Late Ordovician (Ashgillian) glacial deposits in southern Jordan

Abstract

The Late Ordovician (Ashgillian) glacial deposits in southern Jordan, comprise a lower and upper glacially incised palaeovalley system, occupying reactivated basement and Pan-African fault-controlled depressions. The lower palaeovalley, incised into shoreface sandstones of the pre-glacial Tubeiliyat Formation, is filled with thin glaciofluvial sandstones at the base, overlain by up to 50 m of shoreface sandstone. A prominent glaciated surface near the top of this palaeovalley-fill contains intersecting glacial striations aligned E–W and NW–SE. The upper palaeovalley-fill comprises glaciofluvial and marine sandstones, incised into the lower palaeovalley or, where this is absent, into the Tubeiliyat Formation. Southern Jordan lay close to the margin of a Late Ordovician terrestrial ice sheet in Northwest Saudi Arabia, characterised by two major ice advances. These are correlated with the lower and upper palaeovalleys in southern Jordan, interrupted by two subsidiary glacial advances during late stage filling of the lower palaeovalley when ice advanced from the west and northwest. Thus, four ice advances are now recorded from the Late Ordovician glacial record of southern Jordan. Disturbed and deformed green sandstones beneath the upper palaeovalley-fill in the Jebel Ammar area, are confined to the margins of the Hutayya graben, and have been interpreted as structureless glacial loessite or glacial rock flour. Petrographic and textural analyses of the deformed sandstones, their mapped lateral transition into undeformed Tubeiliyat marine sandstones away from the fault zone, and the presence of similar sedimentary structures to those in the pre-glacial marine Tubeiliyat Formation suggest that they are a locally deformed facies equivalent of the Tubeiliyat, not part of the younger glacial deposits. Deformation is attributed to glacially induced crustal stresses and seismic reactivation of pre-existing faults, previously weakened by epeirogenesis, triggering sediment liquefaction and deformation typical of earthquake generated seismites. Deformation, confined to an area of not more than 4 km wide adjacent to the major fault zone, implies earthquake magnitudes of at least 6 (M o). The high authigenic chlorite content of deformed Tubeiliyat sandstones compared to undeformed ones is attributed to a post-seismic hydrothermal system driven by compactional dewatering and hydrofracturing of the bedrock which acted as a groundwater recharge area, supplied by subglacial meltwater from beneath the ice sheet. Fluid movement along glacial seismotectonically reactivated faults infiltrated the adjacent Tubeiliyat sandstones under pressure and elevated geothermal gradient, where chlorite was precipitated from solution.