GSSP for the Anisian-Ladinian Boundary

Definition:

The Base of the Ladinian Stage (Middle Triassic) is defined at the base of a 15 – 20cm thick limestone bed overlying a distinctive groove ("Chiesense groove") of limestone nodules in a shaly matrix, located about 5m above the base of the Buchenstein Beds in the Caffaro River bed (45°49'09.5"N, 10°28'15.5"E), south of the village of Bagolino (Province of Brescia, Northern Italy). The lower surface of the thick limestone bed has the lowest occurrence of the ammonoid Eoprotrachyceras curionii. Secondary global markers in the uppermost Anisian include the lowest occurrence of conodont Neogondolella praehungarica and a brief normal-polarity magnetic zone recognized in closely correlated sections. The GSSP level is bracketed by U-Pb single zircon age data from volcaniclastic horizons, indicating a boundary age of ca. 241 Ma.

Location:

The GSSP is located at Bagolino (Province of Brescia, Northern Italy) in the eastern Lombardian Alps, in Valle del Caffaro between Val Camonica and Valli Giudicarie. The Anisian/Ladinian boundary succession is well exposed at three sites (A – C) in the bed of the river Caffaro, in the surroundings of the Romanterra Bridge, south of the village of Bagolino. The GSSP Site is situated close to the eastern end of Site B and its coordinates are 614.276 / 5075.118 (UTM-grid) and Long./Lat. 45°49'09.5"N, 10°28'15.5"E respectively.
The GSSP Site (= Site B) is situated approximately 150m ESE of the Romanterra Bridge. 		click


Sedimentology:

At Bagolino, the pelagic succession consists from bottom to top of the Angolino Limestone. the Prezzo Limestone, the Buchenstein Beds and the Wengen Beds.
The Buchenstein Beds consist of siliceous pelagic nodular limestones and up to a few decimetres thick volcaniclastic layers which can be traced laterally on a regional scale. A marked change in sedimentation is observed at the top of the Buchenstein Beds with the abrupt switch to the predominantly siliciclastic Wengen Beds.
The Anisian/Ladinian boundary interval comprises the "transitional beds" and the lower part of the Buchenstein Beds and is best exposed at Sites A - C. In particular, at the eastern end of Site B the fully exposed strata from the 57m-level upward can be traced over several tens of metres along strike.
The GSSP level at Bagolino is defined in the lower part of the Buchenstein Beds, around 5m above its base, at the top of a distinct 20-25cm-thick interval of limestone nodules in a shaly matrix in contact (upwards) with several thick limestone beds 		click


Primary Markers:

The GSSP at Bagolino is located at the top of a distinct 20-25cm-thick interval of limestone nodules in a shaly matrix. The Anisian – Ladinian Boundary is defined by the first appearance of the ammonoid genus Eoprotrachyceras, i.e. with the species Eoprotrachyceras curionii at the lower surface of the overlying thick limestone bed.

Secondary Markers:

Conodonts:
The appearance of Neogondolella praehungarica and its co-occurrence with Paragondolella fueloepi in the upper part of the Neogondolella secedensis zone, just below the GSSP level, seems at present to be the only suitable marker for conodont-based correlations.

Other Locations around the World:

Representatives of the genus Eoprotrachyceras have been reported from numerous places including the Triassic successions in North America (Nevada, British Columbia) and Eoprotrachyceras is therefore a suitable marker for trans-Panthalassan correlation.

References:

Brack, P., Rieber, H., Nicora, A., and Mundil, R., 2005. The Global boundary Stratotype Section and Point (GSSP) of the Ladinian Stage (Middle Triassic) at Bagolino (Southern Alps, Northern Italy) and its implications for the Triassic time scale. Episodes 28/4, p. 233 –244.