Catalog - Globorotalia amuria

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Globorotalia amuria

Citation: Globorotalia amuria Scott et al. 1990

Type specimens: Holotype and 32 paratypes registered as TF1635.

Type sample (& lithostrat): Sample O33/f73;

Type age (chronostrat): upper Lillburnian

Type locality: 2.82 km east of Trig. F; Cheviot district, New Zealand

Type repository: Lower Hutt; N.Z Geological Survey foraminiferal collections

Spiral orientation
Shell outline: Weakly elliptical to subcircular. Usually weakly lobulate with broad, shallow re-entrants at sutures. The re-entrant above the aperture is often only slightly larger than its predecessors (Fig. 80A).

Chamber packing and coiling: Commonly there are 4-4.75 chambers in the outer whorl (excluding a very small, kummerform chamberlet which often terminates the chamber sequence). In some individuals there is little difference between the lengths of the last 2 chambers. There are about 5 chambers in the middle whorl; details of the inner whorl are obscured by heavy encrustation. All 32 shells coil sinistrally.

Chamber shape: Late-formed chambers are relatively narrow. The short leading section of the outer margin is only slightly convex initially but joins the low arch of the central section via a zone of increased curvature. The trailing section is poorly defined but, on some chambers, matches the curvature of the leading section, making a symmetrical margin (Fig. 79). This feature, when occurring with a low arched central section and a weakly concave inner margin, gives the chamber a subrectangular appearance. Sections of the inner margin are often aligned but in some the trailing section is slightly elevated. Observations of a few chambers in middle whorls suggest that their outer margins form higher arches and outlines are weakly crescentic. Surfaces of chambers in the outer whorl are slightly convex near the inner margin but curvature increases considerably towards the outer margin. Sutures are weakly impressed.

Axial orientation
Shell outline: Discoidal and inflated. The umbilical face is weakly inflated, radially, and more elevated than the spiral face. Semi-globose shapes are typical (Fig. 80B).

Outline of early whorls: Although these whorls are sometimes inconspicuous, the rate of whorl translation is usually sufficient to form a moderate, occasionally high dome.

Outline of last chamber: The spiral section is short and gently convex to almost linear. The umbilical section is a uniform convex curve in most specimens (Fig. 80B) but occasionally is almost linear. There is no apex. These sections are linked by a relatively broad zone of more rapid curvature near the spiral face. When the umbilical section is convex the periphery occupies a median position (Fig. 79); when it is more linear the periphery lies in the zone of rapid curvature adjacent to the spiral section.

Outline of earlier chambers: Commonly the umbilical extremity of the (n-1)th chamber is as elevated as the last-formed (Fig. 79). This is relates to the negligible difference in the dimensions of these chambers as viewed in spiral orientation. The (n-2)th chamber forms the remainder of the outline. Its umbilical face is less elevated than its successors but the spiral face is usually flattened.

Aperture: A broad, low arch. The lip is usually obscured either by the terminal chamberlet or by encrustation.

Umbilical orientation and wall topography
Sutures are radially directed and slightly convex in the direction of coiling. There is a weak depression about the coiling axis but the umbilicus is either very narrow or closed. Chamber surfaces are convex in the direction of coiling and moderately elevated. Most specimens are very heavily encrusted to the extent that even many pore pits on the last chamber are obscured.

VARIATION Individuals in cuttings at 853.4-862.6 m in Tara-1 offshore well (southeast of Stewart Island; Waiauan) are smaller than those in O33/f73. They are elliptical to subquadrate with about 4 chambers in the outer whorl and are only slightly lobulate. In axial orientation the early whorls form a low dome. Variation in the umbilical outline of the last chamber resembles that in O33/f73. Specimens are less encrusted than in the latter sample and the last chamber is quite densely pustuled (Fig. 80D-F). There is a prominent lip. Specimens from a sidewall core at 514.5 m in Aratika-2 (Greymouth district; Clifdenian) have a very low spire formed by early whorls and are more similar to those in Tara-1 than the type collection material.

DISCRIMINATION Separation from Globorotalia conica is discussed under that species. The aperture in G. zealandica is typically much more highly arched than in G. amuria. This is the major discriminator between the species. The umbilical faces of late-formed chambers in G. zealandica are less inflated radially but elongation of chambers in the direction of coiling tends to be slightly greater. Heavy encrustation is a feature of G. amuria which seldom occurs in G. zealandica.

RELATIONSHIPS The identity of specimens of Globorotalia amuria in O33/f73 has been controversial. Initially, Hornibrook (1961) placed them in G. zealandica. Walters (1965) disagreed and considered that they belonged to the G. miozea lineage while Jenkins (1971) placed them in G. conica, noting that they have much thicker walls than the topotypes from Australia. In our interpretation the great majority of specimens represent G. amuria but 2 are referred to G. conica. Encrustation accounts for much of the difficulty in assessing the identity and relationships of G. amuria as the additional deposits obscure as well as modify the primary architecture. However, while encrustation is likely to reduce the angularity of a ventroconical axial outline and produce some radial inflation of the umbilical walls, it has little effect on the shape of the spiral outline of chambers, particularly the last. Thus the greater width (radial direction) of some late-formed chambers in G. conica, as well as the greater curvature of the trailing section of inner margins, is not attributable to secondary calcification. This is seen in the 2 (encrusted) specimens we identify as G. conica from O33/f73. Additionally, we note slight radial inflation (Fig. 80E) of the last chamber in some lightly encrusted specimens from cuttings at 853.4-862.6 m in Tara-1, suggesting that it is a primary feature.

DISTRIBUTION Clifdenian to lower Waiauan. The lowest in situ record is from Aratika-2 (514.5m). It is present at DSDP Site 593 in core 41-5-76 (Tasman Sea; Lillburnian). The type locality is considered to be upper Lillburnian (mainly keeled sample of G. praemenardii). The northernmost record presently known is in V23/f6536 (Waipukurau district; Waiauan). In contrast, G. conica is widely distributed, sometimes with moderately large populations, in Gisborne East Cape sequences.

References:

Scott, G. H., Bishop, S. & Burt, B. J. (1990). Scott, G. H., Bishop, S. & Burt, B. J. (1990). Guide to some Neogene Globorotalids (Foraminiferida) from New Zealand. New Zealand Geological Survey, Paleontological Bulletin. 61: 1-135. New Zealand Geological Survey, Paleontological Bulletin. 61: 1-135. gs

CATALOG OF ORIGINAL DESCRIPTIONS: Globorotalia amuria Scott et al. 1990

Globorotalia amuria

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