Foraminiferal paleoecology and paleoenvironmental reconstructions of the lower Miocene deposits of the Qom Formation in Northeastern Isfahan, Central Iran

Resumen

The early Miocene Qom
Formation is exposed in the Bagh section, northeast Isfahan, Central
Iran, where it unconformably overlies Oligocene deposits and is
unconformably overlain by the Upper Red Formation. The formation is
mainly represented by carbonate deposits (marl, marly limestone
facies) and subordinate siliciclastic facies. The planktonic
foraminiferal biostratigraphical analysis led to recognition of two
main planktonic foraminiferal zones: 1. the Globigerinoides
primordius Zone in the lowermost part of the section indicating
early Miocene (Aquitanian) and 2. the Globigerinoides trilobus Zone
in the upper most part of the Qom Formation of Burdigalian age
(early Miocene). Composition and abundance of benthic and planktonic
foraminifers were examined for paleoenvironment reconstruction and
paleoecology. The Q-mode cluster analysis performed on the benthic
foraminifers led us subdivide the section into three distinct
benthic foraminiferal clusters: I. the Cibicidoides ungerianus
cluster in the lower part of succession, indicating predominantly
inner neritic environments; II. the Lenticulina orbicularis cluster
in the middle part of section, indicative of middle-outer neritic
environments, and III. the Elphidium-Amphistegina cluster recorded
in the upper part of the formation and characterizing a typical
outer neritic environment. The planktonic foraminifers are
interspersed in the upper part of the section with relatively low
diversity, indicating open marine shelf segments, whereas the lower
part of the section is characterized by an abundance of benthic
foraminifers. As a point to consider for paleoecology and
paleoenvironment, abundant although with low diversity benthic
foraminifera with small tests, and calculated data based on these
clusters are indicative of high nutrient and high oxygen environment
with normal marine salinity during the early Miocene at the Bagh
section.