HOLLOW CARBONATE PEBBLES - A CASE-STUDY OF SELECTIVE SECONDARY POROSITY GENERATION, NEOGENE, ISRAEL

Neogene conglomerates are very common in Israel. In a few conglomerate localities in the hilly region of Galilee, northern Israel, a large n umber 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, w hich alternate with reddish-brown argillaceous mudstones. The hollow p ebbles constitute, in fact, only a special case of a wider range of re lated features. The most relevant observations related to these occurr ences are the following. (1) The clasts are polymictic, consisting of various limestones, dolomites, and minor cherts. The hollow pebbles an d related features, however, occur only in porous dolomites. (2) Calci tized, dedolomitic hard rims characterize all the affected pebbles. Th e thickness of these rims varies between a few mm and 1-2 cm. (3) In p art of the affected pebbles, the dolomitic core (inside the calcitic r ims) is preserved intact. In others, only the hard rim is preserved, a nd 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 seque nce of events leading to the production of the described occurrences i s inferred to be the calcitization of porous dolomite pebbles, resulti ng in the formation of dense rims around porous cores. The next step i nvolved the dissolution of the inner core of the pebble by acidic solu tions, leaving the calcitic rims almost intact. The restriction of hol low pebbles (and related features) occurrences to conglomerate strata which are interbedded with argillaceous mudstones points to a causal r elationship between the two. Accordingly, the precipitation of calcite during the first step is attributed to the establishment of anoxic mi croenvironments within the porewater soon after the accumulation of th e argillaceous material. The reversal of conditions from carbonate pre cipitation to dissolution (during the second step) seems to be related to a change from reducing to oxidizing microenvironments, following t he emergence of the fluviatile sediments from the aquatic environments . The critical requirements for obtaining the hollow pebbles seem, the refore, to be the presence of a 'suitable' lithological type of pebble , namely, porous dolomite, combined with a special sequence of deposit ional and diagenetic environments. The apparent rarity of this combina tion seems to account for the lack of documentation on this phenomenon to this date.