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Sediment gravity flow deposits of an Ordovician deep-sea fan system (western Precordillera, Argentina)—Discussion
Sedimentary Geology, 72 (1991) 157-164
157
Elsevier Science Publishers B.V., Amsterdam
Discussion
Sediment gravity flow deposits of an Ordovician deep-sea fan system (western Precordillera, Argentina) Discussion Gustavo Gonzfilez-Bonorino Seroicio Geolbgico National, Ao. Santa Fe 1548, 13P, 1060 Buenos Aires, Argentina Received September 28, 1990; revised version accepted April 10, 1991
Spalletti et al. (1989) described Ordovician deep-water terrigenous deposits in the Precordillera of central western Argentina. Their conclusions were (i) that most of the strata were sedimented from sediment gravity flows, (ii) in a submarine fan system, (iii) developed on the continental slope and rise of a passive-type margin, and (iv) that the mid-ocean ridge lay so close to the continental margin as to dramatically pond sedimentation. Little sedimentological information has been published on these rocks so the descriptions in Spalletti et al. are welcome. The adequacy of the evidence presented in support of points (i) and (ii) can be judged by the reader. My greatest concern is with the incomplete and biased referencing of previous work and regional information that may lead readers not familiar with the geology of this area to think that point (iii) is an original contribution of this paper, or previous papers by the same working group, and that point (iv) is substantiated by regional data. The section summarizing the regional geology includes only seven references, one to a 19th century author and three to papers by this same working group dealing with the stratigraphy of the studied area. A recent synthesis in English by Ramos et al. (1986) is not cited here. The presentation poorly reflects the amount of research carried out on the lower Paleozoic of the Precordillera. This section in part (p. 288) reads: "Cingolani et al. (1986) established [my italics] three regional facies associations for the Middle to 0037-0738/91/$03.50
© 1991 - Elsevier Science Publishers B.V.
Upper Ordovician in the Precordillera ... : a) an eastern ... platform, b) a central ... continental slope, and c) a western ... basin plain environment". In fact this reconstruction was published at least 17 years ago (cf. Gonzfilez Bonorino and Middleton, 1973, 1976; Gonzfilez Bonorino, 1976; none of these papers are cited) and included in synthetic papers (e.g. Baldis et al., 1982). Furthermore, Gonzfilez Bonorino (1975; not cited) described deposits and measured paleocurrents in exposures 30 km along strike from those dealt with in Spalletti et al. and interpreted them as Ordovician continental slope facies. A first mention of a passive margin setting occurs in the concluding remarks (p. 298), conveying the impression of an original proposal. It reads: " T h e paleogeographic reconstruction, together with the absence of volcanic arc derived clastic material in the P T F [their rocks] (Spalletti, in preparation) and with the ocean ridge nature of the basalt lava flows all suggest that the Ordovician basin of the Precordillera was located on a passive continental margin." This paragraph contains important omissions and misrepresentations of previous work. (a) A passive margin setting for the Cambro-Ordovician was previously postulated in Gonzfilez Bonorino and Middleton (1973), Gonzfilez Bonorino (1976), Baldis and Bordonaro (1984) and Bond et al. (1984); only Baldis and Bordonaro (1984) is cited elsewhere but not in connection with a passive margin setting. (b) The absence of volcanic detritus in the deep-water
158
DISCUSSION
deposits in general was previously pointed out by Ramos et al. (1986) who are not acknowledged. (c) The Ordovician basalts were, in fact, interpreted by Ramos et al. (1986) as generated from a subducting mid-ocean ridge and their arguments for convergence are not confronted by Spalletti et al. The regional paleogeographic reconstruction postulates severe sediment-ponding by a mid-ocean ridge lying relatively close to the rifted margin. The authors do not discuss how the complexly deformed strata were palinspastically restored, nor do they provide an explanation for the anomalously low spreading rate over roughly 150 m.y. They also assumed that the basaltic rocks are intercalated in basin plain deposits, not taking into account evidence of the basaltic lavas and associated intrusions being largely in slope deposits (e.g. Gonzfilez Bonorino, 1975; Ramos et al., 1986). In addition, the authors do not counter the arguments for convergence in Ramos et al. (1986). In conclusion, the paleogeographic reconstruction conflicts with regional data and is not substantiated by new information in the paper. However tedious it may be, no matter how frustrated one may feel reading our novel, brilliant idea in the words of others, a thourough survey of the literature and proper acknowledgement of previous research, are fundamental to scientific progress as well as ethically correct.
References Baldis, B. and Bordonaro, O., 1984. Cfimbrico y Ordovlcico en la sierra Chica de Zonda y cerro Pedernal, provincia de San Juan. G6nesis del margen continental en la Precordillera. IX Congr. Geol. Argentino, 4: 190-207. Baldis, B.A., Beresi, M.S,, Bordonaro, O. and Vaca, A., 1982. Sintesis evolutiva de la Precordillera argentina. V Congr. Latinoamer. Geol., Act., IV: 399-445. Bond, G.C., Nickerson, P.A. and Kominz, M.A., 1984. Breakup of a supercontinent between 625 Ma and 555 Ma: new evidence and implications for continental histories. Earth Planet. Sci. Lett., 70: 325-345. Gonzfilez Bonorino, G., 1975. Sedimentologica de la Formaci6n Punta Negra y algunas consideraciones sobre la geologia regional de la Precordillera de San Juan y Mendoza. Asoc. Geol. Argentina, Rev., XXX: 223-246. Gonzfilez Bonorino, G., 1976. Acerca de la existencia de la Protoprecordillera de Cuyo. VI Cong. Geol. Argentino., Act., I: 101-107 Gonzfilez Bonorino, G. and Middleton, G.V., 1973. The Precordillera of west central Argentina: evidence for an early Paleozoic continental margin. Geol. Soc. Am., Abstr. Prog., 5 (7): 641. Gonzfilez Bonorino, G. and Middleton, G.V., 1976. A Devonian submarine fan in western Argentina. J. Sediment. Petrol., 46: 56-69. Ramos, V.A., Jordan, T.E., Allmendinger, R.W., Mpodozis, C., Kay, S.M., Cort6s, J.M. and Palma, M., 1986. Paleozoic terranes of the central Argentine-Chilean Andes. Tectonics, 5: 855-880. Spalletti, L.A., Cingolani, C.A., Varela, R. and Cuerda, A.J., 1989. Sediment gravity flow deposits of an Ordovician deep-sea fan system (western Precordillera, Argentina). Sediment. Geol., 61: 287-301.
Sediment gravity flow deposits of an Ordovician deep-sea fan system (western Precordillera, Argentina) reply Luis A. Spalletti, Carlos A. Cingolani, R i c a r d o Varela a n d A l f r e d o J. C u e r d a Centro de Investigaciones Geolbgicas, Calle 1 n. 644 (1900) La Plata, Argentina
Received March 19, 1991; revised version accepted April 10, 1991
The purpose of the paper that was subjected to discussion by Gonzfilez Bonorino, was to analyze the characteristics of the sediments making up the Portezuelo del Tontal Formation (PTF) through the definition of the sedimentary facies, facies
associations and divisions in order to infer the transportation and depositional mechanisms (conclusion (i) as stated by Gonzfilez Bonorino), as well as the depositional system (conclusion (ii) of our paper, according to Gonzfilez Bonorino).
157
Elsevier Science Publishers B.V., Amsterdam
Discussion
Sediment gravity flow deposits of an Ordovician deep-sea fan system (western Precordillera, Argentina) Discussion Gustavo Gonzfilez-Bonorino Seroicio Geolbgico National, Ao. Santa Fe 1548, 13P, 1060 Buenos Aires, Argentina Received September 28, 1990; revised version accepted April 10, 1991
Spalletti et al. (1989) described Ordovician deep-water terrigenous deposits in the Precordillera of central western Argentina. Their conclusions were (i) that most of the strata were sedimented from sediment gravity flows, (ii) in a submarine fan system, (iii) developed on the continental slope and rise of a passive-type margin, and (iv) that the mid-ocean ridge lay so close to the continental margin as to dramatically pond sedimentation. Little sedimentological information has been published on these rocks so the descriptions in Spalletti et al. are welcome. The adequacy of the evidence presented in support of points (i) and (ii) can be judged by the reader. My greatest concern is with the incomplete and biased referencing of previous work and regional information that may lead readers not familiar with the geology of this area to think that point (iii) is an original contribution of this paper, or previous papers by the same working group, and that point (iv) is substantiated by regional data. The section summarizing the regional geology includes only seven references, one to a 19th century author and three to papers by this same working group dealing with the stratigraphy of the studied area. A recent synthesis in English by Ramos et al. (1986) is not cited here. The presentation poorly reflects the amount of research carried out on the lower Paleozoic of the Precordillera. This section in part (p. 288) reads: "Cingolani et al. (1986) established [my italics] three regional facies associations for the Middle to 0037-0738/91/$03.50
© 1991 - Elsevier Science Publishers B.V.
Upper Ordovician in the Precordillera ... : a) an eastern ... platform, b) a central ... continental slope, and c) a western ... basin plain environment". In fact this reconstruction was published at least 17 years ago (cf. Gonzfilez Bonorino and Middleton, 1973, 1976; Gonzfilez Bonorino, 1976; none of these papers are cited) and included in synthetic papers (e.g. Baldis et al., 1982). Furthermore, Gonzfilez Bonorino (1975; not cited) described deposits and measured paleocurrents in exposures 30 km along strike from those dealt with in Spalletti et al. and interpreted them as Ordovician continental slope facies. A first mention of a passive margin setting occurs in the concluding remarks (p. 298), conveying the impression of an original proposal. It reads: " T h e paleogeographic reconstruction, together with the absence of volcanic arc derived clastic material in the P T F [their rocks] (Spalletti, in preparation) and with the ocean ridge nature of the basalt lava flows all suggest that the Ordovician basin of the Precordillera was located on a passive continental margin." This paragraph contains important omissions and misrepresentations of previous work. (a) A passive margin setting for the Cambro-Ordovician was previously postulated in Gonzfilez Bonorino and Middleton (1973), Gonzfilez Bonorino (1976), Baldis and Bordonaro (1984) and Bond et al. (1984); only Baldis and Bordonaro (1984) is cited elsewhere but not in connection with a passive margin setting. (b) The absence of volcanic detritus in the deep-water
158
DISCUSSION
deposits in general was previously pointed out by Ramos et al. (1986) who are not acknowledged. (c) The Ordovician basalts were, in fact, interpreted by Ramos et al. (1986) as generated from a subducting mid-ocean ridge and their arguments for convergence are not confronted by Spalletti et al. The regional paleogeographic reconstruction postulates severe sediment-ponding by a mid-ocean ridge lying relatively close to the rifted margin. The authors do not discuss how the complexly deformed strata were palinspastically restored, nor do they provide an explanation for the anomalously low spreading rate over roughly 150 m.y. They also assumed that the basaltic rocks are intercalated in basin plain deposits, not taking into account evidence of the basaltic lavas and associated intrusions being largely in slope deposits (e.g. Gonzfilez Bonorino, 1975; Ramos et al., 1986). In addition, the authors do not counter the arguments for convergence in Ramos et al. (1986). In conclusion, the paleogeographic reconstruction conflicts with regional data and is not substantiated by new information in the paper. However tedious it may be, no matter how frustrated one may feel reading our novel, brilliant idea in the words of others, a thourough survey of the literature and proper acknowledgement of previous research, are fundamental to scientific progress as well as ethically correct.
References Baldis, B. and Bordonaro, O., 1984. Cfimbrico y Ordovlcico en la sierra Chica de Zonda y cerro Pedernal, provincia de San Juan. G6nesis del margen continental en la Precordillera. IX Congr. Geol. Argentino, 4: 190-207. Baldis, B.A., Beresi, M.S,, Bordonaro, O. and Vaca, A., 1982. Sintesis evolutiva de la Precordillera argentina. V Congr. Latinoamer. Geol., Act., IV: 399-445. Bond, G.C., Nickerson, P.A. and Kominz, M.A., 1984. Breakup of a supercontinent between 625 Ma and 555 Ma: new evidence and implications for continental histories. Earth Planet. Sci. Lett., 70: 325-345. Gonzfilez Bonorino, G., 1975. Sedimentologica de la Formaci6n Punta Negra y algunas consideraciones sobre la geologia regional de la Precordillera de San Juan y Mendoza. Asoc. Geol. Argentina, Rev., XXX: 223-246. Gonzfilez Bonorino, G., 1976. Acerca de la existencia de la Protoprecordillera de Cuyo. VI Cong. Geol. Argentino., Act., I: 101-107 Gonzfilez Bonorino, G. and Middleton, G.V., 1973. The Precordillera of west central Argentina: evidence for an early Paleozoic continental margin. Geol. Soc. Am., Abstr. Prog., 5 (7): 641. Gonzfilez Bonorino, G. and Middleton, G.V., 1976. A Devonian submarine fan in western Argentina. J. Sediment. Petrol., 46: 56-69. Ramos, V.A., Jordan, T.E., Allmendinger, R.W., Mpodozis, C., Kay, S.M., Cort6s, J.M. and Palma, M., 1986. Paleozoic terranes of the central Argentine-Chilean Andes. Tectonics, 5: 855-880. Spalletti, L.A., Cingolani, C.A., Varela, R. and Cuerda, A.J., 1989. Sediment gravity flow deposits of an Ordovician deep-sea fan system (western Precordillera, Argentina). Sediment. Geol., 61: 287-301.
Sediment gravity flow deposits of an Ordovician deep-sea fan system (western Precordillera, Argentina) reply Luis A. Spalletti, Carlos A. Cingolani, R i c a r d o Varela a n d A l f r e d o J. C u e r d a Centro de Investigaciones Geolbgicas, Calle 1 n. 644 (1900) La Plata, Argentina
Received March 19, 1991; revised version accepted April 10, 1991
The purpose of the paper that was subjected to discussion by Gonzfilez Bonorino, was to analyze the characteristics of the sediments making up the Portezuelo del Tontal Formation (PTF) through the definition of the sedimentary facies, facies
associations and divisions in order to infer the transportation and depositional mechanisms (conclusion (i) as stated by Gonzfilez Bonorino), as well as the depositional system (conclusion (ii) of our paper, according to Gonzfilez Bonorino).