Santa Marta Formation - Stratigraphy

Stratigraphy

The Santa Marta Formation was deposited during the Santonian and Campanian ages of the Late Cretaceous. It overlies the Gustav Group laid down during the Barremian and Santonian ages and is succeeded by the Snow Hill Island Formation of late Campanian age. Together, the Santa Marta Formation, Snow Hill Island Formation, the overlying López de Bertodano Formation (deposited from the late Campanian age of the Late Cretaceous to the early Paleocene epoch of the early Paleogene), and the Sobral Formation (deposited during the early Paleocene) form the Marambio Group.

Originally the formation was subdivided into three informal members termed the Alpha, Beta, and Gamma members. The names were later changed to the Lachman Crags, Herbert Sound, and Rabot members. The Lachman Crags and Herbert Sound members, named after the areas in which they outcrop, are found in the northern part of James Ross Island. Both members are late Campanian in age. The Lachman Crags Member, the older of the two, is around 500 meters thick. The lower section of the member consists of tuffaceous mudstone while the upper section consists of tuffaceous turbidites formed by underwater avalanches. Bioturbation is evident in tuff beds throughout the member due to the disruption of sediments by benthic life during the time of deposition. The Herbert Sound member is also around 500 meters thick and also can be divided into two distinct sections. Channeled debris flows interbedded with turbides make up the lower portion of the member and are overlain by fine sandstones (followed by coarser sandstones and coquinas) that make up the upper portion of the member.

The depositional environment is thought to have been a system of abyssal fans radiating out from a large river delta. The rapid aggradation of sediments from the delta produced a steep delta slope, which may have resulted in occasional debris flows that formed the turbides. A high degree of tectonic activity in the region at the time may explain the intermittent tuff beds throughout the formation.

The Rabot Member of the Santa Marta Formation is confined to the southeastern part of James Ross Island and dates back to the early to late Campanian. Outcroppings of the member are separated from those of other members in the northern part of the island. Originally the member was regarded as its own formation, and now it is considered to be the lateral equivalent of both the Lachman Crags and Herbert Sound members. Like the Lachman Crags and Herbert Sound members, the Rabot member consists of mudstones and beds of tuff that are often highly bioturbated, and also consists of rare conglomerates. Recently a fourth member has been assigned to the formation called the Hamilton Point Member. The beds of this member used to be considered part of the upper portion of the Rabot member, but now are considered to be their own distinct member.