Saint-Chinian Formation

The Saint-Chinian Formation is a geological formation composed of shales with limestone inclusions, dating from the Lower Ordovician (Tremadocian).

Saint-Chinian Formation
Stratigraphic range: Tremadocian PreꞒ Ꞓ O S D C P T J K Pg N
Geragnostus mediterraneus
Type	Geologic formation
Underlies	La Maurerie Formation
Overlies	La Dentelle Formation
Thickness	around 500 m
Lithology
Primary	Shale
Other	Sandstone, Limestone
Location
Coordinates	43.418624°N 2.945366°E / 43.418624; 2.945366
Region	Occitanie
Country	France
Type section
Named for	Saint-Chinian
Saint-Chinian Formation (France)

It is one of the six geological formations from the Saint-Chinian Cambro-Ordovician basin, comprising, from the most ancient to the most recent : La Dentelle Formation, Saint-Chinian Formation, La Maurerie Formation, Cluse de l'Orb Formation and Setso member, Foulon Formation and Landeyran Formation. This formation outcrops on the southern flanks of the Montagne Noire in Southern France.^[1]

The formation received its name from the small city of Saint-Chinian, a commune located in the Hérault department in Occitania.

It is dated from the biostratigraphic sub-zone characterized by Taihungshania miqueli and Asaphelina barroisi berardi.^[1]

Paleoenvironment

The Saint-Chinian Formation was deposited in a deep-marine setting along the southern margin of the Montagne Noire during the Tremadocian (Early Ordovician). The succession is dominated by dark grey to black shales interbedded with fine-grained sandstones and thin limestone layers, interpreted as distal turbidites. These lithologies reflect sedimentation on an outer shelf to slope environment, characterized by low energy conditions and episodic gravity flows.^[2]

The thin nature of the silico-clastics deposits, the presence of slump structures, graded bedding, and synsedimentary deformation combined with a trilobite fauna dominated by members of the Asaphidae family, indicates periodic slope instability, possibly linked to tectonic activity. The fauna, including trilobites and cephalopods, suggests oxygenated bottom waters and an open marine setting.^[3][4]

Biogeographically, the assemblages show affinities with other Tremadocian faunas from northern Gondwana, including those of Morocco, Bohemia, and the Iberian Peninsula. The formation provides important data for regional paleogeographic reconstructions and for understanding early Ordovician biodiversification in peri-Gondwanan basins.^[5]

Paleofauna

Shales of the Saint-Chinian Formation bears in certain areas accumulations of phosphated nodules, each potentially bearing a trilobite. Those are the so-called "schistes à gateaux"^[6]..

Nautiloids

Genus	Species	Locality	Age	Notes	Images
Cyclostomiceras	C. thorali	Euloma filacovi Zone	Late Tremadocian	A cyclostomiceratid ellesmerocerid[7]
Saloceras	S. chinianense	Euloma filacovi Zone	Late Tremadocian	An eothinoceratid ellesmerocerid[7]
	S. murvielense	Euloma filacovi Zone	Late Tremadocian	An eothinoceratid ellesmerocerid[7]
	S. pradense	Taihungshania miqueli acme Subzone	Late Tremadocian	An eothinoceratid ellesmerocerid[7]
Rioceras	R. escandei	Euloma filacovi Zone ; Asaphelina barroisi berardi and Taihungshania miqueli Subzone	Late Tremadocian	A rioceratid ellesmerocerid[7]
Felinoceras	F. constrictum	Euloma filacovi Zone	Late Tremadocian	A rioceratid ellesmerocerid[7]
Annbactroceras	A. martyi	Euloma filacovi Zone	Late Tremadocian	A baltoceratid orthocerid[7]
	A. felinense	Euloma filacovi Zone	Late Tremadocian	A baltoceratid orthocerid[7]
Bactroceras	B. mourguesi	Euloma filacovi Zone	Late Tremadocian	A baltoceratid orthocerid[7]
Cochlioceras	C.? aff. roemeri	Euloma filacovi Zone ; Asaphelina barroisi berardi and Taihungshania miqueli Subzone	Late Tremadocian	A baltoceratid orthocerid[7]
Thoraloceras	T. bactroceroides	Euloma filacovi Zone ; Asaphelina barroisi berardi and Taihungshania miqueli Subzone	Late Tremadocian	A troedsonellid dissidocerid[7]
Lobendoceras	L. undulatum	Euloma filacovi Zone ; Asaphelina barroisi berardi and Taihungshania miqueli Subzone	Late Tremadocian	A protocameroceratid endocerid[7]

Trilobites

Genus	Species	Locality	Age	Notes	Images
Paramegalaspis	P. immarginata	Prades-sur-Vernazobre	Late Tremadocian-Early Arenig	An asaphid[8]
Asaphellus	A. frequens	Prades-sur-Vernazobre	Late Tremadocian-Early Arenig	An asaphid[8]
	A.? sp.	Félines-Minervois	Late Tremadocian-Early Arenig	An asaphid[8]
Aocaspis	A. macrophtalma	La Rouvelane, près de Prades-sur-Vernazobre	Late Tremadocian-Early Arenig	An nileid[9]
Geragnostus	G. occitanus	Saint-Chinian	Early Arenig	A geragnostid[10]
	G. callavei	Saint-Chinian	Early Arenig	A geragnostid[10]
	G. pusio	Saint-Chinian	Early Arenig	A geragnostid[10]
Micragnostus	M. languedocensis	Saint-Chinian	Early Arenig	A micragnostid[10]
Trinodus	T. chinianensis	Saint-Chinian	Early Arenig	A trinodid[10]
	T. corpulentus	Saint-Chinian	Early Arenig	A trinodid[10]
	T. abruptus	Saint-Chinian	Early Arenig	A trinodid[10]
	T. oviformis	Saint-Chinian	Early Arenig	A trinodid[10]

• Trilobites:^[11]

  Members from the Asaphida order and the Asaphidae family are commons :

  • Asaphelina barroisi barroisi,
  • Asaphelina barroisi berardi,
  • Niobella ligneresi,
  • Taihungshania miqueli,
  • Ampyx priscus,^[12]
  • Ampyxinella ;

  The Agnostida order is present, with:

  • Arthrorhachis;^[13]

  Ptychopariida order:

  • Euloma filacovi,
  • Solenopleuropsis ribeiro;^[14]

  Phacopida order :

  • Platycalymene.^[13]

Echinoderms

Genus	Species	Locality	Age	Notes	Images
Arauricystis	A. primaeva			A cornute stylophoran[15]
	A. occitana			A cornute stylophoran[15]
Cothurnocystis	C. fellinensis			A cornute stylophoran[15]
	C. courtessolei			A cornute stylophoran[15]
Phyllocystis	P. blayaci			A cornute stylophoran[15]
	P. crassimarginata			A cornute stylophoran[15]
Thoralicystis	T. griffei			A cornute stylophoran[15]
Trigonocarpus	T. singularis			A cornute stylophoran[15]
Chauvelicystis	C. spinosa			A cornute stylophoran[15]
	C. vizcainoi			A cornute stylophoran[15]
Galliaecystis	G. lignieresi			A cornute stylophoran[15]
	G. ubaghsi			A cornute stylophoran[15]
Amygdalotheca	A. griffei			A cornute stylophoran[15]
Anatifopsis	A. trapeziiformis			A mitrate stylophoran[15]
Balanocystites	B. escandei			A mitrate stylophoran[15]
Peltocystis	P. cornuta			A mitrate stylophoran[15]
Vizcainocarpus	V. dentiger			A mitrate stylophoran[15]
Chinianocarpos	C. thorali			A mitrate stylophoran[15]
Minervaecystis	M. vidali			A solute homoistelean[15]
Balantiocystis	B. thorali			An eocrinoïd[15]
Aethocrinus	A. moorei			A cladide crinoïd[15]
Macrocystella	M. azaizi			A dichoporite rhombifere. Two subspecies are present : M. a. paucicristatus and M. a. multicristata [15]
Hemicystites	H. boehmi			A edrioasteroïd[15]
?Pyrgocystis	?P. sp.			A edrioasteroïd[15]
Chinianaster	C. levyi			A goniactinid stelleroïd[15]
Villebrunaster	V. thorali			A goniactinid stelleroïd[15]
Ampullaster	A. ubaghsi			A goniactinid stelleroïd[15]
Pradesura	P. jacobi			A stenurid stelleroïd[15]

Other fossil remains of marine animals are known from the formation:^[16]

• brachiopods ;
• molluscs :
  • bivalves,
  • rostroconchs,
  • monoplacophorans ;
• gastropods ;
• graptolites.