Hollow carbonate pebbles: a case study of selective secondary porosity generation, Neogene, Israel

• Published in: Sedimentary geology 1996-06, Vol.103 (3), p.161-174
• Main Authors: Kafri, Uri, Sass, Eytan
• Format: Article
• Language: English
• ISSN: 0037-0738; 1879-0968
• DOI: 10.1016/0037-0738(95)00085-2

Neogene conglomerates are very common in Israel. In a few conglomerate localities in the hilly region of Galilee, northern Israel, a large number of the pebbles have internal cavities. This puzzling phenomenon has not been documented previously in Israel or elsewhere. The studied conglomerates occur as lithified, calcite-cemented lenticular beds, which alternate with reddish-brown argillaceous mudstones. The hollow pebbles constitute, in fact, only a special case of a wider range of related features. The most relevant observations related to these occurrences are the following. (1) The clasts are polymictic, consisting of various limestones, dolomites, and minor cherts. The hollow pebbles and related features, however, occur only in porous dolomites. (2) Calcitized, dedolomitic hard rims characterize all the affected pebbles. The thickness of these rims varies between a few mm and 1–2 cm. (3) In part of the affected pebbles, the dolomitic core (inside the calcitic rims) is preserved intact. In others, only the hard rim is preserved, and the core is empty. These are the hollow pebbles sensu stricto. (4) Late-stage calcite spar occurs in some of the voids and in veins that cross the groundmass of the conglomerates. The first step in the sequence of events leading to the production of the described occurrences is inferred to be the calcitization of porous dolomite pebbles, resulting in the formation of dense rims around porous cores. The next step involved the dissolution of the inner core of the pebble by acidic solutions, leaving the calcitic rims almost intact. The restriction of hollow pebbles (and related features) occurrences to conglomerate strata which are interbedded with argillaceous mudstones points to a causal relationship between the two. Accordingly, the precipitation of calcite during the first step is attributed to the establishment of anoxic microenvironments within the porewater soon after the accumulation of the argillaceous material. The reversal of conditions from carbonate precipitation to dissolution (during the second step) seems to be related to a change from reducing to oxidizing microenvironments, following the emergence of the fluviatile sediments from the aquatic environments. The critical requirements for obtaining the hollow pebbles seem, therefore, to be the presence of a ‘suitable’ lithological type of pebble, namely, porous dolomite, combined with a special sequence of depositional and diagenetic environments. The apparent ra