QUARTZ-PEBBLE CONGLOMERATES: DO THEY FORM DURING DIAGENESIS?

In quartz-pebble conglomerates (QPC) more than 90% of the clasts consist of vein quartz, chert, and quartzite. Thick, laterally extensive QPC are well-known in the rock record, but no such deposits are forming in modern environments. QPC occurrence, as measured by records in the AGI GeoRef database, increases backward through time from the Tertiary through the Precambrian. The inverse relationship between QPC abundance and stratigraphic age is seen both in numbers of reported QPC and in QPC as a percentage of all conglomerate records. This is in direct contrast to the record for sedimentary rocks as a whole, which show a well-documented decrease in preserved volume with increasing geologic age.

We propose that progressive diagenesis, whereby labile clasts have a greater probability of selective destruction over time, provides the best explanation for this distribution. Evidence in support of this hypothesis comes from petrologic comparison of polymict and quartz-pebble conglomerates. Precambrian and Tertiary conglomerates with similar sources and sedimentology have vastly different clast populations, with non-quartzose clasts much more abundant in the younger rocks. In addition, petrologic comparison of similar-aged QPC and polymict conglomerates shows that QPC have consistently higher proportions of diagenetic secondary matrix and pressure-solved grain contacts. We do not propose that all QPC originate diagenetically, and acknowledge that protracted transport, intense chemical weathering, and sediment recycling may all produce QPC. We conclude, however, that diagenetic factors can play an important role in QPC formation by preferentially destroying less durable clasts, and that this may provide the best explanation in cases where independent evidence for an alternative mechanism is lacking and appropriate petrologic indicators are present.