5.1. Chronostratigraphy of the Menqu section
The Nieniexiongla Formation and the overlain Pupuga Formation can be
divided by their distinct characteristics of lithology, bioclastics and
δ13C values (Han, Hu, Li, & Garzanti, 2016; Han, Hu,
Kemp, & Li, 2018).
The Lithiotis -rich interval occurs in the top of the Pupuga
Formation. This horizon appears extensively in the Lower Jurassic
successions in the Tethys realm and can be used as a mark of
biostratigraphic correlation (Han, Hu, Li, & Garzanti, 2016). The
Pupuga Formation consists mainly of medium- to thick-bedded (20-50
cm-thick) limestones with abundant bioclastics, e.g., corals, sponges
and bivalves. In contrast, the Nieniexiongla Fm. is composed mainly of
thin to medium bedded (~10 cm-thick) oolitic grainstones
with a few sandstone intercalations (Han, Hu, Li, & Garzanti, 2016).
The upper Nieniexiongla Formation reveals an abundance of storm deposits
(Han, Hu, Kemp, & Li, 2018). The base of Nieniexiongla Formation is
coincided with the onset of negative excursion of the carbon isotope,
which was interpreted as the onset of Toarcian Oceanic Anoxic Event
(Han, Hu, Kemp, & Li, 2018). A negative carbon isotope excursion of
~2.5‰ is recorded in organic matter, and a small
magnitude of about ~-1.5‰ is recorded in
δ13Ccarb (Han, Hu, Kemp, & Li, 2018).
In Menqu section, all the above outlined transitions were found at
~11.7 m. From 0 to 11.7 m of the section, the
lithologies are mainly medium- to thick-bedded bioclastic grainstones
with a limestone bed yielding rich Lithiotis (Fig. 2d). The
radiation of the Lithiotis fauna is a global sedimentary event
during the Pliensbachian (Franceschi et al., 2014), which demonstrates
an Early Jurassic age of this interval. From 11.7 m to top of the
section, the lithologies change dramatically as thin to medium bedded
limestones with sandstone intercalations (Fig. 2). Hummocky
cross-beddings were found in these sandstone layers, indicating strong
storm wave influences. δ13Ccarb values
changed significantly at ~12 m to average 2.19‰ in the
interval below 11.7 m and 1.40‰ above. These observations allow us to
date the Menqu section as Early Jurassic age and reveal that the
boundary of Pupuga and Nieniexiongla formations is at
~11.7 m. All the studied oolitic grainstones are from
the Lower Jurassic Nieniexiongla Formation.