Print this pageSedimentologic and Chemo-Stratigraphic Recognition of Third-Order Sequences in Resedimented Carbonate: The Middle Jurassic Vajont Limestone, Venetian Alps, Italy
ZEMPOLICH, WILLIAM G., and ELISABETTA ERBA
The Middle Jurassic Vajont Limestone is predominantly composed of oolite that was shed from the Friuli Platform and resedimented by gravity flows into the Belluno Basin. Detailed petrographic, geochemical and biostratigraphic study of a 325 m-thick section reveals stratigraphic variation in nonskeletal and skeletal grain content and {13}C and {18}O composition of radial calcite ooids. Nannofossil and ammonite data indicate that deposition occured during the Late Aalenian-Early Bajocian over a period of approximately 2 million years.
Point-count analysis indicates that Vajont gravity flows are predominantly
composed of nonskeletal grains (average = 89 
percent
by volume) with lesser
amounts of skeletal grains (11 percent). Nonskeletal grain types of the Vajont
include ooids (44 percent), peloids (30 percent), oncoids (7 percent), grain
fragments (4 percent) and grapestone (3.75 percent
). Overall, the Vajont
exhibits an upwards increase in skeletal, peloid and grapestone content and a
concomittant decrease in ooid and oncoid content. Within the stratigraphic
section, Vajont grain contents and {13}C compositions define four
decameter-scale nonskeletal and skeletal-rich sequences. Each nonskeletal and
skeletal-rich sequence pair is interpreted to represent an individual cycle of
transgressive and highstand sedimentation (<3rd-order). The absence of a
definitive lowstand compositional signal, i.e. skeletal-dominated gravity flows,
between the lower and upper pair of nonskeletal and skeletal-rich sequences is
attributed to the sustained flooding of the Friuli Platform during a long-term
(3rd- order) rise in global sea-level during the Middle Aalenian-Early Bajocian.