Depositional environments and diagenesis of north Gondwana Ediacaran clays (Zenifim and Elat Conglomerate Formations, Israel)

Abstract

A thick sequence of arkosic sandstones and conglomerates, locally interbedded with volcanic rocks, was deposited in the piedmont region and within intramountain basins of the northern Arabian-Nubian Shield (ANS) during the late Neoproterozoic Ediacaran period. We have studied the geochemistry and clay mineralogy of these sediments in outcrops and drill cores along a ∼200 km long transect in southern Israel, in order to constrain their depositional environments and the diagenetic processes that have affected them. Whole rock trace element data for the Israeli Ediacaran sediments indicates magmatic island arc provenance, consistent with a source in the ANS. Major elements data indicate the importance of pedogenic processes in the source area and modification by grain size sorting during transport and deposition. In the southernmost part of the study area, clay assemblages of the Ediacaran formations include significant proportions of the mineral illite–smectite (I/S), including of randomly interlayered (R0) structure. In the central and northern parts of the study area, the entire Ediacaran section is dominated by illite, while the minor I/S found in this area is always ordered, and changes from R1- to R3-type structure with increasing depth. This transition from R1- to R3-type I/S seems to shallow towards the north. These trends of clay minerals composition along the Ediacaran strata reflect increased paleo-thermal gradient and/or burial depths toward the north, which is correlated with the formation of late Devonian – Carboniferous thermal structure over the Levant area. The pristine, I/S dominated clay assemblage found in the Ediacaran sediments of southernmost Israel is remarkable given these sediments' relatively old age and offers a unique opportunity to probe the Ediacaran climate in this region, which resided at low latitudes at that time. This I/S dominated, kaolinite lacking, clay assemblage, together with relatively low chemical index of alteration values found for the Ediacaran sediments of southern Israel, reflect limited chemical weathering of ANS basement rocks under relatively dry, potentially cool, climatic conditions. The persistence of such climatic conditions within the tropical belt is consistent with a series of late Ediacaran glaciation events, although negating a complete, prolonged 'Snowball Earth'-type global glaciation.