Page 134 - Geology of Carbonate Reservoirs

DEPOSITIONAL ENVIRONMENTS AND PROCESSES 115

Figure 5.2 Photomicrograph of ooids and peloids with meniscus cement typical of the
vadose diagenetic environment in a Holocene eolianite from Cancun, Mexico. Note that the

meniscus cement occurs only at grain contacts. The grains are 1.5 mm in diameter.
ments produce calcitic constituents and heterotrophs. In both cases taxonomic
diversity is usually high because the beach setting is typical of ramps and some open
shelves, where normal oceanic temperature, salinity, and nutrients reach the shore-
line unimpeded by rims at the platform margin. Diversity notwithstanding, the

beach environment is a difficult place to live except for those animals able to burrow
to escape being swept away by breaking waves and swash. Some plants thrive in
dunes, but only those with root systems adapted to anchor the plant against being
swept away by waves or storms. Most skeletal constituents in beaches are derived
from the nearshore subtidal environment. In the temperate setting of the western
Mediterranean Sea around the Balearic Islands, Posidonia seagrass meadows grow
at tens of meters water depth and provide an environment for large populations of
mollusks, echinoderms, bryozoans, red algae, benthic foraminifera, and calcareous
epiphytes. Fragments of those organisms make up nearly the entire sand fraction in
beaches and dunes along the shoreline (Fornos and Ahr, 1997, 2006 ). In the tropical
Caribbean Sea around Isla Cancun, Yucat á n, aragonitic ooids are being formed in
the nearshore environment and then transported onto the beaches and dunes. Trace
fossils typical to coastal dune environments include plant roots, burrows by land -
dwelling animals, and footprints of creatures that walked or crawled over the dunes.
Plant roots penetrate directly downward through the upper dune facies forming
sharp angles with inclined crossbeds. Rhizocretions (also called root casts, rhizo-
morphs, or rhizoliths) are commonly associated with dune exposure surfaces and,
along with caliche, provide additional features for the top of the idealized dune
sequence. Body fossils commonly found in dune deposits include air - breathing