Manjeri Formation

Belingwe bell Manjeri Formation Shale MHZ141b 35... 

Bclingwe belt Manjeri Formation Shale MHZ220B 35... 

Belingwe bell Manjeri Formation Mudstone MHZ037Bb 36... 

Belingwc bcli Manjeri Formation Siltstone MHZDOBBid 36... 

Belingwe belt Manjeri Formation Black chert MHZON 36... 

Bclingwe bell Manjeri Formation Shale MHZI26a 36... 

Belingwe beli Manjeri Formation Ironstone TR43 36... 

The Manjeri Formation is overlain by Reliance Formation (ultra-) mafic volcanic rocks (Fig. 3b). The contact between the two formations is a high-strain zone of 1-5 m width over-lain by relatively unstrained pillow lavas. It has been interpreted as (1) conformable but accommodating layer-parallel sUp during refolding of the belt (Blenkinsop et al. 1993), or (2) a thrust or detachment surface that accommodated obduc-tion of an oceanic plateau (Kusky Kidd 1992). 

The carbonate horizons of the Cheshire and Manjeri Formations contain extensive well-preserved stromatolites. These were formed in shallow or intertidal waters, as demonstrated by interbedded ripple-marked and mud-cracked silts. Texturally, the limestones show many indications of organic activity and gas release structures (Martin et al. 1980). The simplest uni-formitarian explanation is that the structures were built by cyanobacteria. Isotopically, carbon from kerogen in the stromatolites typically has 6 C = -25 to -30%o. This implies but does not prove fractionation by rubisco of carbon captured from the atmosphere-ocean system. Carbonate in Cheshire limestones is typically close to 0%o, suggesting that carbon in the atmosphere-ocean system was modulated by rubisco and dominated by oxygenic photosynthesis on a planetary scale (Fig. 1) at least by this date. It should be noted that the c. 3 Ga Steep Rock stromatolites are similar (work by Abell, Grassineau and Nisbet). 

The Manjeri Formation is the lowest part of the Ngezi Group in the Belingwe belt (Bickle Nisbet 1993), Zimbabwe, one of the least deformed... 

Stable isotope results are shown in Table 2 and Figures 4 and 5 and as a profile through the Nercmar core in the Manjeri Formation at the Unconformity National Monument outcrop (Fig. 6). 

To summarize, the most probable explanation of the C isotope spread in the shallow-water facies of the Manjeri Formation (Spring Valley and Shavi sediments and Rupemba (Spring Valley) stromatolites), and in the Cheshire Formation stromatohtes and black shales is thus that carbon isotope fractionation records a diverse ecology that existed based primarily on oxygenic photosynthesis, but with associated organisms... 

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