Page 82 - Carbonate Facies in Geologic History

Diagenetic Changes in Carbonate Sediments 69

resulting in a lamination contrary to gravity. Such stromatolitic structure is com-
monest in the intertidal zone.
SMF -21 Spongiostrome mudstone fabric (Plates XI A, XIV)
Tuffeted algal fabric is preserved in fme lime mud sediment in tidal ponds.
SMF-22 Micrite with large onkoids (Plate XV A)
Wackestone or floatstone texture. This is a quiet-water sediment with algal balls
composed oflight organic matter which is later calcified or which traps fme detrital
lime mud. They are typical of shallow water backreef environment, found typically
on edges of ponds or channels.
SMF -23 Unlaminated homogenous unfossiliferous pure micrite (Plate XV C)
This lime mudstone is mainly deposited in somewhat saline or evaporative tidal
ponds. Selenite crystals may form at random or in rosettes in this sediment.
SMF-24 Coarse lithoclastic-bioclastic rudstone or floatstone (Plate XVI)
The clasts are generally of unfossiliferous micrite or calcisiltite, the matrix variable
but sparse. Crossbedding and edgewise arrangement of pebbles may occur. The
sediment is normally termed intraformational limestone pebble conglomerate. It is
formed as a lag deposit in tidal channels.

Diagenetic Changes in Carbonate Sediments

Chapter I mentions the critical importance of diagenesis in interpreting carbonate
facies and the extensive research being conducted in this field. In view of Ba-
thurst's recent excellent synthesis (1971) of this important subject, there follows
only an outline of the many diagenetic effects which can be recognized. Since
many of these effects can be confused with original depositional grains and fabric,
any petrographic study of carbonate in thin sections cannot avoid consideration
of diagenesis. The alteration of carbonate sedimentary particles and matrix is a
continuing process. It begins during deposition and continues long after burial
and the first stages oflithification.
1. Diagenesis in shallow marine water.
a) In lime muds:
In Holocene fine-grain carbonate sediments there is little or no carbonate mineral
alteration but only slight compaction and fabric rearrangement as the soupy mud
settles on the sea bottom. The change in density is from about 80% water saturation in
the upper 20 cm to about 50% water saturated pore space after the bottom ooze has
, settled. As far down as measurements have been taken in such mud (7-8 m) no addi-
tional compaction has been observed. Decay of entombed organic matter in relatively
impermeable water-saturated mud results in a reducing environment. Apparently little
more has happened to carbonate muds produced and left in the marine environment
during the last 5000 years. Thin sections of ancient limestones show that no additional
compaction normally occurs after burial in the subsurface.
b) In lime sands:
(1) Void-filling cement evenly surrounding sand grains may be precipitated as fibrous
aragonite or high Mg calcite grain-skin or isopachous cement in the form of tiny
palisadelike crystals.
(2) Beach rock is formed as lime sands in the splash zone are cemented by aragonite
and Mg calcite, the same minerals present in wholely marine cements.
(3) Aragonite crystal growth hardens pellets and cements grapestone lumps in areas of
moderate water circulation. Even ooid formation may be viewed as a process of
submarine cementation by the plastering of aragonite needles, tangential to a nu-
cleus moving in marine water.
(4) Particle surface alteration (micrite rinds). The process is one of infilling with micritic
carbonate, of tiny algal or fungi borings on lime sand grains. The process goes on