A review of 3.7 Ga stromatolites from the Isua Supracrustal Belt, West Greenland

Abstract

This paper reviews the origin of putative stromatolites within 3.7 Ga meta-dolostones from the Isua Supracrustal Belt (Greenland) as either bona fide biogenic structures, as other (abiogenic) types of primary sedimentary structures, or as the products of structural deformation.

Regional mapping shows that primary structures are preserved locally across a range of lithology, specifically within the hinge zones of fold structures. Examples of such primary structures – bedding, cross-bedding, and edgewise conglomerate – within the fold structure that hosts the stromatolites, which have been discounted by some as the products of high strain, are here re-affirmed to be primary structures. Importantly, this demonstrates the potential of localised exceptional preservation. Critically, bedding and cross-bedding are defined by a combination of compositional variations and grain size that, together with an asymptotic geometry of the latter present an uniquely compelling case against the formation of these features through shearing and deformational tectonic forces.

The stromatolites can be discriminated from either primary sedimentary flame structures caused by sediment loading, or sand volcanoes caused by dewatering. Similarly, they can be discriminated from necking structures formed by tectonic boudinage on geometrical grounds, nor are they boudins themselves as they are asymmetrical and irregularly spaced. They are also clearly distinct from small-scale fold structures because of the repeated flat-base versus conical-top geometry of the stromatolites across four individual layers.

Instead, the putative stromatolites display characteristic features consistent with formation as biogenic structures. They occur in specific horizons that lie within a well-preserved stratigraphy, have flat bases and coniform to domical upper surfaces, display internal laminations, and are made of dolomite with REE + Y and stable isotopic values consistent with deposition in a biologically-influenced shallow sea. They are strained structures, elongated along the plunge direction of the regional synformal anticlinal fold in which they occur (i.e., the X-axis of the local strain ellipsoid). Importantly, 3-D observations show that the coniform stromatolites taper in the finite extension direction and that overlying mica-rich meta-dolomite beds lap onto the stromatolites in all dimensions.

In conclusion, the Isua stromatolites conform to known biogenic structures from throughout the geological record, both in terms of their overall setting, but also their unique geometry that is clearly distinct from any other known mechanism of formation. Their preservation is best explained by their occurrence within a fold hinge that has experienced plane strain in a small domain that escaped infiltration by H₂O-rich metamorphic fluids into siliceous meta-dolostones.