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A dolomitized shelfedge hardground in the northern Gulf of Mexico—comment
Sedimentary Geology, 68 (1990) 307-308
307
Elsevier Science Publishers B.V., Amsterdam
Discussions A dolomitized shelfedge hardground in the northern Gulf of Mexico comment G e r a l d M. F r i e d m a n Department of Geology, Brooklyn College, Brooklyn, N. K 11210 (USA), and Northeastern Science Foundation, Inc., affiliated with Brooklyn College of the City University of New York, Rensselaer Center of Applied Geology, 15 Third Street P.O. Box 746, Troy, N. Y. 12181-0746, U.S.A. Accepted July 9, 1990
Poppe et al. (1990) missed the opportunity to relate their findings to methane-derived outer continental shelf hardgrounds which extend along a large segment of the Atlantic margin of N o r t h America associated with former shorelines (Hathaway and Degens, 1968; Allen et al., 1969; Friedman et al., 1971; Friedman and Sanders, 1979; Friedman, 1988). Their contribution to this knowledge is (1) that this hardground extends beyond the Atlantic Ocean into the Gulf of Mexico, and (2) that the composition of the methane-generated carbonate consists in places of dolomite, instead of the more c o m m o n aragonite and the less common calcite. Radiocarbon dating of the matrix of samples of carbonate hardgrounds resembling beachrock yields 15,600 +_ 250 years off the New Jersey coast and 20,000 years B.P. off the coast of New England. These ages compare with 29,130 _+ 285 years B.P. for the Gulf of Mexico and as the literature points out these ages date precursor marsh gases. As an example, the radiocarbon age of molluscan shells enclosed in the 15,600 _+ 250 year hardground is 4390_+ 120 years, hence much younger than that of the enclosing matrix. At first sight, the radiocarbon ages give what appears to be an impossible result: cementation of " y o u n g " shells by "old" cement. Assuming that the radiocarbon measurements are correct, they require some explanation. One possible way to explain them invokes the idea that the actual precipitation of the cement occurred less than 4390 years ago, 0037-0738/90/$03.50
© 1990 - Elsevier Science Publishers B.V.
but that the carbon in the aragonite did not come directly out of seawater (in which case the carbon would be "new", that is, 4390 years old), but rather was derived from a source of older carbon. Analyses of stable-carbon isotopes show that the cement is enriched in the light stable isotope (12C); its 613C value is - 4 4 . 8 . This contrasts with a 613C value of - 8.4 for the molluscan shells of the rock. The strongly negative 813C value of the cement suggests that carbon of the aragonite cement came from methane (CH4); no other material is known in which the lighter carbon isotope has been that much enriched. This cementation may have been controlled by methane originating from tidal marshes, now submerged. In tidal marshes, methane is abundantly generated as a marsh gas. On the Atlantic continental shelf, marsh sediments underlie the lithified shelfedge hardgrounds. During the most recent transgression, coastal sediments came to overlie submerged tidal marshes. After the methane escaped from the marsh sediments, it was oxidized to the bicarbonate ion, which then combined with calcium to precipitate as carbonate. Another interesting observation is that these outer continental shelf hardgrounds resemble beachrock. In our original papers, however, we felt (see Allen et al., 1969, p. 144) that " t h e enrichment of 12C mitigates against the beachrock hypothesis. We know of only one beachrock sample from which the stable isotope ratios have been determined" and its value is + 3.3 (Gavish and
308
G.M. FRIEDMAN
F r i e d m a n , 1969). More recent work, still u n p u b lished, of beachrock from the same M e d i t e r a n e a n coast as that studied b y Gavish a n d F r i e d m a n (1969) has revealed that beachrock in places is a m e t h a n e - g e n e r a t e d dolostone c o m p a r a b l e to that which Poppe et al. (1990) discuss. Poppe et al. (1990) assigned a Pleistocene age to the h a r d g r o u n d based on age dates of the precursor methane. T h e y state " t h e h a r d g r o u n d was preserved d u r i n g the H o l o c e n e transgression". O u r r a d i o c a r b o n dates based o n the m o l l u s c a n inclusions relate lithification to the Holocene. This observation parallels our new data form beachrocks in which the enclosed m o l l u s c a n shells are likewise y o u n g e r t h a n the r a d i o c a r b o n dates obtained for m e t h a n e - g e n e r a t e d lithified dolomite rock.
References Allen, R.C., Gavish, E., Friedman, G.M. and Sanders, J.E., 1969. Aragonite-cemented sandstone from outer continen-
tal shelf off Delaware Bay: submarine lithification mechanism yields product resembling beachrock. J. Sediment. Petrol. 39: 136-149. Friedman, G.M., 1988. Methane-derived aragonitic carbonates formed by subduction-induced pore-water expulsion along the Oregon/Washington margin: discussion and reply: Geol. Soc. Bull. 100: 622-623. Friedman, G.M, and Sanders, J.E., 1979. Principles of Sedimentology. John Wiley, New York, N.Y., 792 pp. Friedman, G.M,, Sanders, J.E., Gavish, E. and Allen, R.C., 1971. Marine lithification mechanism yields rock resembling beachrock. In: O.P. Bricker (Editor), Carbonate Cements. Johns Hopkins University Press, Baltimore, Md., pp 50-53. Gavish, E. and Friedman, G.M., 1969. Progressive diagenesis in Quaternary to Late Tertiary carbonate sediments: sequence and time scale. J. Sediment. Petrol., 39: 980-1006. Hathaway, J.C. and Degens, E.T., 1968. Methane-derived marine carbonates of Pleistocene age. Geol. Soc. Am., 1968 Annu. Meet., Progr. Abstr., pp. 129-130. Poppe, L.J., Circe, R.C., and Vuletich, A.K., 1990. A dolomitized shelfedge hardground in the northern Gulf of Mexico: Sediment. Geol. 66: 29-44.
307
Elsevier Science Publishers B.V., Amsterdam
Discussions A dolomitized shelfedge hardground in the northern Gulf of Mexico comment G e r a l d M. F r i e d m a n Department of Geology, Brooklyn College, Brooklyn, N. K 11210 (USA), and Northeastern Science Foundation, Inc., affiliated with Brooklyn College of the City University of New York, Rensselaer Center of Applied Geology, 15 Third Street P.O. Box 746, Troy, N. Y. 12181-0746, U.S.A. Accepted July 9, 1990
Poppe et al. (1990) missed the opportunity to relate their findings to methane-derived outer continental shelf hardgrounds which extend along a large segment of the Atlantic margin of N o r t h America associated with former shorelines (Hathaway and Degens, 1968; Allen et al., 1969; Friedman et al., 1971; Friedman and Sanders, 1979; Friedman, 1988). Their contribution to this knowledge is (1) that this hardground extends beyond the Atlantic Ocean into the Gulf of Mexico, and (2) that the composition of the methane-generated carbonate consists in places of dolomite, instead of the more c o m m o n aragonite and the less common calcite. Radiocarbon dating of the matrix of samples of carbonate hardgrounds resembling beachrock yields 15,600 +_ 250 years off the New Jersey coast and 20,000 years B.P. off the coast of New England. These ages compare with 29,130 _+ 285 years B.P. for the Gulf of Mexico and as the literature points out these ages date precursor marsh gases. As an example, the radiocarbon age of molluscan shells enclosed in the 15,600 _+ 250 year hardground is 4390_+ 120 years, hence much younger than that of the enclosing matrix. At first sight, the radiocarbon ages give what appears to be an impossible result: cementation of " y o u n g " shells by "old" cement. Assuming that the radiocarbon measurements are correct, they require some explanation. One possible way to explain them invokes the idea that the actual precipitation of the cement occurred less than 4390 years ago, 0037-0738/90/$03.50
© 1990 - Elsevier Science Publishers B.V.
but that the carbon in the aragonite did not come directly out of seawater (in which case the carbon would be "new", that is, 4390 years old), but rather was derived from a source of older carbon. Analyses of stable-carbon isotopes show that the cement is enriched in the light stable isotope (12C); its 613C value is - 4 4 . 8 . This contrasts with a 613C value of - 8.4 for the molluscan shells of the rock. The strongly negative 813C value of the cement suggests that carbon of the aragonite cement came from methane (CH4); no other material is known in which the lighter carbon isotope has been that much enriched. This cementation may have been controlled by methane originating from tidal marshes, now submerged. In tidal marshes, methane is abundantly generated as a marsh gas. On the Atlantic continental shelf, marsh sediments underlie the lithified shelfedge hardgrounds. During the most recent transgression, coastal sediments came to overlie submerged tidal marshes. After the methane escaped from the marsh sediments, it was oxidized to the bicarbonate ion, which then combined with calcium to precipitate as carbonate. Another interesting observation is that these outer continental shelf hardgrounds resemble beachrock. In our original papers, however, we felt (see Allen et al., 1969, p. 144) that " t h e enrichment of 12C mitigates against the beachrock hypothesis. We know of only one beachrock sample from which the stable isotope ratios have been determined" and its value is + 3.3 (Gavish and
308
G.M. FRIEDMAN
F r i e d m a n , 1969). More recent work, still u n p u b lished, of beachrock from the same M e d i t e r a n e a n coast as that studied b y Gavish a n d F r i e d m a n (1969) has revealed that beachrock in places is a m e t h a n e - g e n e r a t e d dolostone c o m p a r a b l e to that which Poppe et al. (1990) discuss. Poppe et al. (1990) assigned a Pleistocene age to the h a r d g r o u n d based on age dates of the precursor methane. T h e y state " t h e h a r d g r o u n d was preserved d u r i n g the H o l o c e n e transgression". O u r r a d i o c a r b o n dates based o n the m o l l u s c a n inclusions relate lithification to the Holocene. This observation parallels our new data form beachrocks in which the enclosed m o l l u s c a n shells are likewise y o u n g e r t h a n the r a d i o c a r b o n dates obtained for m e t h a n e - g e n e r a t e d lithified dolomite rock.
References Allen, R.C., Gavish, E., Friedman, G.M. and Sanders, J.E., 1969. Aragonite-cemented sandstone from outer continen-
tal shelf off Delaware Bay: submarine lithification mechanism yields product resembling beachrock. J. Sediment. Petrol. 39: 136-149. Friedman, G.M., 1988. Methane-derived aragonitic carbonates formed by subduction-induced pore-water expulsion along the Oregon/Washington margin: discussion and reply: Geol. Soc. Bull. 100: 622-623. Friedman, G.M, and Sanders, J.E., 1979. Principles of Sedimentology. John Wiley, New York, N.Y., 792 pp. Friedman, G.M,, Sanders, J.E., Gavish, E. and Allen, R.C., 1971. Marine lithification mechanism yields rock resembling beachrock. In: O.P. Bricker (Editor), Carbonate Cements. Johns Hopkins University Press, Baltimore, Md., pp 50-53. Gavish, E. and Friedman, G.M., 1969. Progressive diagenesis in Quaternary to Late Tertiary carbonate sediments: sequence and time scale. J. Sediment. Petrol., 39: 980-1006. Hathaway, J.C. and Degens, E.T., 1968. Methane-derived marine carbonates of Pleistocene age. Geol. Soc. Am., 1968 Annu. Meet., Progr. Abstr., pp. 129-130. Poppe, L.J., Circe, R.C., and Vuletich, A.K., 1990. A dolomitized shelfedge hardground in the northern Gulf of Mexico: Sediment. Geol. 66: 29-44.