BORED HIATUS COBBLES AS PALEOENVIRONMENTAL AND STRATIGRAPHIC INDICATORS: AN EXAMPLE FROM THE UPPER CRETACEOUS OF SOUTHERN ISRAEL (MENUHA FORMATION, SANTONIAN)

Hiatus cobbles have been significant substrates for boring and encrusting organisms through the Phanerozoic. They provide large, relatively stable calcareous surfaces in systems where sedimentation is minimal. Diverse sclerobiont communities have inhabited hiatus cobbles since the Cambrian, so they have been important contributors to our understanding of the evolution of these ecological systems. Here we review hiatus cobble formation and colonization, and then describe an example from southern Israel where hiatus cobbles are critical for interpreting the paleoenvironment of an Upper Cretaceous sedimentary sequence. These cobbles are found at the base of the Menuha Formation (Santonian, Mount Scopus Group) unconformably above the top of the Zihor Formation (Turonian-Coniacian, Judea Group) exposed in the Makhtesh Ramon region of the Negev Highlands. The cobbles are almost entirely composed of micritic limestone, and many are exhumed cemented burrow-fills apparently from 10-20 meters of upper Zihor Formation strata removed by erosion. There are also a few cobbles of dolomitic limestone and rare vertebrate bone. The average cobble is roughly 1000 cubic centimeters in volume, and many are exhumed cemented burrow-fills. The cobbles are moderately to heavily bored by bivalves (producing Gastrochaenolites) and worms (forming Trypanites), and some have remnant attachments of oysters. They are densely arrayed in a single layer, often touching each other or only a few centimeters apart. The sclerobionts associated with the cobbles, along with their hydrodynamic arrangement, strongly suggest that these cobbles accumulated in very shallow water above normal wavebase. Most of them (77%) are encrusted on their top surfaces only, indicating that they were bored in place and not later delivered to a deeper environment by submarine currents. The rest of the Menuha Formation above is a chalk with relatively few macrofossils (primarily shark teeth and oysters) and a few trace fossils (Planolites and Thalassinoides are the most common). Sorting out its depositional environment has been contentious, but the hiatus cobbles show that at least the initial deposits were in very shallow water. This has important implications for the development of the Syrian Arc structures in this region, especially the Ramon Monocline.