Xiaowa Formation
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"Guanling biota" redirects here. For the Anisian-age geological formation, see
Guanling Formation .
The Xiaowa Formation is a Carnian -age geological formation found in southern China . It is a sequence of limestone and marls from the Carnian stage of the Triassic . Its lower section was previously known as the Wayao Formation or Wayao Member of the Falang Formation (a nomenclature still used by some authors). In 2002, the Wayao Member was renamed and raised to the Xiaowa Formation to prevent confusion with an Eocene unit of the same name.[1] Crinoids and marine reptiles are abundant in the Xiaowa Formation, forming a lagerstätte known as the Guanling biota . Ammonoids and conodonts found in the formation constrain its age to the early Carnian.[2] [1] [3] [4] [5] ichthyosaurs , thalattosaurs , placodonts , and Odontochelys [4] Carnian Pluvial Event , around 234 million years ago.[6] [7]
Contents
Geology
The Xiaowa Formation has three members. The lower member is relatively thin but is also very fossiliferous. It begins with thick-bedded grey biomicrite (fine-grained fossiliferous limestone) interbedded with greenish shale . Bivalves and crinoid fragments are the most common fossils in the biomicrite layers, which sometimes grade upwards to dark grey laminated marls. The lower section of the lower member represents a relatively well-oxygenated pelagic environment. The lower member then transitions to a section of darker and more clastic layers indicative of anoxic conditions and reduced reef activity. Most of the articulated crinoids and vertebrate fossils of the Guanling biota hail from a dark grey micrite at the base of the lower member's upper section. This is followed by dark grey marls and black shale rich in bivalves, ammonoids , and slightly radioactive clay minerals . The lower member concludes with a sequence of dark grey laminated marls incorporating conodont fossils and silty quartz grains.[1] [4]
The middle member of the Xiaowa Formation is by far the thickest unit and includes thick-bedded grey limestone and marl layers interbedding with each other. Sediment deformation is characteristic of layers in this member, while fossils are represented mainly by occasional bivalves and ammonoids. The middle member represents a deep-water environment influenced by tectonic events which disturb sediment layers and create distant turbidites that periodically supply increased clastic material. The upper member is mostly dominated by laminated limestone. Though marl interbedding and fossils are practically absent, silty to sandy quartz grains are common and dominate the last few meters of the formation. This member represents a shallower ocean environment (likely raised by tectonic uplift) supplied with dust from terrestrial areas[1] [4]
Paleobiota
The Xiaowa Formation encompasses several biostratigraphic zones. The Protrachyceras costulatum Zhuganpo Formation continues into the first few meters of the Xiaowa Formation's lower member. However, the rest of the lower member (including the Guanling biota) belongs to the Trachyceras multituberculatum Austrotrachyceras triadicum T. aon or T. aonoides zone of the western Tethys (Europe). The middle member of the Xiaowa Formation belongs to the Sirenites cf. senticosus [2] [1] [3] [5]
Bivalves
Brachiopods
Cephalopods
Cephalopods of the Xiaowa Formation Taxon Species Notes Arctosirenites A. canadensis A trachyceratid ammonoid which occurs alongside T. multituberculatum. [2] A. columbianus A trachyceratid ammonoid which occurs alongside T. multituberculatum.[2] Austrotrachyceras A. triadicum A trachyceratid ammonoid which occurs alongside T. multituberculatum.[2] [5] Buchites B cf. aldrovandii A buchitid ammonoid which occurs in the S. cf. senticosus zone.[1] [4] Clionites C. cf. zeilleri A clionitid ammonoid which occurs in the P. costulatum zone.[1] [4] Enoploceras ?E . sp.A rare tainoceratid nautiloid tentatively referred to Enoploceras.[2] Guanlingoceras G. guanlingensis A trachyceratid ammonoid[9] Hauerites H. cf. himalayanus A rare ammonoid which occurs in the T. multituberculatum zone.[1] [4] Paratrachyceras P. cf. hoffmani A trachyceratid ammonoid which occurs in the T. multituberculatum zone.[1] [4] Protrachyceras P. costulatum A trachyceratid ammonoid, namesake of the P. costulatum zone[2] [1] [4] P. deprati A trachyceratid ammonoid which occurs in the oldest part of the Lower Member.[1] [4] P. douvillei A trachyceratid ammonoid which occurs in the T. multituberculatum zone.[1] [4] P. cf. douvillei A trachyceratid ammonoid which occurs in the P. costulatum zone.[1] [4] P. ladinum A trachyceratid ammonoid which occurs in the T. multituberculatum zone.[1] P. longiangense A trachyceratid ammonoid which occurs in the T. multituberculatum zone.[1] P . spAn unnamed species of trachyceratid ammonoid which occurs in the S. cf. senticosus zone.[2] [1] [4] Simonyceras S. simonyi A ussuritid ammonoid.[3] Sirenites S. cf. senticosus A trachyceratid ammonoid, namesake of the S. cf. senticosus zone.[1] [4] Sibyllites S. cf. tenuispinosus A tropitid ammonoid which occurs in the S. cf. senticosus zone.[1] S. sp.An unnamed species of tropitid ammonoid which occurs in the S. cf. senticosus zone.[1] Trachyceras T. aonoides A trachyceratid ammonoid, namesake of the T. aonoides zone.[3] T. cf. aon A trachyceratid ammonoid which occurs in the T. multituberculatum zone.[1] [4] T. multituberculatum An abundant trachyceratid ammonoid, namesake of the T. multituberculatum zone.[2] [1] [4] [5] T. sinensis A trachyceratid ammonoid which occurs in the T. multituberculatum zone.[1] T. uraniae A trachyceratid ammonoid which occurs alongside T. multituberculatum.[2] [5] T. sp. AAn unnamed species of trachyceratid ammonoid similar to Protrachyceras douvillei.[2] T. sp. BAn unnamed species of trachyceratid ammonoid similar to Protrachyceras deprati.[2] T . sp.An unnamed species of trachyceratid ammonoid which occurs in the S. cf. senticosus zone.[1] [4] Yakutosirenites Y. mulanae An early ammonite [10]
Conodonts
Echinoderms
Fish
Reptiles
References
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