Page 95 - Carbonate Facies in Geologic History

82 Outline of Carbonate Petrography

45. Algal mat mm laminites forming stromatolites (Plate XIV)
Mats consisting of various assemblages of mostly blue-green algae and dia-
toms provide a semiresistant open network into which lime mud and sand grains
infiltrate and are trapped. The mucilaginous secretions of the algae and diatoms
in association with fine filaments act as binding agents. No trace of the calcareous
internal organic structure remains except "Girvanella tubules", but fine lamina-
tion is preserved reflecting the intermittent mud accumulation (Davies in Logan
et aI., 1970, p.169-205; Kendall and Skipwith, 1968; Neumann et al., 1970, p.285).
46. Spongiostrome fabric in stromatolites (Plate XIA)
Certain modern algal growths (e.g., Scytonema) resulting in tiny bush-tufted
mats whose fabric is calcified and preserved as irregularly anastomosing ftlamen-
tous structure (Johnson, 1961, PI. 103; Davies in Logan et aI., 1970, p.190).
47. Bulbous algal stromatolites (Plate XIV A)
Includes the typical "Collenia or Cryptozoan" forms of stromatolites, laterally
linked hemispheroids. The bulbous form results from sustained energetic but
intermittent algal growth, probably over original protrusions of substrate such as
arched mud-cracked polygons. Tidal ranges above half a meter or so are neces-
sary (Logan et aI., 1964; Kendall and Skipwith, 1968; Davies in Logan et aI., 1970,
Fig. 20 and Fig. 22).
48. Fenestral laminate fabric (Loferites, birdseye limestone) (Plates XII B,
XIII A, XIX A)
Laminate lime mudstone or dolomite riddled with parallel aligned pores;
larger ones with stromatactoid shapes; in places with geopetal fills of internal
sediment. Sediment is commonly pelleted. The structure results from desiccation
and is found best developed in areas of tidal flats where persistent inundation is
combined with maximum exposure (Fischer, 1964, p.124; Tebbutt et aI., 1965).
49. Small scale (cm thick) layers of micro graded pelleted sediment (Plates lIB,
XI,XII)
Characterized by a clastic texture and normal "fining-upward" size grading of
tiny clasts and peloids. Such cm graded beds commonly occur on natural levees
and slightly higher areas of better drainage 0n the flats. The cm grading of sand-
size grains may be capped by a micrite layer which could represent deposited lime
mud or coarser sediment whose texture was obliterated by micritization or ca-
liche processes (Shinn et aI., 1969; Wood and Wolfe, 1969).
50. Mud balls and soft pebbles
These are semi-hardened globs formed by erosion of pre-existing lime mud or
balled-up sediment by digging arthropods. They may be rounded through rolling
by gentle current action and may form armored mud balls when shells or twigs
adhere to them (AGI Glossary, 1973, p.130).

51. Edgewise conglomerate (Plate XVI)
Flat pebble conglomerate, imbricate lithoclasts accumulated in places in fan-
like, swirly patterns due to strong currents. They fill channels and pot holes. The
clasts are mainly derived from the dried-out lime mud on levees and algal mats,
the chips being abraded, moulded and redeposited in areas of current on floors of
tidal channels (Shinn et aI., 1969).