Page 92 - Carbonate Facies in Geologic History

Lime Mud Mounds on Shelves and Shelf Margin Slopes 79

26. Festoon, medium-scale cross bedding
Cross beds in sets up to 1/2 m thick; caused by scour and subsequent fill by
megaripples formed in moderately strong currents. Beds exhibiting this structure
are generally very lenticular and often truncate lower beds. The structures are
common in outer, seaward parts of shelves, downslope from beaches where long-
shore currents exist (Thomson and Thomasson, 1969, p.62; Stricklin and Smith,
1973).
27. Onkoids in lime sand or in lime mud matrix (Plates IXB, XV A)
Balls with concentric lamellae, commonly irregular and crinkled; formed by
coating of particles by algae, foraminifera, serpulids; where occurring in micritic
matrix they are seen in widely distributed beds. Such particles must have formed
in areas of moderate circulation but were deposited over broad areas of shallow,
standing water. When seen in sandy matrix the balls may have been formed and
deposited in channels or near shoals where water movement was swifter than
normal (SMF -13 and SMF -22).
28. Local small lime mud mounds
Small, rounded, well-defined, algal-foraminiferal micrite accumulations (not
more than 1-2 m in diameter). These occur generally in well-bedded limestone
deposited in shallow marine water on shelves or on gentle slopes off shelf margins.
29. Calcareous concretion zones (commonly in marls)
Prevalent in shallow basinal shales equivalent to shelf carbonates. They are
considered to be syngenetic or early diagenetic in origin, are formed within the
substrata and represent a slowdown in the normal argillaceous sedimentation
(Weeks, 1957, p.98; Waage, 1964, p.550).

Lime Mud Mounds on Shelves and Shelf Margin Slopes
(Standard Facies Belts 4 and 7)

Several of these sedimentary structures are discussed in more detail in later Chap-
ters.
30. Collapse breccia and vein patterns within micrite core (Plate XXIIB)
Thorough syngenetic brecciation, mud flowage, with superimposed later brec-
ciation and development of vein calcite is common in lime mud buildups. It is
probably caused by early dissolution, collapse, and slumping of the mixture of
lime mud and fragile shelly components. This may have occurred through grav-
ity-slumping under marine conditions or upon exposure to meteoric water when
the mass was at or near the sea surface (Heckel and Cocke, 1969; Choquette and
Traut, 1963).
31. Stromatactoid structure (Plate XVIII B)
Originally open-space sedimentary structures characterized by horizontal or
nearly flat bottoms and by irregular, digitate upper surfaces. The original cavities
are generally filled by internal micritic sediment or by sparry calcite in patterns
indicating void-filling precipitation. The structure occurs only within lime mud
buildups. The origin of the cavities is much discussed in the literature (see Chap-
ter V) and may be due to slumping of partly lithified lime mud, settling of rigid