The Hesseltal Formation or Blackcoloured Formation is a
Late Cretaceous (late
Cenomanian
The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in the s ...
to early
Turonian) geological formation from northern
Germany. It consists of lithified
marl
Marl is an earthy material rich in carbonate minerals, clays, and silt. When hardened into rock, this becomes marlstone. It is formed in marine or freshwater environments, often through the activities of algae.
Marl makes up the lower part o ...
s and
limestone, with a unique series of
black shales deposited in
anoxic conditions during the
Cenomanian-Turonian boundary event.
It provides an important record of the fauna of the proto-
North Sea basin. The formation's diversity of fossil fish is thought to be due to a consequence of an expanding
oxygen minimum zone that caused
mass mortalities among different communities of fish, including both those associated with warm surface waters and those associated with colder waters from
upwelling. The anoxic conditions allowed for detailed preservation of some of these fish, including bony fish with their stomach contents preserved and even a few partial body fossils of cartilaginous fish.
Fossil
ammonites with preserved soft parts are also known.
The formation's deposition also documents the
Plenus Cold Event, an enigmatic, brief but severe cooling event that occurred during the otherwise rapid global warming from the Cenomanian-Turonian boundary event, possibly as a consequence of changing ocean circulation.
Paleobiota
Most fish are documented in Diedrich (2012) and a species list from Amalfitano ''et al'' (2020).
Bony fish
Chondrichthyans
Reptiles
References
{{Reflist
Geologic formations of Germany
Shale formations
Cretaceous Germany
Cenomanian Stage
Turonian Stage