General geology The formation is a relatively thin band of dark dolomite and shale, approximately in total thickness. It extends about from east to west along the northern edge of Monte San Giorgio and across the
Swiss-Italian border towards Besano. The organic matter can be characterized as
type II kerogen, enriched in
hopane and
porphyrin compounds, though strongly depleted in
Carbon-13. These
biomarkers, when combined, indicate that most of the organic material was derived from
cyanobacteria. Other sediments and rock types are uncommon in the formation. Thin layers of fine-grained laminated dolomite with a white color extend over a wide area. They have very little organic matter and instead contain shell fragments and peloids. This white dolomite likely represents distal
turbidite deposits, collapsed from nearby carbonate sources. A similar origin is inferred for massive (unbedded) dolomite layers, which have a porous texture and heterogenous grain sizes. There is some evidence of reworking, as thin dolomite layers rarely show wavy layering or are ripped up into clasts by deep currents. The black shale layers occasionally preserve bands of
chert, derived from radiolarian blooms. Numerous narrow
bentonite layers (
volcanic tuffs) occur throughout the formation. They are mostly composed of
illite and
montmorillonite, with occasional crystals of
sanidine. Unlike most Triassic tuffs from the Southern Alps,
plagioclase crystals are completely absent. Disarticulation and reorientation of Grenzbitumenzone fossils have favored the presence of weak and oxygenated bottom currents. Early evidence for bottom currents was controversial and perhaps based on a misdrawn illustration, == Paleobiota ==