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RbSr radiometric age of late Precambrian fossil-bearing and associated rocks from Sinai
Earth and Planetary Science L etters, 24 ( 1974) 136 - 140
© North-Holland Publishing Company, Amsterdam - Printed in The Netherlands
Rb-Sr
RADIOMETRIC
AGE OF LATE PRECAMBRIAN FOSSIL-BEARING ASSOCIATED ROCKS FROM SINAI
AND
ARYEH E. SHIMRON Department of Geology, Hebrew University, Jerusalem {Isreal)
and DOUGLAS G. BROOKINS Department of Geology, The University of New Mexico, Albuquerque, N.M. (U.S.A.)
Received July 9, 1974 Revised version received October 2, 1974
Six samples of metamorphic rocks from three essentially coeval stratigraphic units, two of which contain Precambrian microfossils, have been analyzed by the Rb-Sr whole-rock radiomet;ic method. Least-squares regression of the data yields an isochron date of 934 ± 80 m.y. with initial 87Sr/86Sr = 0.7007 ± 0.0011. This date may reflect the approximate age of formation of these rocks or, alternately, a time of major metamorphism. Regardless, the date is significant in that it (a) is in agreement with the 900-m.y. date for rocks containing similar types of fossils from Bitter Springs, Australia, and (b) coupled with structural evidence, supports the time equivalence of these rocks with 1000-m.y. old rocks ("Kibaran cycle") of Saudi Arabia.
1. Introduction The Sinai Peninsula forms an important land bridge between the Asian and African continents and a critical geologic region between the two continents separating along the Red Sea rift axis. The recent discovery of Precambrian microfossils in certain formations in Sinai and the lack of reliable radiometric age data for the Precambrian rocks in the region stimulated the present work. This paper discusses some of the wider implications of the R b - S r radiometric age obtained and suggests a possible correlation with the Saudi Arabian Precambrian.
2. General geology The specimens collected and analyzed belong to the Sinai supracrustal series comprising the Samra (youngest), Beda and Um Zariq (oldest) formations [1 ].
They form the major portion of the Wadi Kyd metamorphic belt in southeast Sinai (Fig. 1). Possible similar groups of rocks have previously been referred to as the Hammamat, Dokhan and Shadli series by Schiirmann [2], in that order which are also found in the Precambrian of Egypt. According to Sch/Jrmann, the Shadli rocks (oldest) are a regional metamorphic series separated from the Dokhan rocks by a widespread regional unconformity which in turn are separated from the youngest Hammamat rocks by another major unconformity [2, p. 10]. Broad regional correlations on this basis were made by Schiirmann which, however, are found unacceptable for the following reasons: (a) Recent detailed work in the region by one of us (A.E.S.) has shown that the three formations in Sinai show probable contemporaneous polyphase deformation and progressive metamorphism. (b) Black slates from the Dokhan (Beda formation equivalent?) and Shadli (Um Zariq or Samra formation equivalent?) contain filamentous and coccoid blue-
R b - S r AGE DATING OF ROCKS FROM SINAI
137
w
+ + , , ,+ . . . . . . . .
L;.-+
+
• ~
:-
"+i
72!
PT
\
L[GEI~D m
I......~ g r a n i t i c rocks Samra Formation; nornfele~ s l a t e , marblep graywacke OeOa Formation; andasits, conglomeratet pyroclastics, t u r O i o l t a s Um Lariq ~ormation; amphibollte, schist ~ g s r n e t - s t a u r o l i t e e t c . ) Knayah Formation; regenerated infrac~ustsl gneisses anO migmatltea ( i n c l . Border a i o r i t a a ) fault s~7
samnle
locatlon
Fig. 1. Generalized geology, WadS Kyd area (blown up), southern Sinai Peninsula.
.'/,~'~.,
138
green algae [3] which are similar and thus unlikely to originate from Precambrian formations separated (in the same immediate area) by a widespread time span. (c) The boundaries between the formations are gradational and characteristic lithologies from one are also found in another formation (i.e., black slates, marbles, conglomerates, tuffs). The metamorphic grades are similar and progressive for all the formations concerned. (d) The specimens dated do not convincingly confirm the geological evidence in the sense that considerable scatter about the isochron is noted. However, the postulated unconformities can in all probability be attributed to structural discordances brought about by complex polyphase deformation (4 phases of folding) and Tertiary tectonics whereby low and high-grade metamorphic rocks have locally been brought into tectonic juxtaposition. Minor sedimentary unconformities were, however, present between the formations before the onset of orogenesis. The present subdivision of the three rock units into formations is based on progressive sedimentary facies changes and directions of younging shown in the ubiquitous sedimentary structures. The structurally lower-most Um Zariq formation is essentially a pelitic facies showing high amphibolite facies metamorphism with garnet, andalusite and staurolite as the index minerals (Spec. 835). This formation is followed upwards by the mainly volcanogenic-conglomeratic Beda formation (Specs. 714K3 and 737R) and finally by the uppermost, predominantly arenaceous Samra formation (Specs. 709K1 and 709K2, 191R). The latter two are mainly metamorphosed in the greenschist facies but both show progressive metamorphism to biotite grade eastwards and Beda rocks to biotite-garnet grade near the Um Zariq formation. All the analyzed rocks are black carbonaceous slates grading to meta-greywacke, with the exception of 835 which is a black porphyroblastic schist. The whole fold belt is marginally rimcned by higher-grade infracrustal gneisses and migmatites, the palingenetic portions of which are intrusive into the lower grade supracrustal rocks. The former are the "regenerated" basement rocks which comprise the Knayah formation (probably equivalent to Schiirmann's Mitiq series). Contact metamorphism due to the deep-level emplacement of younger granitic plutons
A.E. SHIMRON AND D.G. BROOKINS
has affected the southeastern boundary of the Beda and Samra formations. Cordierite hornfelses are present along the southeastern boundary of the Kyd metamorphic belt and a slight spotting may be present in some black slates in the vicinity of specimens 709K1, 709K2, and 737R.
3. Analytical procedures and results Rb and Sr contents and Sr isotopic compositions reported in Table 1 were obtained by standard methods. Sr isotopic composition was determined from both spiked and unspiked samples; agreement between runs using each method is 0.0003 (95% confidence level). All isotopic ratios are based on S6Sr/aaSr = 0.I 194; and aTSr/a6Sr for Eimer and Amend standard SrCO3 = 0.7080. Total Rb and Sr were determined by both standard isotope dilution techniques and by replicate X-ray spectrography; G-l, G-2, GSP-1, and
/ 0,~
87SR
714K3
73~ 031
030 0
+
79710R 9K2
709K~/~/
87R=/86SR 1=
2'
Fig. 2. Possible whole-rock isochron, samples from the Samra (709K1, 709K2, 791R), Beda (714K3, 737R), and Um Zariq (835) rock units (see text for discussion) from the Wadi Kyd area, Sinai Peninsula (see Fig. 1).
Rb-Sr AGE DATING OF ROCKS FROM SINAI TABLE 1 Rb-Sr analytical data Unit name
Sample 87Sr/86Sr
Rb (ppm)
Sr STRb/ (ppm) 86Sr
Samra Samra Samra Beds Beda Um Zariq
709K1 709K2 791R 714K3 737R 835
70.3 8.1.5 105.2 32.5 71.9 92.2
248.9 0.82 164 1.43 233.8 1.29 62.6 1.51 182 1.15 104.1 2.55
0.7115 0.7171 0.7147 0.7252 0.7160 0.7333
BCR-1 were used as standards for the latter. The errors for the 87Rb/a6Sr data in Table 1 are 2% (95% level). The decay constant for 87Rb is assumed to be 1.39 X 10-11/yr. The isochron (Fig. 2) was calculated from the data in Table 1 using the least-squares method of York [4]. The error assigned to the age is based on one standard deviation.
4. Interpretation of the geochronological age The predominantly east-west trend of the metasedimentary rocks in Sinai [1] and the 934 + 80-m.y. date suggest that these rocks can at least tentatively be equated with the Saudi Arabian "Kibaran cycle" [5, 6] with a given date of about 1000 m.y. on metamorphosed greenstones and metasediments. The latter are also east-west trending. Fleck et al. [7] point out R b - S r whole-rocks dates in Saudi Arabia ranging between 759 + 36 m.y. to 958 +- 22 m.y. on tonalitic and granodioritic gneisses which cut rocks folded at least three times. The Sinai analogue to the latter rocks may be: (a) the regenerated and refoliated basement gneisses and migmatites (Knayah formation) which cut across the polyphasaly folded supracrustal rocks, or (b) the granodioritic gneisses (Elat gneiss, Vatir gneiss; [1]) which were emplaced after the first and into the second phase of deformation in the Elat area (Fig. 1), but which are also found in large masses in Sinai (Wadi Vatir and Wadi Zalaqa areas). Brown (written communication) has called the author's (D.G.B.) attention to a R b - S r whole-rock isochron [9] date of 622 m.y. for igneous rocks which cut rocks in Saudi Arabia considered to be equivalent to the
139 Shadli Series in Sinai. This 622-m.y. date sets a minimum on the supracrustal rocks. However, this minimum can be considerably extended since Brown et al. [6] refer to Saudi Arabian syntectonic granitic rocks with dates of 800-650 m.y. These schlieric and gneissose rocks show many similarities to the widespread Sinai syntectonic dioritic rocks (Border diorites) which are clearly intrusive into and spacially related to the Knayah formation migmatites and gneisses, and also into the supracrustal formations. The 934 + 80-m.y. date obtained for the fossil bearing and associated rocks from the Sinai supracrustal formations is presumably not far from the age of formation of these rocks as Shimron and Horowitz [3] have pointed out that the algae are similar to fauna from Bitter Springs, Australia, which have tentatively been assigned a 900-m.y. date by Nagy [10; see also Schopf and Blacic, 111. Further, algae of about this age or possible younger (e.g. 700-900 m.y.) have also been reported from Jersey Channel Islands by Squire [12]. The 934 + 80-m.y. date from Sinai may, however, be a minimum date if some autometamorphic event such as a younger slight contact metamorphic event (as mentioned above) caused an effective age lowering by K- (and Rb-) addition or partial recrystallization. It should perhaps also be pointed out that evidence for more primitive filamentous blue-green algae has also been found in the Swaziland and Bulawayan sequences with ages exceeding 3 billion years [13].
Acknowledgements One of us (D.G.B.) acknowledges partial financial support from the National Science Foundation Grant GA-36133. Mr. W. Doering, U.S. Geological Survey, kindly provided the Rb and Sr X-ray fluorescence analyses. B. Mukhopadhyay, S.L. Bolivar, and M.J. Lee assisted in the chemistry and mass spectrometry. Field work in the Sinai Peninsula (A.E.S.) was made possible by a grant from the Bat-Sheva de Rothschild Formation, tire Hebrew University and others. The work continued at the Dept. of Geology, University of Glasgow whereby the partial financial support from the Micheal and Anna Wix Trust is gratefully acknowledged.
140
A.E. SHIMRON AND D.G. BROOKINS
References 1
2
3 4 5 6
7 R.J. Fleck, W.R. Greenwood, D.G. Hadley and W.C. Prinz, Age and origin of tonalite-granodiorite gneisses, western A.E. Shimron, The Precambrian structural and metamorSaudi Arabia, EOS Trans. Am. Geophys. Union 54 (1973) 12~ phic history of the Elat area, Ph. 1~. Thesis (unpublished), 8 V. Kazmin, Precambrian of Ethiopia, Nature 230 (1971) Hebrew University, Jerusalem (1973). 176. H.M.E. Schiirmann, The Precambrian along the Gulf of 9 H. Lenz, F. Bender, C. Besand, W. Harre, H. Kreuzer, P. Suez and the northern part of the Red Sea (E.J. BriU, Leiden, Muller and I. Wendt, The age of early tectonic events in the 1966). zone of the Jordan geosuture, Int. Geol. Congr. 24th Sess., A.E. Shimron and A. Horowitz, Precambrian organic microMontreal 3 (1972) 371. fossils from Sinai, Pollen and Spores 14 (1972) 333. 10 L.A. Nagy, Transvaal stromatolite: first evidence for the D. York, Least-squares fitting of a straight line, Can. J. diversification of cells about 2.2 × 109 years ago, Science Phys. 44 (1966) 1079. 183 (1974) 514. G.F. Brown, Tectonic Map of the Arabian Peninsula, Map 11 J.W. Schopf and J.M. Blacic, New Microorganisms from AP-2, Jiddah, Saudi Arabia (1972). the Bitter Springs Formation (Late Precambrian) or the north-central Amadeus Basin, Australia, J. Paleontol. 45 G.F. Brown and R.G. Coleman, The tectonic framework of the Arabian Peninsula, Int. Geol. Congr. 24th Sess., (1971) 925. 12 A.D. Squire, Discovery of Late Precambrian trace fossils in Montreal 3 (1972) 300. Jersey, Channel Islands, Geol. Mag. 110 (1973) 223. 13 K.A. Kvenvolden, Organic geochemistry of early Precambrian sediments, Int. Geol. Congr. 24th Sess., Montreal 1 (1972) 31.
© North-Holland Publishing Company, Amsterdam - Printed in The Netherlands
Rb-Sr
RADIOMETRIC
AGE OF LATE PRECAMBRIAN FOSSIL-BEARING ASSOCIATED ROCKS FROM SINAI
AND
ARYEH E. SHIMRON Department of Geology, Hebrew University, Jerusalem {Isreal)
and DOUGLAS G. BROOKINS Department of Geology, The University of New Mexico, Albuquerque, N.M. (U.S.A.)
Received July 9, 1974 Revised version received October 2, 1974
Six samples of metamorphic rocks from three essentially coeval stratigraphic units, two of which contain Precambrian microfossils, have been analyzed by the Rb-Sr whole-rock radiomet;ic method. Least-squares regression of the data yields an isochron date of 934 ± 80 m.y. with initial 87Sr/86Sr = 0.7007 ± 0.0011. This date may reflect the approximate age of formation of these rocks or, alternately, a time of major metamorphism. Regardless, the date is significant in that it (a) is in agreement with the 900-m.y. date for rocks containing similar types of fossils from Bitter Springs, Australia, and (b) coupled with structural evidence, supports the time equivalence of these rocks with 1000-m.y. old rocks ("Kibaran cycle") of Saudi Arabia.
1. Introduction The Sinai Peninsula forms an important land bridge between the Asian and African continents and a critical geologic region between the two continents separating along the Red Sea rift axis. The recent discovery of Precambrian microfossils in certain formations in Sinai and the lack of reliable radiometric age data for the Precambrian rocks in the region stimulated the present work. This paper discusses some of the wider implications of the R b - S r radiometric age obtained and suggests a possible correlation with the Saudi Arabian Precambrian.
2. General geology The specimens collected and analyzed belong to the Sinai supracrustal series comprising the Samra (youngest), Beda and Um Zariq (oldest) formations [1 ].
They form the major portion of the Wadi Kyd metamorphic belt in southeast Sinai (Fig. 1). Possible similar groups of rocks have previously been referred to as the Hammamat, Dokhan and Shadli series by Schiirmann [2], in that order which are also found in the Precambrian of Egypt. According to Sch/Jrmann, the Shadli rocks (oldest) are a regional metamorphic series separated from the Dokhan rocks by a widespread regional unconformity which in turn are separated from the youngest Hammamat rocks by another major unconformity [2, p. 10]. Broad regional correlations on this basis were made by Schiirmann which, however, are found unacceptable for the following reasons: (a) Recent detailed work in the region by one of us (A.E.S.) has shown that the three formations in Sinai show probable contemporaneous polyphase deformation and progressive metamorphism. (b) Black slates from the Dokhan (Beda formation equivalent?) and Shadli (Um Zariq or Samra formation equivalent?) contain filamentous and coccoid blue-
R b - S r AGE DATING OF ROCKS FROM SINAI
137
w
+ + , , ,+ . . . . . . . .
L;.-+
+
• ~
:-
"+i
72!
PT
\
L[GEI~D m
I......~ g r a n i t i c rocks Samra Formation; nornfele~ s l a t e , marblep graywacke OeOa Formation; andasits, conglomeratet pyroclastics, t u r O i o l t a s Um Lariq ~ormation; amphibollte, schist ~ g s r n e t - s t a u r o l i t e e t c . ) Knayah Formation; regenerated infrac~ustsl gneisses anO migmatltea ( i n c l . Border a i o r i t a a ) fault s~7
samnle
locatlon
Fig. 1. Generalized geology, WadS Kyd area (blown up), southern Sinai Peninsula.
.'/,~'~.,
138
green algae [3] which are similar and thus unlikely to originate from Precambrian formations separated (in the same immediate area) by a widespread time span. (c) The boundaries between the formations are gradational and characteristic lithologies from one are also found in another formation (i.e., black slates, marbles, conglomerates, tuffs). The metamorphic grades are similar and progressive for all the formations concerned. (d) The specimens dated do not convincingly confirm the geological evidence in the sense that considerable scatter about the isochron is noted. However, the postulated unconformities can in all probability be attributed to structural discordances brought about by complex polyphase deformation (4 phases of folding) and Tertiary tectonics whereby low and high-grade metamorphic rocks have locally been brought into tectonic juxtaposition. Minor sedimentary unconformities were, however, present between the formations before the onset of orogenesis. The present subdivision of the three rock units into formations is based on progressive sedimentary facies changes and directions of younging shown in the ubiquitous sedimentary structures. The structurally lower-most Um Zariq formation is essentially a pelitic facies showing high amphibolite facies metamorphism with garnet, andalusite and staurolite as the index minerals (Spec. 835). This formation is followed upwards by the mainly volcanogenic-conglomeratic Beda formation (Specs. 714K3 and 737R) and finally by the uppermost, predominantly arenaceous Samra formation (Specs. 709K1 and 709K2, 191R). The latter two are mainly metamorphosed in the greenschist facies but both show progressive metamorphism to biotite grade eastwards and Beda rocks to biotite-garnet grade near the Um Zariq formation. All the analyzed rocks are black carbonaceous slates grading to meta-greywacke, with the exception of 835 which is a black porphyroblastic schist. The whole fold belt is marginally rimcned by higher-grade infracrustal gneisses and migmatites, the palingenetic portions of which are intrusive into the lower grade supracrustal rocks. The former are the "regenerated" basement rocks which comprise the Knayah formation (probably equivalent to Schiirmann's Mitiq series). Contact metamorphism due to the deep-level emplacement of younger granitic plutons
A.E. SHIMRON AND D.G. BROOKINS
has affected the southeastern boundary of the Beda and Samra formations. Cordierite hornfelses are present along the southeastern boundary of the Kyd metamorphic belt and a slight spotting may be present in some black slates in the vicinity of specimens 709K1, 709K2, and 737R.
3. Analytical procedures and results Rb and Sr contents and Sr isotopic compositions reported in Table 1 were obtained by standard methods. Sr isotopic composition was determined from both spiked and unspiked samples; agreement between runs using each method is 0.0003 (95% confidence level). All isotopic ratios are based on S6Sr/aaSr = 0.I 194; and aTSr/a6Sr for Eimer and Amend standard SrCO3 = 0.7080. Total Rb and Sr were determined by both standard isotope dilution techniques and by replicate X-ray spectrography; G-l, G-2, GSP-1, and
/ 0,~
87SR
714K3
73~ 031
030 0
+
79710R 9K2
709K~/~/
87R=/86SR 1=
2'
Fig. 2. Possible whole-rock isochron, samples from the Samra (709K1, 709K2, 791R), Beda (714K3, 737R), and Um Zariq (835) rock units (see text for discussion) from the Wadi Kyd area, Sinai Peninsula (see Fig. 1).
Rb-Sr AGE DATING OF ROCKS FROM SINAI TABLE 1 Rb-Sr analytical data Unit name
Sample 87Sr/86Sr
Rb (ppm)
Sr STRb/ (ppm) 86Sr
Samra Samra Samra Beds Beda Um Zariq
709K1 709K2 791R 714K3 737R 835
70.3 8.1.5 105.2 32.5 71.9 92.2
248.9 0.82 164 1.43 233.8 1.29 62.6 1.51 182 1.15 104.1 2.55
0.7115 0.7171 0.7147 0.7252 0.7160 0.7333
BCR-1 were used as standards for the latter. The errors for the 87Rb/a6Sr data in Table 1 are 2% (95% level). The decay constant for 87Rb is assumed to be 1.39 X 10-11/yr. The isochron (Fig. 2) was calculated from the data in Table 1 using the least-squares method of York [4]. The error assigned to the age is based on one standard deviation.
4. Interpretation of the geochronological age The predominantly east-west trend of the metasedimentary rocks in Sinai [1] and the 934 + 80-m.y. date suggest that these rocks can at least tentatively be equated with the Saudi Arabian "Kibaran cycle" [5, 6] with a given date of about 1000 m.y. on metamorphosed greenstones and metasediments. The latter are also east-west trending. Fleck et al. [7] point out R b - S r whole-rocks dates in Saudi Arabia ranging between 759 + 36 m.y. to 958 +- 22 m.y. on tonalitic and granodioritic gneisses which cut rocks folded at least three times. The Sinai analogue to the latter rocks may be: (a) the regenerated and refoliated basement gneisses and migmatites (Knayah formation) which cut across the polyphasaly folded supracrustal rocks, or (b) the granodioritic gneisses (Elat gneiss, Vatir gneiss; [1]) which were emplaced after the first and into the second phase of deformation in the Elat area (Fig. 1), but which are also found in large masses in Sinai (Wadi Vatir and Wadi Zalaqa areas). Brown (written communication) has called the author's (D.G.B.) attention to a R b - S r whole-rock isochron [9] date of 622 m.y. for igneous rocks which cut rocks in Saudi Arabia considered to be equivalent to the
139 Shadli Series in Sinai. This 622-m.y. date sets a minimum on the supracrustal rocks. However, this minimum can be considerably extended since Brown et al. [6] refer to Saudi Arabian syntectonic granitic rocks with dates of 800-650 m.y. These schlieric and gneissose rocks show many similarities to the widespread Sinai syntectonic dioritic rocks (Border diorites) which are clearly intrusive into and spacially related to the Knayah formation migmatites and gneisses, and also into the supracrustal formations. The 934 + 80-m.y. date obtained for the fossil bearing and associated rocks from the Sinai supracrustal formations is presumably not far from the age of formation of these rocks as Shimron and Horowitz [3] have pointed out that the algae are similar to fauna from Bitter Springs, Australia, which have tentatively been assigned a 900-m.y. date by Nagy [10; see also Schopf and Blacic, 111. Further, algae of about this age or possible younger (e.g. 700-900 m.y.) have also been reported from Jersey Channel Islands by Squire [12]. The 934 + 80-m.y. date from Sinai may, however, be a minimum date if some autometamorphic event such as a younger slight contact metamorphic event (as mentioned above) caused an effective age lowering by K- (and Rb-) addition or partial recrystallization. It should perhaps also be pointed out that evidence for more primitive filamentous blue-green algae has also been found in the Swaziland and Bulawayan sequences with ages exceeding 3 billion years [13].
Acknowledgements One of us (D.G.B.) acknowledges partial financial support from the National Science Foundation Grant GA-36133. Mr. W. Doering, U.S. Geological Survey, kindly provided the Rb and Sr X-ray fluorescence analyses. B. Mukhopadhyay, S.L. Bolivar, and M.J. Lee assisted in the chemistry and mass spectrometry. Field work in the Sinai Peninsula (A.E.S.) was made possible by a grant from the Bat-Sheva de Rothschild Formation, tire Hebrew University and others. The work continued at the Dept. of Geology, University of Glasgow whereby the partial financial support from the Micheal and Anna Wix Trust is gratefully acknowledged.
140
A.E. SHIMRON AND D.G. BROOKINS
References 1
2
3 4 5 6
7 R.J. Fleck, W.R. Greenwood, D.G. Hadley and W.C. Prinz, Age and origin of tonalite-granodiorite gneisses, western A.E. Shimron, The Precambrian structural and metamorSaudi Arabia, EOS Trans. Am. Geophys. Union 54 (1973) 12~ phic history of the Elat area, Ph. 1~. Thesis (unpublished), 8 V. Kazmin, Precambrian of Ethiopia, Nature 230 (1971) Hebrew University, Jerusalem (1973). 176. H.M.E. Schiirmann, The Precambrian along the Gulf of 9 H. Lenz, F. Bender, C. Besand, W. Harre, H. Kreuzer, P. Suez and the northern part of the Red Sea (E.J. BriU, Leiden, Muller and I. Wendt, The age of early tectonic events in the 1966). zone of the Jordan geosuture, Int. Geol. Congr. 24th Sess., A.E. Shimron and A. Horowitz, Precambrian organic microMontreal 3 (1972) 371. fossils from Sinai, Pollen and Spores 14 (1972) 333. 10 L.A. Nagy, Transvaal stromatolite: first evidence for the D. York, Least-squares fitting of a straight line, Can. J. diversification of cells about 2.2 × 109 years ago, Science Phys. 44 (1966) 1079. 183 (1974) 514. G.F. Brown, Tectonic Map of the Arabian Peninsula, Map 11 J.W. Schopf and J.M. Blacic, New Microorganisms from AP-2, Jiddah, Saudi Arabia (1972). the Bitter Springs Formation (Late Precambrian) or the north-central Amadeus Basin, Australia, J. Paleontol. 45 G.F. Brown and R.G. Coleman, The tectonic framework of the Arabian Peninsula, Int. Geol. Congr. 24th Sess., (1971) 925. 12 A.D. Squire, Discovery of Late Precambrian trace fossils in Montreal 3 (1972) 300. Jersey, Channel Islands, Geol. Mag. 110 (1973) 223. 13 K.A. Kvenvolden, Organic geochemistry of early Precambrian sediments, Int. Geol. Congr. 24th Sess., Montreal 1 (1972) 31.