Ranikothalia sindensis Zone in Late Paleocene Biostratigraphy
1991; Micropaleontology Press; Volume: 37; Issue: 1 Linguagem: Inglês
10.2307/1485746
ISSN1937-2795
Autores Tópico(s)Geology and Paleoclimatology Research
ResumoThe foraminiferal genus Ranikothalia characterizes the Upper Paleocene (Thanetian) succession of Pakistan as well as other locations thruoughout the world. In addition, it sometimes extends into the Lower Eocene. A closer examination of Upper Paleocene species from various localities demonstrates that the open spiral complanate evolute forms may all be identified as Ranikothalia sindensis (Davies), as illustrated here from the Upper Paleocene Lockhart Limestone and the Lower Eocene Margala Hill Limestone from the Kala Chitta Range, northern Pakistan. In view of this demonstration of its stratigraphic value, the Ranikothalia sindensis Partial Range Zone should be incorporated within the Upper Paleocene (Thanetian) in the Paleogene stratigraphic correlation scale of Cavelier and Pomerol (1983). INTRODUCTION Cavelier and Pomerol (1983, 1986) published a Paleogene stratigraphic correlation scale which combined the most valuable data of various fossil groups, including both planktonic and benthonic larger foraminiferal zonations. The stratigraphic distribution of the benthonic larger foraminiferal zones is shown in column 11 of text-figure 2. Several Nummulites zones are indicated from the Early Eocene to the Late Oligocene, thus drawing attention to the absence of the genus Nummulites in the Paleocene. This paper intends to show the value of the genus Ranikothalia in the Paleogene biostratigraphy. DISCUSSION The genus Ranikothalia is very closely related to Nummulites, but it can be distinguished by its thickened, fan-shaped marginal cord and evolute complanate test. The thick marginal cord is accepted by all modem workers as a characteristic feature of Ranikothalia, when compared with Operculina, Assilina and Nummulites, in which the marginal cord is never developed to any similar extent. The genus Ranikothalia (type-species Nummulites nuttalli Davies, 1927, p. 266, pl. 18, fig. 3 from the Upper Paleocene Ranikot Formation at Jhirak, Sind, Lower Indus Basin, Pakistan) characterizes the Upper Paleocene (Thanetian) succession of Pakistan and elsewhere in the world, but it does extend sometimes into the Lower Eocene. Caudri (1944, p. 368) originally erected the genus based on studies of specimens from the Upper Paleocene Ranikot beds at Thal, in the Samana Range of northern Pakistan, when comparing these to the Caribbean fauna from Venezuela. Although several species have been recorded in the literature from this stratigraphic interval the American-Caribbean region and the IndoPakistan sub-continent, as well as from Mali, and Turkey, closer examination suggests that the majority should be identified as Ranikothalia sindensis (Davies), which exhibits an essentially operculine, complanate test. A careful perusal of the available data suggests that the following grouping of Ranikothalia species can be made: a) Nummulites-like thick forms characterized by the presence of septal filaments and thick marginal cord, e.g., Ranikothalia nuttalli (Davies) and Ranikothalia bermudezi (Palmer). Closer examination of these forms may reveal that they belong to a single species. b) Operculine complanate forms having evolute expanding spiral with a thick marginal cord and absence of true septal filaments, e.g., Ranikothalia sindensis (Davies) = Ranikothalia soldadensis (Vaughan and Cole). c) Nummulitid forms which appear to be intergrading between the above mentioned groups, e.g., Ranikothalia sahnii Davies, with initial nummulitic (involute) spiral and operculine final structure. Although the thick marginal cord is common to Ranikothalia sindensis (Davies) and to Ranikothalia nuttalli (Davies), the former is typically a complanate, evolute form without septal filaments, whereas the latter form is typically involute with septal filaments. This significant difference in morphological character is important to precise systematic position of the genus Ranikothalia. I consider, however, that the operculine individuals of R. sindensis in fact give a distinctive character to the genus Ranikothalia with respect to the genus Nummulites. From Pakistan, Davies (1927, p. 266, pi. 18, figs. 3-4; pl. 19, figs. 7-9; pl. 19, figs. 10-12; pl. 20, figs. 1-4; 1930, p. 68, pl. 10, figs. 12, 14) and Davies and Pinfold (1937, pl. 3, figs. 1-2; pl. 4, fig. 21; pl. 6, figs. 19-20), illustrated the original material named Nummulites nuttalli, Nummulites thalicus and Nummulites sindensis from the Upper Paleocene strata in the Samana Range and the Salt Range (northern Pakistan) and Sind (southern Pakistan). These are assigned to the genus Ranikothalia by virture of their thick marginal cord. Ranikothalia sindensis (Davies) can, however, be distinguished by its rapidly expanding spiral in contrast to the gradually increasing spiral of R. nuttalli and R. thalicus, and by the absence of true septal filaments. Berggren (1974) reported Ranikothalia bermudezi (Palmer) from the Upper Paleocene, Horizon III, of Mali (as Operculinoides bermudezi). Drooger (1960, pl. 5, figs. 1-14) has illustrated a flattened form, Ranikothalia soldadensis (originally Operculinoides soldadensis Vaughan and Cole 1941), from the Paleocene of French Guyana, while Butterlin and Monod (1969, pl. 3, fig. 8) have illustrated specimens identified as Ranikothalia sahnii (Davies) micropaleontology, vol. 37, no. 1, pp. 77-85, pls. 1-4, 1991 77 This content downloaded from 157.55.39.163 on Sun, 20 Nov 2016 04:15:16 UTC All use subject to http://about.jstor.org/terms A. A. Butt: Ranikothalia sindensis zone in Late Paleocene biostratigraphy TEXT-FIGURE 1 Location map of the Salt Range, Samana, Kala Chitta and Hazara Ranges, northern Pakistan. from the Lower Eocene of the Taurus Range, Turkey, all of which demonstrate the operculine complanate condition. The typical Ranikothalia sahnii (Davies 1952, p. 155, pl. 1, fig. 5) was originally described and illustrated from the Upper Paleocene of Togo, West Africa, and clearly exhibits an initial nummulitic spiral (thick, involute) with a final operculine (complanate) spiral, as noted above. In ex mination of a large number of Ranikothalia sindensis specimens of the Kala Chitta Range and Hazara Mountains, northern Pakistan, I found them to be abundant in the Upper Paleocene and very rare in the Lower Eocene strata. The major y of specimens are flattened and almost identical to the figured specimens of Drooger (1960) and Butterlin and Monod (1969). Since the name Ranikothalia sindensis (Davies) has
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