Page 309 - Carbonate Facies in Geologic History

296 Shoaling upward Shelf Cycles and Shelf Dolomitization

Crest of shelf margin:
Oolitic coated pellet, lump-bearing well-sorted and partly cemented grainstone, the main
reservoir facies. Pisolitic, onkoid, lump-bearing coarse-grained packstone with abundant
codiacean and dasycladacean remains, few bioclasts (Plate XB).
Slope and basin:
Peloid mudstone-wackestone with argillaceous partings grading down to brown lime-
stone with few bioclasts, the typical lower Smackover lime mudstone and dark shale.
Extensive dolomitization has affected the shelf facies of the Smackover, partic-
ularly in Texas, and is responsible for development of considerable porosity and
permeability and good oil reservoir rock.
Very similar Jurassic cycles are also known from the Lias of Lorraine where
they were first well described by Kliipfel in 1917. These have been discussed more
recently by Hallam (Duff et aI., 1967). The Helvetic nappes of central Switzerland
contain thick Lower and Middle Cretaceous limestone whose sedimentary cycles
of this type were described by Fichter in 1934. The upward shoaling pattern of
one of these cycles was described in detail by Ziegler (1967).

Hard Grounds and Emersion Surfaces

Many oolitic grainstone cycles are capped by hard ground surfaces which are
widely traceable and form an integral part of the cyclic history. These surfaces can
be formed under both marine and subaerial conditions and represent secular
stillstands in sedimentation or major regressions punctuating depositional his-
tory. They are marked by numerous early diagenetic features which have been
thoroughly described both in Europe and North America. Recognition of the
significance of shallow marine hard ground surfaces stems from work by Shinn
(1969) on Holocene beds in the Persian Gulf and also by application of this study
to Jurassic strata in France (Purser, 1969, 1972).
Criteria indicative of lithification of hard ground. under marine conditions
include: isopachous druse or palisade intergranular cement, organic planation of
carbonate surfaces by browsing invertebrates, surfaces pitted by echinoids, bor-
ings in hardened sediment by pholad bivalves, sparry calcite geopetals in borings
which have been scoured by later borers, oysters plastered on hardened rock,
reworking of hardened pebbles from the surface, and common micritization of the
upper centimeters of the surface by algal and bacterial action. In additon, below
such marine hard grounds evidence of a pronounced slow-down of sedimentation
may exist. More abundant burrows may appear or there may be more common
vertical burrows which formed in the slowly hardening substrate. Concentration
of glauconite, phosphatic nodules, and trace amounts of iron pyrites and man-
ganese oxide are prevalent along such surfaces. Oxidation of the increased iron
content on later weathering commonly results in a reddish zone marking the hard
ground whether or not there was originally oxidation by subaerial exposure.
Emersion surfaces showing subaerial exposure occur at the tops of some
shoaling cycles and are marked by certain petrographic characteristics controlled
in part by climate. Sands may show evidence of pendent or meniscus cements if
lithified in the vadose zone. On the other hand, intertidal splash zone areas form
beach rock, aragonitic micrite coatings, and other features indicative of true