Precambrian Research 101 2000 69 – 78
Letter
Setting and origin for problematic rocks from the \ 3.7 Ga Isua Greenstone Belt, southern west Greenland: Earth’s
oldest coarse clastic sediments
Christopher M. Fedo
Department of Geology, Bell Hall, George Washington Uni6ersity, Washington, DC
20052
, USA Received 14 September 1999; accepted 18 November 1999
Abstract
Whether or not coarse detrital sedimentary rocks occur within the \ 3.7 Ga Isua Greenstone Belt IGB, southern west Greenland, has been debated for some time. Repeated, regional metamorphic, deformational, and metasomatic
events have obscured most protolith lithologies leading to misunderstandings about the stratigraphy and environ- ments of deposition. Rocks here interpreted as meta-conglomerate crop out in a fault-bounded structural domain that
is lower in strain relative to adjacent domains. The meta-conglomerate has a strike length of 1 km and is 10 m thick. Bed thickness ranges from 10 cm to more than 1 m, and beds may be either framework- or matrix-supported.
A poorly sorted and variably rounded polymict assemblage of framework clasts consisting of meta-chert, BIF, and a variety of mafic volcanic rock fragments are set into a matrix of biotite + quartz + garnet schist; clast compositions
indicate reworking of adjacent stratigraphic units. A lack of structural similarity in framework clasts, range of grain sizes, range of rounding, and polymict composition demand a primary sedimentary origin for the deposit. Inferred
depositional processes include traction and debris flow, which would be consistent with subaerial or shallow subaqueous environments, although the limited extent of the meta-conglomerate warrants interpretational caution.
© 2000 Elsevier Science B.V. All rights reserved.
Keywords
:
Isua; Archean; Greenstone; Sedimentary; Conglomerate www.elsevier.comlocateprecamres
1. Introduction
The presence of coarse detrital sedimentary rocks within the ‘stratigraphy’ of the Isua
supracrustal succession [Isua Greenstone Belt IGB Appel et al., 1998] has been postulated
many times beginning with early geologic descrip-
Corresponding author. Tel.: + 1-202-9946964; fax: + 1- 202-9940450.
E-mail address
:
cfedogwu.edu C.M. Fedo 0301-926800 - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 1 - 9 2 6 8 9 9 0 0 1 0 0 - X
tions Allaart, 1976; Bridgwater et al., 1979; Dim- roth, 1982; Nutman et al., 1984. Despite the
apparent recognition of such coarse rocks, there consistently has been concern that some of them
could in fact represent severely tectonized rocks of uncertain protolith Bridgwater et al., 1979; Nut-
man et al., 1984, whose current outcrop appear- ance
merely resembles
metamorphosed ‘conglomerate’. Lack of consensus on this very
basic matter still exists, and leads to significant confusion when discussing and interpreting the
setting in which these historically problematic rocks were formed.
One problem in assessing possible origins for supposed clastic sedimentary rocks at Isua is the
occurrence of several types of coarse-grained rocks, especially those termed ‘conglomeratic
structures’ by Nutman et al. 1984, who recog- nized three specific types, namely 1 ‘flat pebble
conglomerate structure’; 2 ‘round pebble con- glomerate structure’ and 3 ‘conglomeratic struc-
ture in the felsic formation of sequence A’. The three different types of conglomerate are exposed
in ‘Sequence A’ Nutman et al., 1984; Nutman, 1986, which represents a poly-deformed and
lithologically diverse set of rocks that comprise most of the supracrustal succession of the IGB.
Based on new detailed stratigraphic by C.M. Fedo and regional structural by J.S. Myers
mapping in the main study area Fig. 1, it has become clear that the stratigraphy and protolith
lithologies as outlined by Nutman et al. 1984 needed significant revision Rosing et al., 1996;
Appel et al., 1998 in order to better interpret the paleogeography. Utilizing new data collected dur-
ing three field seasons 1997 – 1999, the purpose of this letter is to 1 re-examine the stratigraphic
setting; 2 critically evaluate whether or not coarse detrital sediments occur within the belt,
and 3 discuss the paleogeographic implications.
2. Geologic framework
