Geologia da croácia

Geologia Croatica

61/2–3

73–103

14 Figs.

1 Tab.

Zagreb 2008 73

Abiogenic, microbial and hybrid authigenic carbonate crusts: components of Precambrian stromatolites
Robert Riding

Department of Earth and Planetary Sciences, University of Tennessee, 1412 Circle Drive, Knoxville, TN 37996, USA; ([e-mail])

Geologia Croatica
ABSTRACT
Authigenic seafloor carbonate crusts include fenestrate microbialite, thrombolite, and four types here designated: Fine-grained Crust, Sparry Crust, Hybrid Sparry Fine-grained Crust, and Sparry Crust plus Coarse Grains. Each of the latter four types includes at least some layered examples that have generally been regarded as stromatolites. Recognition and interpretation of these various deposits assists understanding of stromatolite development. Sparry Crust is common in the Late Archaean-Mesoproterozoic. It includes botryoidal fans and other crystal pseudomorphs, microdigitate stromatolite, dendrite, isopachous laminite, and herringbone calcite. Although differing in primary mineralogy and bedform, these are all characterized by coarse sparry, commonly radial fibrous, fabric and appear light coloured in thin-section. They have commonly been referred to as seafloor cement, although they formed at the open sediment-water interface rather than as void-fills. Two of them in particular, isopachous laminite and microdigitate “tufa”, typically form isopachous layers with good vertical inheritance and have been regarded as stromatolites. In contrast to Sparry Crust, Fine-grained Crust has fine-grained (micritic, clotted, peloidal, filamentous) microfabric that appears dark in thin-section, and irregular uneven layering with relatively poor inheritance. Mixed crusts, composed of millimetric alternations of Sparry and Fine-grained crust, are here termed Hybrid Sparry Finegrained Crust. Sparry Crust with coarse allochthonous grains – here termed Sparry Crust plus Coarse Grains – includes some examples that have been