Coprolite Carbonate Sequence
Stratigraphy: Use or Euphemism
PEEBLES, ROSS G.
In the carbonate-evaporite Arab Formation (Upper Jurassic) of the southern
Arabian Gulf, coprolites are frequent constituents of the carbonates as pellet
grains in limestones and as pellet ghosts in dolostones. In particular, the
crustacean coprolite Favreina and the gastropod coprolite Prethocoprolithus are
common and diagnostic of the Arab Formation. These coprolites have been used as
indicators of very shallow marine and intertidal paleoenvironments and as
biostratigraphic markers for the Late Jurassic. It has also been noticed that
their occurrence is frequently constrained to specific horizons, making them
useful for stratigraphic correlation between wells. Recent, high-resolution
sequence
stratigraphic
analysis
of the Arab Formation has revealed that Favreina
and Prethocoprolithus occupy distinct and predictable locations within Arab
Formation depositional cycles or sequences (parasequences). Favreina occurs
rarely and is restricted to thin transgressive lag deposits found at the base of
sequences and at hardgrounds/firmgrounds associated with condensed sections or
maximum flooding surfaces. In both cases Favreina is found overlying dense,
nonporous facies (marginal marine evaporites or dolomites) and can be associated
with permeability baffles or barriers. Prethocoprolithus on the contrary is
frequently the dominant grain in transgressive carbonate sands (pellet
packstones/grainstones) formed during marine flooding over intertidal settings
and as highstand pellet packstones/grainstones deposited when the depositional
sequence
has shallowed (aggraded) to intertidal conditions. These
Prethocoprolithus sands are commonly porous and permeable, with the pores and
pore network preserved by
well
-developed rim cements that protect the good
interparticle (intercrystalline) and pelmouldic porosity during burial.