Page 96 - Carbonate Facies in Geologic History

Intertidal Environments, Tide Flats, Channels, Levees, Ponds, Beaches 83

52. Lenses of cross-bedded lithoclastic-bioclastic calcarenite
Occurring isolated in finer-grained, planar-bedded sediments and represent-
ing channels cut down in tidal flats and filled with coarse sediment. Such sediment
occurs often in intrachannel bars (Shinn et aI., 1969).
53. Rill marks
Narrow, deep grooves formed when small dendritic channels bifurcate up-
stream; commonly found in subaerial portions of beaches, sand bars, and sand
flats (Shrock, 1948, p.128; Pettijohn and Potter, 1964, p. 332).
54. Raindrop prints
Small, shallow, circular to elliptical, and vertical or slanting depressions or
crater-like pits surrounded by a slightly raised rim, formed in soft and relatively
fine sediment on tidal flats by impact of falling raindrops (Shrock, 1948, p.141-
146).
55. Crinkly laminae from plant rootlets
Rootlets penetrating both vertically and horizontally, shoving aside and lifting
fine layers of sediment and imparting a characteristic wispy, wrinkled, fabric. The
disturbance of normally laminated sediment by plant growth on tidal flats is
common. In carbonate sediment the roots and roothairs may be preserved by
precipitated sheathes (Shinn et aI., 1969, Fig.21).
56. Preserved roots and peat seams
Organic tubes, both calcified and carbonized, in carbonate tidal flat and
marsh sediment. Coal and peats occur as remnants of extensive swamps at the
shoreward edge of tidal flats in tropical climates (Shinn et aI., 1969, Fig. 21 B).
57. Algal mat blisters from rain showers
Sudden rain soaking rubbery, algal mats causes them to become impermeable.
Gases released within the mat through organic decay raises immediate blisters
following rain. The blisters may be calcified and preserved in situ or dry up and
peel off to form small circular clasts (Ginsburg, personal communication).
58.Peeled and curled mud chips
Thin films of lime mud and raised layers of algal mats when dried out are
curled into flakes. These may be buried more or less in situ by rapidly deposited
sediment or they may be redeposited in nearby channels and rills as intraclasts
(lithoclasts) (Shinn et aI., 1969).
59. Mud cracks
Positive expression of desiccation mud cracks appearing as polygonal forms
on bedding planes, or cracks filled with later sediment and preserved as raised
ridges (casts) on undersides of beds. In cross section, desiccation cracks are com-
monly seen as vertical v-shaped cracks (Pettijohn and Potter, 1964, p. 323).
60. Dolomite and limestone crusts
Hardened crusts present within subdivisions of the supratidal-intertidal zone
where most frequent intermittent wetting and drying occurs; related to sedimen-
tary highs such as levees and beach ridges, or encircling edges of ponds. Forma-
tion is due to evaporation and precipitation of carbonate minerals in concen-
trated brines in capillary zone at the surface. This is a favored place for dolomiti-