Page 132 - Geology of Carbonate Reservoirs

DEPOSITIONAL ENVIRONMENTS AND PROCESSES 113

middle shoreface environment represents a transition between breaking waves on
the landward shore and the deeper water of the shallow neritic domain; conse-
quently, sedimentary textures are less well sorted and may contain a mud fraction.
These rocks may include packstones along with grainstones. The lower shoreface
environment is mud - rich and dominated by wackestones to mudstones.
Grain types in beach – dune complexes may consist of nonskeletal sands, skeletal
sands, or both. Most tropical beaches, except those around reefs, consist of nonskel-
etal sands rich in aragonitic components such as ooids and peloids. As we already
mentioned, temperate to cool - water beaches usually consist of bioclastic sands
without aragonitic constituents.
There are distinctive sedimentary structures that mark the berm, upper, middle,

and lower shoreface zones in beaches. The typical beach profile consists of a nearly
level berm at the storm high - water line, a seaward - sloping zone called the upper
shoreface that may include ridges and runnels parallel to the shoreline, a longshore
bar and trough zone called the middle shoreface, and a burrowed, transitional zone
between beach and open sea called the lower shoreface, or shoreface toe. Berms
result when storm waves erode and redeposit shoreface sands above mean high -
water level. They are intimately associated with the upper shoreface environment,
where the dominant sedimentary structures are low - angle, inclined, parallel to sub-

parallel beds. Rill marks and ripple crossbeds are common but difficult to identify
in borehole cores or limited outcrop exposures. Middle shoreface sedimentary struc-

tures mainly reflect the strong unidirectional scour of the longshore current system.
Concave - upward, festoon, or trough crossbeds are the typical structure, and they
represent migrating large - scale ripples that move with the longshore current. Car-
bonate sand is transported parallel to the shoreline to produce ridges and swales,
or runnels. Middle shoreface topography is marked by distinctive submarine ridges
and troughs oriented parallel to the shore. Lower shoreface sedimentary structures
are characterized by small - scale (centimeter) oscillation ripples and bioturbation.
The ripples are formed by the oscillatory motion induced by the passage of deeper -
water swells. The bioturbation in this case is in the form of burrows, mainly feeding
traces of shallow neritic marine organisms. Most feeding traces are horizontal to
subhorizontal, but because the burrows are commonly large (centimeter size and
larger) in this environment, they are relatively easy to identify in slabbed borehole
cores.
Sedimentary structures in dunes can be gigantic in scale. Carbonate dunes are
reported to range in height from 2 or 3 m to over 30 m and may have total formation
thicknesses that range from 6 to over 300 m (McKee and Ward, 1983 ). A distinguish-
ing characteristic of dunes is their large - scale sedimentary structures, particularly
the crossbeds in the lower parts of dunes. McKee (1966) measured individual dunes
in the White Sands National Monument, New Mexico and found that they have
tabular crossbeds stacked to heights exceeding 10 m and extending over many tens
of meters horizontally. According to his findings, the windward portions of dunes

are rarely preserved. Large tabular and spillover crossbeds in the lower parts of
dunes pass vertically into smaller scale features that include trough crossbeds and
current ripple marks. Individual trough crossbed sets in middle and upper dune
segments may be 1 m or more thick and their orientation may deviate from the

average orientation of the larger tabular crossbeds below, depending on paleo - wind
directions. This general tendency to exhibit large, tabular and spillover crossbeds