Burrow
Cementation Trends in Shoreface Sediments of the
Zorn, Marilyn E.1, George
Pemberton1,
Isotopic analyses of authigenic siderite
and calcite cements within in situ and transported Rosselia socialis
burrows from shoreface deposits in the Upper Cretaceous Horseshoe Canyon
Formation, Alberta, Canada reveal the early cementation history of the burrow
and geochemical conditions of the initial sedimentary environment. Transverse,
incremental sampling of the bulbous, in situ and spindle-like,
transported forms reveals systematic isotopic deviation of d13C and d18O around the burrow
core, representing accretionary records of evolving pore-water conditions. As
the number of isotopic deviations recorded in bulbous specimens is equal to
those observed in spindle-shaped burrows, both the in situ and
transported burrows likely underwent similar periods of cementation.
Transportation of the spindle-like burrows without disaggregation indicates
that cementation occurred in close proximity to the sediment water interface
prior to entrainment, but before lithification of the surrounding sediments.
These findings suggest that burrow-facilitated cementation can occur during
very shallow burial or even at the sediment-water interface. Similar
conditions are inferred in Ophiomorpha networks in the proximal offshore
to distal lower shoreface where accumulated organic matter is prevalent and the
degree of anaerobisis increases. This results in early cementation that affects
permeability and porosity trends that can be detrimental to reservoir
properties. This early siderite cementation is not present in Ophiomorpha
systems in the proximal lower shoreface to foreshore zones where the burrows
would be an integral part of the reservoir dynamics. This "toe of the
shoreface" early cementation should be evaluated in any reserve
estimations.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California