Journal of African Earth Sciences, Vol. 15, No. 2, pp. 21%236, 1992. 0899-5362192 $5.00+0.00 Printed in Great Britain © 1992 Pergamon Press Lid A carbonate-banded iron formation transition in the Early Protorezoicum of South Africa I. W. HXl~ICa, D. LAMPRECHT,W. ALTERMANNand U. E. HORSTMANN Department of Geology,University of Stellenbosch, Stellenboseh 7600, Republic of South Africa (First received 3rd June, 1992; revised form received 12th August, 1992) Abstract - Seven new and two re.surveyedstratigraphic sections through the important carbonate-BIF transitionin GriqualandWest are presentedand comparedwith six publishedsections. Lateralcorrelation within this zone is attemptedbut the variability was found to be too great for meaningfulsubdivision. Substantial lithologicalirregularity is the only unifyingcharacter of this zone, for which the new name Finsch Member (Formation)is proposed. Vertical and lateral lithologicalvariations as well as chemical changes across this zone are discussed with reference to environmental aspects. Local and regional considerations lead to the conclusion that fresh water-sea water mixing occurred in a shallowingbasin. INTRODUCTION environmental aspects thought to be recorded by the vertical transition and the lateral correlation The apparently conformable Early Proterozoic are discussed. transition from the Campbellrand carbonates to the Kuruman Iron Formation in the northern Cape NEW PROFILES Province is a boundary that deserves careful scru- tiny. Motivation Assuming that it is tectonically undisturbed all In 1987, a very detailed (cm scale) lithological over (which is not certain at all, as will be shown and chemical survey of the transition zone in the below), the change in composition from carbonate Finsch open cast mine was conducted (Lamprecht, to Fe-silicate is enough reason to carefully record 1993). These revealed that 24 m of alternating the transition at as many sites as possible. Only a cherts and ferruginous mudstones intervened few cores are available. Man-made outcrops are between a lower 9 m thick sequence of BIF macro- confined to the Finsch open cast mh~e and the cycles and the upper continuous BIF sequence. access road to the relay station at Gakarosa Then, published sections of this transition beacon (Fig. 1). Natural surface exposures with (Beukes, 1978; 1980a; 1980b; 1983), some of some topographic expression are numerous, but which are still the best available and are therefore karsting in the underlying carbonates has included in Fig. 11, did not record a prominent commonly disturbed and recemented the beds in silici-clastic zone in the transition north of the the iron formation. Shales, mudstones and black Griquatown Fault (Fig. 1). tuffites in Iron formation, when pyrite-bearing, From 1988 to 1990, indisputable tectonic dupli- weather very rapidly and make extremely poor cation was discovered in the Kuruman and outcrops. However, compared to core sections, Griquatown Iron Formations, south of the Griqua- surface outcrops allow an accurate assessment of town Fault (Fig. I) (Altermann and H~Ibich, 1990). the superposition of bedding, the type of bed- Bedding-parallel brecciation and intensely folded forms, as well as the larger types of internal zones were discovered in new profiles measured bedding features. Thus slumping, contortions, north of the Griquatown Fault during 1988 and are tectonic folding, paleo-erosion features, local shear reported on below. These findings cast some doubt cleavage, brecciation, as well as dislocation on the stratigraphic correlations attempted parallel to and across bedding, are recognized with previously from N to S across hundreds of Idlo- more confidence in surface and mine outcrops. metres ofbanded Iron formations andbased almost This contribution puts on record seven new exclusively on borehole data (Beukes, op. cit). transition profiles and two resurveyed ones. We try When Altermann and Herbig (I 991) found shallow to correlate laterally the lithologies of 15 sections water limestone facies in the upper part of the (Figs 1 and 1 I) over 450 km of strike. Changing Campbellrand Subgroup south of the Griquatown 217 218 I. W. I-LU~ICH,D. LAMPRECI-IT, W. ALTERMANNand U. E. HORSTMANN fault zone, where basinal facies had been assumed considered for the two chemical sequences and so far (e.g., Beukes, 1980a), it became clear that their transition. More information on the com- the environmental interpretations had to be re- paratively narrow transition was therefore needed. Olifantshoek Sequence =Ad 2~4° Ongeluk Lava Koegas Subgroup l Pomfret Griquatown Iron Form. i j 26 ° _ Kuruman Iron Formation Campbellrand Subgroup / 50 km g -27 ° 27 °_ V, ~tn- _28° 28 ° - Kimberley O :.vi:.. Gri uatown I I/ -29 ° i~i \ Griquatown Fault iSpioenkop " aap,~a% O~"at°n II We 3APETOWN~ 22 ° 23 ° I I Fig. 1. Outcrop area of Asbesheuwels Subgroup (Kuruman and Griquatown Iron Formations) showing 15 section and profile sites of the transition zone (full squares). AD = Adelaide BH AD-5; Ts = Tsineng; Ku = Kurumankop; Wh = White Bank, Bh Wb-98; Sp = Spitsberg; Ga = Gakarosa; De = Derby, BH DI-1; GL = Gladstone; Da = Danielskuil: Ou = Ouplaas; Wa ffi Warrendale; Fi = Flnsch Mine; Ho = Hopefleld; Kn = Klein Naute; We = Westerberg; BH W- 1. A carbonate-banded iron formation Wansition in the Early Protorczoicum of South Africa 219 The Finsch Profile being >7 but <9. Cyclicity was probably seasonally Chemical analyses of the major and trace controlled. River influx was low and biogenic acti- element contents of 50 selected samples covering vity at the sediment-water interface high, with slow all lithological changes observed in this profile burial rates. (Fig. 2), reveal that in the iron formation i.e. units Carbonate precipitation is enhanced and cycli- "e" and '~", K, Na, Ca, Rb and Sr have lower average city drastically reduced in unit '"o". Better shelter- concentrations as compared with core analyses ing and even lesser clastic influx, if any, are indi- from Griqualand West BIF and others world wide, cated. (Horstmann and H~bich, submitted) whereas A1 is The first occurrence of regular, microbanded enriched. No detectable difference to the remaining cherty layers in unit "c" is accompanied by major and trace elements has occurred. Twenty decreasing biogenic remains (Fig. 2b, graphs). Thus, subsections were measured to 0.5 cm accuracy on conditions either became too harsh for living forms eight benches (from 112 m to 208 m) in Finsch or their remains were oxidized before burial Mine (Fig. 2a). Correlation from one subsection to because of too low burial rates. Clastic input was the next was controlled using sets of marker virtually absent and pH dropped below 7, to allow mesobands. Care was taken to stay away from chert to precipitate. A stronger, at least periodic karsted and slumped areas and from several thin but very sluggish fresh water inflow or excessive kimberlite dykes. Dips to the west are extremely periodic rain into a practically closed environment low, measurable by extrapolation only. The litho- is indicated. The cyclicity is pronounced and was logical description in Fig. 2b is used in the follow- probably seasonally (evaporation) controlled. Fe- ing discussion. Mg precipitation is registered in ankerite-siderite What follows is a resum~ of conclusions drawn mineralization. from a detailed environmental analysis by D. F. This trend is continued with a massive ankeritic Lamprecht (1993). chert in unit "d". Throughout units "c" and "d" the Unit "a" has beds of microscopic pellets and Fe/Mg ratio steadily increases upwards (Fig. 2b, sedimentary breccias. These and cryptalgal mats, graphs). As from unit "e" upwards, SiO 2 and Fe20 a with conophyton-like structures only a few mm are the dominant major oxides together making up high, point to shallow water and periodically higher some 88 % of banded ironstone (units "e" and ']") kinetic energy, in spite of the upward increasing and 84 % of mudstones (units "f' to "i") in Fig. 2b frequency of black, pyritic shales. It is concluded (Table 1). This means that a very sudden and that this unit was deposited in mainly tranquil drastic change occurred in the source supplying waters, with anoxic conditions underneath the solute to the water body as well as in redox and pH sediment/water interface, producing black shales conditions. At the same stratigraphic level the that were periodically stirred up. A sheltered bay or preservation of stilpnomelane mesobands of lagoon behind a barrier island or carbonate reef is volcaniclastic origin is recorded in the profile (La envisaged. Conditions were mainly marine, the pH Berge, 1966a + b). The distribution of these tuffites Table 1. Density and length weighted averages of major element oxides in the transition zone. Unit* j h+i g f e e+J/2 Aver. ftoj SiO 2 46.04 61.67 64.54 18.01 57.57 50.02 46.46 TiO 2 0.06 0.12 0.6 0.07 nd 0.04 0. 17 AI203 1.31 2.41 12.89 1.62 0.26 0.93 3.56 Fe203 38.71 27.9 7.93 64.06 36.8 38.02 38.12 MnO 0.19 0.36 0.04 2.36 0.03 0.15 1.03 MgO 2.15 0.7 1.43 0.91 0.71 1.63 0.87 CaO 2.61 0.99 0.46 1.18 1.67 1.98 0.98 Na~O 0.21 0.08 0.12 0.1 0.09 0.11 0. I K20 0.08 0.08 5.92 0.24 nd 0.05 0.94 P205 0.08 0.07 0.05 0.68 0.07 0.07 0.29 LOI 7.53 4.4 3.9 8.39 2.87 5.85 5.76 H20 1.84 2.57 1.4 1.77 0.46 1.34 2.12 Tot. 100.81 101.35 99.28 99.39 100.53 100.37 100.4 * See stratigraphic units in Fig.