This formation, a thin but fossiliferous succession of dolomite and black shale, is famous for its preservation of Middle Triassic (Anisian–Ladinian) marine life including fish and aquatic reptiles.

[1] The formation is a relatively thin band of dark dolomite and shale, approximately 5 to 16 metres (16 to 52 ft) in total thickness.

It extends about 10 kilometres (6.2 mi) from east to west along the northern edge of Monte San Giorgio and across the Swiss-Italian border towards Besano.

[1][5] In individual outcrops, the Grenzbitumenzone overlies the lower part of San Salvatore Dolomite, a thick and widespread carbonate-rich formation.

At its upper extent, the Grenzbitumenzone grades into the San Giorgio Dolomite, a formation with fewer fossils and a lower concentration of organic matter.

The width of laminae in these dolomite layers vary widely from sub-millimeter to sub-centimeter scales, as a function of mineral or grain size variation.

[2][9] The organic matter can be characterized as type II kerogen, enriched in hopane and porphyrin compounds, though strongly depleted in Carbon-13.

[2] The formation is representative of a small intraplatform basin, a deep and stable marine environment which would have been positioned among shallow-water reefs and carbonate platforms.

The carbonate platforms themselves are preserved in thick sequences, such as the San Salvatore Dolomite further north and west, and the Esino Limestone further east.

[1] This system of carbonate platforms and basins developed along a western tongue of the Tethys Ocean, which transgressed eastwards during the Middle Triassic.

Early studies argued that it was pseudoplanktonic (attached to floating objects) or washed in from shallower areas, congruent with an anoxic Grenzbitumenzone seabed.

[13][11][14][15] Early evidence for bottom currents was controversial and perhaps based on a misdrawn illustration,[11][1][4] but further specimen sampling supports the same general conclusion.

Soft tissue preservation is rare but not unheard of: it includes calcified shark cartilage, phosphatized coprolites and gut contents, and organic remnants of reptile scales.

This may be due to the low sedimentation rate of the Grenzbitumenzone relative to the Meride Limestone, providing more time for the influence of bottom currents prior to burial.

Fossil completeness may have been enhanced by the relatively small and isolated nature of the Grenzbitumenzone basin, protected from the influence of stronger marine currents.