purely submarine conditions (Newell et aI., 1953; Jacka et
aI., 1972). Secondary dolomitization is extensive in some parts of the slope rocks,
particularly in brecciated talus beds. No dolomite pebbles are seen in the boulder
beds of the Rader debris flow and other slides which ranged far into the basin.
Dolomitization must therefore have followed the time in which the boulders were
emplaced.