Chalk and Related Deep-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 752

Chapter 25
Chalk and Related Deep-Marine
Carbonates
Charles E. Savrda 1
Department of Geology and Geography, Auburn University, Auburn, Alabama, USA
1 Corresponding author: e-mail: [e-mail]
1. INTRODUCTION
This chapter focuses on the ichnology of those marine carbonates that are
composed mainly of the remains of calcareous nannoplankton, principally
coccolithophores, that is, carbonate ooze, chalk, and nannofossil limestone.
Because coccolithophores did not emerge until the Early Jurassic, this class
of carbonates has a relatively restricted stratigraphic range. Nonetheless, these
deposits are locally important elements of Early Jurassic to modern marine suc-
cessions; they bear records of paleoenvironmental, paleoceanographic, and
paleoclimatic conditions, and they commonly are of economic significance,
particularly to the petroleum industry ( Scholle, 1977 ).
Nannofossil ooze and its variably lithified equivalents (chalk and limestone)
are commonly referred to as “deep-water” and “pelagic” carbonates, to distin-
guish them from facies formed in shallow carbonate-platform settings. How-
ever, this terminology requires qualification. Through most of post-Triassic
time, deposition of carbonate ooze occurred in relatively deep, low-energy
marine settings distant from significant sources of siliciclastic sediments but
above the carbonate compensation depth (CCD). These settings, typically
beneath 1-5 km of water in modern oceans, include continental slopes and rises,
abyssal plains, and tops and flanks of ridges and plateaus. However, at times in
the late Mesozoic and earliest Tertiary, but particularly during the Cretaceous,
comparable coccolith-rich muds also accumulated in relatively shallow set-
tings, including continental shelves and epeiric basins, some of which were
influenced substantially by storm-induced or other bottom currents. This incur-
sion of typically deep-water facies into shallower realms is attributed to (1) high
sea-level stands, which resulted in siliciclastic sediment starvation ( Hancock,
1975 ), and (2) lower Mg/Ca ratios in Cretaceous sea water, which significantly
enhanced coccolith production ( Stanley et al., 2005 ). In most settings, carbonate