Unusual preservation of Ophiomorpha in middle Miocene rocks ofWadi Zablah, east Matruh area, northwestern Egypt

Abstract

Unusual biogenic structures in the form of hollow burrows are preserved in middle Miocene sandy limestones (calcarenites) exposed inWadi Zablah, east of the town ofMersaMatruh along the EgyptianMediterranean coast. This calcarenite unit is about 30 m thick and lies in the middle part of the Marmarica Formation. The burrows commonly are up to 2.5 m long and can reach a length of 3 m in some outcrops. The internal diameter ranges from 1 to 5 cm. In most cases there is a hard cemented zone surrounding the hollow burrows that ranges in thickness from 0.2 to 0.4 cm.In several burrows where the hard cemented zone is preserved, a knobby sculpture is visible on the internal surfaces of the burrows. This is considered to be a negative mold of the pelleted wall of Ophiomorpha. This inverted knobby sculpture is formed by the cemented zone, which permitted the trace fossils’ preservation, the original Ophiomorpha wall having been dissolved away during or subsequent to lithification by subsequent weathering processes. The presence of these burrows in association with marine fossils suggests a littoral to very shallow shelf environment.