Origin, palaeoecology and stratigraphic significance of bored and encrusted concretions from the Upper Cretaceous (Santonian) of southern Israel

Abstract

Reworked concretions 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 reworked concretions since the Ordovician, so they have been important contributors to our understanding of the evolution of these ecological systems. Here, we describe reworked concretions from southern Israel where they are critical for interpreting the stratigraphy and paleoenvironment of an Upper Cretaceous sedimentary sequence. These cobblesized concretions (averaging roughly 1,000 cm 3 ) are found at the base of the Menuha Formation (Santonian to lower Campanian, Mount Scopus Group) unconformably above the top of the Zihor Formation (Turonian-Coniacian, Judea Group) exposed in the Ramon region of the Negev Highlands. The concretions are almost entirely composed of micritic limestone, and many are exhumed, cemented burrow-fills apparently from 10-20 m of upper Zihor Formation strata removed by erosion. There are also a few cobbles of dolomitic limestone and rare vertebrate bone.The cobbles are moderately to heavily bored by bivalves (producing Gastrochaenolites) and worms (forming Trypanites), and a few have cemented 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 wave base. 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). These reworked cobbles show that the initial deposits of the Menuha Formation accumulated in very shallow water. This has important implications for the development of the Syrian Arc structures in this region, especially the Ramon Monocline.

Microconchid-dominated cobbles from the Upper Devonian of Russia: Opportunism and dominance in a restricted environment following the Frasnian–Famennian biotic crisis

“…Such pioneering colonization by boring biota has previously been detected in many other mobile hard substrates, as in Middle Jurassic concretions and Upper Cretaceous cobbles . Interestingly, as in the present case, the Upper Cretaceous cobbles were also colonized first by biota producing the boring Trypanites (see Wilson et al, 2012). As many encrusters directly cover the empty borings, it is evident that they colonized the cobbles after the borers vanished.…”

“…This type of cobble must have originated via burrowing of firm substrate by some invertebrates, followed by burrow cementation and exhumation due to erosion of surrounding shelly material. Such cobbles are reminiscent of the so-called hiatus concretions (Voigt, 1968; see also for a thorough review) or reworked concretions (Fürsich et al, 1992;Wilson et al, 2012). Late Devonian (Frasnian) hiatus concretions are also well known in the USA (Baird, 1976(Baird, , 1981.…”

Analysis of bioerosion in clasts from a Miocene rocky-shore, Concepción Formation, Veracruz, México

“…8). Similar cases with several cycles of bioerosion of pebbles-boulders have been reported and associated with eustatic sea-level changes in Eocene-Pliocene outcrops from Europe (Radwański, 1970;Roniewicz, 1970;Wilson, 1985Wilson, , 1986Wilson, , 1987Gibert and Martinell, 1992;Siggerud et al, 2000;Wilson and Taylor, 2001;Taylor and Wilson, 2003;Johnson et al, 2011;Wilson et al, 2012;Baarli et al, 2013). Clasts that show only one side with traces of bioerosion are those that were not removed or are those with other faces only briefly exposed.…”

Kimmeridgian hardground-sequence boundary from the Mesozoic margin of the Holy Cross Mountains (central Poland): implications for the evolution of the northern Tethyan carbonate shelf

“…The composition of the borings, and especially the domination of Gastrochaenolites, indicate that colonization of the clasts generally proceeded in a shallow-water environment (e.g., Bromley 1994;Wilson et al 2012), where the hydrodynamics were strong enough to overturn the clasts, as evidenced by the borings and encrustations present on Deposition of the clasts occurred in a shallow, subtidal environment, possibly often in restricted areas where gastropods, serpulids, and bivalves dominated. The ferruginous outer parts of the clasts and ferruginous oxidative spots, along with laminae and aureoles around the borings and cracks, could have been a consequence of surface oxidation of pyrite grains during weathering and reworking of the clasts on an oxygenated seafloor in extremely shallow water.…”