Discoscaphites iris, ZONE

Landman, Neil H., 2004, Cephalopods From The Cretaceous / Tertiary Boundary Interval On The Atlantic Coastal Plain, With A Description Of The Highest Ammonite Zones In North America. Part 2. Northeastern Monmouth County, New Jersey, Bulletin of the American Museum of Natural History 2004 (287), pp. 1-107 : 37-38

DISCOSCAPHITES IRIS ZONE

The Discoscaphites iris Zone in New Jersey is not equally fossiliferous everywhere. The ammonites and associated invertebrate fauna are most abundant in the Manasquan River Drainage, central Monmouth County (Landman et al., in prep. a). The discoscaphites are larger and more numerous at this site than anywhere else. Ammonites are much less abundant at Parkers Creek, Crosswicks Creek, and the Inversand Pit.

This difference in ammonite abundance may reflect preservational factors, notably degree of reworking, as previously noted. However, it may also reflect the preferred habitat of the living organisms in terms of water depth. If we assume that sea level was 50 m above the present level at the K/T boundary ( Olsson et al., 2002), we can calculate the approximate water depth at each site in New Jersey by subtracting out its present elevation, assuming that there has been little tectonic movement in the region since the end of the Cretaceous ( Miller et al., 2004) and neglecting such factors as sediment loading and compaction. According to these calculations, the Manasquan River Drainage was 20 m deep, whereas all the oth­ er sites were 30–50 m deep. Ammonites and other invertebrates may have preferred a shallow water habitat.

In New Jersey, the D. iris Zone occupies the uppermost Maastrichtian and is truncated on top by an unconformity spanning the K/ T boundary. Fossils from the D. iris Zone are reworked into the overlying Paleocene strata. This same pattern appears on the Gulf Coastal Plain.

Kennedy et al. (2001) described the ammonites in the Brazos River section in Falls County, Texas. These ammonites represent the D. iris Zone and consist mostly of Eubaculites carinatus and Discoscaphites sphaeroidalis Kennedy and Cobban, 2000 , which is closely related to D. iris . These ammonites range through the upper 11 m of the Corsicana Formation and are preserved with their original aragonitic shell. In contrast, the basal part of the Kincaid Formation yields phosphatic, glauconitic fragments of Eubaculites and Discoscaphites , which were presumably reworked from the underlying Corsicana Formation. According to Hansen et al. (1987), the formational contact marks the K/ T boundary (although see Kennedy et al., 2001, for alternative viewpoints on the position of the boundary).

The D. iris Zone is present at the classic locality of the Owl Creek Formation on Owl Creek, Tippah County, Mississippi ( Kennedy and Cobban, 2000). The Owl Creek Formation at this site consists of dark greenish gray, micaceous, calcareous, argillaceous, slightly glauconitic sand and is unconformably overlain by the Paleocene Clayton Formation, the base of which is a hard, fossiliferous, sandy limestone ( Sohl, 1960: 39). The base of the Clayton Formation contains reworked fossils of Cretaceous age, including Discoscaphites . In Hardeman County, Tennessee, the base of the Clayton Formation consists of approximately 1.5 m of marl containing Paleocene fossils as well as reworked fossils from the Owl Creek Formation ( Sohl, 1960: 23).

Stephenson (1955) and Campbell and Lee (2000) described the Owl Creek Formation from Crowley’s Ridge in Stoddard and Scott counties, southeastern Missouri. The formation at this site consists of yellowish brown and greenish gray, glauconitic, finely micaceous sand or sandy clay and yields Sphenodiscus pleurisepta , Eubaculites carinatus , and Discoscaphites iris . The formation is unconformably overlain by the Clayton Formation, the base of which consists of palegreen, sandy, very glauconitic clay containing a mixture of Cretaceous and Paleocene fossils.

In the Western Interior, the D. iris Zone correlates with the dinosaur­bearing strata of the Lance and Hell Creek formations and their equivalents. The confirmed highest occurrence of Jeletzkytes nebrascensis , and hence the top of the J. nebrascensis Zone , is in the lowermost part of the Hell Creek Formation, South Dakota ( Hartman and Kirkland, 2002; Cochran et al., 2003). All of the ammonites above the basal Hell Creek Formation are fragmentary specimens and lie well below the Cretaceous/Tertiary boundary. Hoganson and Murphy (2002) reported a fragment of Discoscaphites cf. D. conradi or Jeletzkytes cf. J. nebrascensis from the Breien Member of the Hell Creek Formation in south­central North Dakota, the top of which is 46–61 m below the Hell Creek/Fort Union formational contact. Hartman and Kirkland (2002: 292) reported a fragment of Hoploscaphites ? from the Fort Rice unit in the middle of the Hell Creek Formation above the Breien Member, and speculated that this specimen was probably the youngest ammonite in the Western Interior. Jeletzky and Clemens (1965) reported a fragment of the early whorls of a scaphite from the Lance Formation approximately 330 m above the top of the Fox Hills Formation in eastern Wyoming. These occurrences of fragmentary scaphites could represent an extension of the J. nebrascensis Zone or evidence of a higher, as yet poorly documented zone ( Kennedy et al., 1998). However, even these occurrences are probably below the D. iris Zone on the Gulf and Atlantic Coastal Plains.