Page 89 - Carbonate Facies in Geologic History

76 Outline of Carbonate Petrography

5. Load casts
A sole mark, bulbous, mammillary, or papilli forms which are downward
protrusions of sand produced by load deformation in underlying hydroplastic
mud; due to yielding under unequal load (Pettijohn and Potter, 1964, p.318;
Kuenen, 1953, p.1048 and 1058). Termed "flow cast" by Shrock (1948, p.156).
6. Groove casts and tool markings
Sole marks, rounded or sharp-crested rectilinear ridges produced by filling of
grooves (Pettijohn and Potter, 1964, p. 311; Shrock, 1948, p.162).
The grooves may be produced by engraving tools such as shells, sand grains,
pebbles, and logs swept over firm lutite bottoms by currents. The marks are
preserved as casts on bases of overlying beds (Dzulymski and Sanders, 1959;
Pettijohn and Potter, 1964, p. 348).
7. Conglomerate channel fills
Bodies of carbonate, chert, and sandstone pebble clasts which have flattish
tops and shallow convex bases. These may interrupt the normal thin and planar
bedded basinal and slope strata (young, 1970, p.2304-2305).
8. Glide surfaces
Observed within uniform and thinly bedded lime mudstones and calcisiltites
as major discontinuities in sequence, formed probably as a result of large-scale
slippage or slumping of strata without much internal deformation. Such struc-
tures may be of great lateral extent, masses of several hundred feet long appar-
ently having slid across the stable part of an exactly similar section. Contacts are
sharp. 1. Harms (personal communication) believes that some of these could be
shallow channels cut in fine-grained sediment by dense brines and penecontempo-
raneously filled in by the fine sediment (Garrison and Fischer, 1969, p. 38; Wilson,
1969, p. 9-11).
9. Soft sediment slumps
These are rarer in carbonates than in clastic sediments (Plate XXVII). Convo-
lute bedding: wavy or contorted laminations that diminish upward and down-
ward within a given sedimentational unit (Kuenen, 1952, p.31 and 1953, p.1056;
Pettijohn and Potter, 1964, p.292). Flame structures: the mud plumes separating
the downward bulging load pockets or load casts of sand and sand-shale interface.
Also described as "streaked-out ripples" (Pettijohn and Potter, 1964, p. 305).
10. Mn-Fe crusts and nodules
Carbonate lithification on the present-day sea floor mainly of globigerinid
oozes, commonly with an admixture of benthonic skeletal matter occurs at depths
ranging from about 200-3500 m. In places there exists a superficial crust overlying
essentially unconsolidated sediments. Some of these crusts, nodules, and coatings
are rich in manganese and iron oxide resulting from continuous solution and
reprecipitation processes within the chemical gradient that exists near the sedi-
ment-water interface (Price, 1967; Garrison and Fischer, 1969; Morgenstein,
1973).
11. Subsolution forming in situ conglomerates (Plate XXIX B)
A solution rubble of limestone crusts formed in situ on the sea floor and
perhaps in places moved slightly downslope. Some such blocks or pebbles are