Paper No. 1
Presentation Time: 9:00 AM-6:30 PM
MICROBIALLY MEDIATED IRON-OXIDE BANDS AT PETRA (SOUTHERN JORDAN) ARE OXIDATION PRODUCTS OF LATE DIAGENETIC SIDERITE
At Petra, a world-famous archaeological site, ancient Arabs carved elaborate edifices into quartz arenite of the Cambrian Umm Ishrin Formation. The carvings reveal elaborate, scalloped patterns of diagenetic iron oxide that most geologists would call Liesegang banding. We have seen similar patterns developed in the Triassic Shinarump Member of the Chinle Formation and the Jurassic Navajo Sandstone of the Colorado Plateau, USA. In all these rocks, accumulation of iron oxide along joints indicates that they developed after lithification, during late diagenesis. As in both of the North American sandstones, iron oxide pseudomorphs after siderite (FeCO3
) are spatially associated with the scalloped patterns in the Umm Ishrin Formation at Petra. SEM images of the convex edges of dense iron-oxide bands from Petra reveal structures closely resembling the twisted stalks of the iron-oxidizing microbe Gallionella
. The scallops that comprise the patterns in the Umm Ishrin Formation are an order of magnitude larger than those within the Colorado Plateau sandstones. Large, three-dimensional manganese-oxide dendrites are present in the Jordanian sandstones, and, as in the Navajo Sandstone, these structures are associated with preserved (unoxidized) manganese-rich carbonates.
Unlike the Shinarump Member (where siderite developed early due to methanic floodplain deposits), the fluvial Umm Ishrin was deposited before the appearance of land plants. The Navajo Sandstone was also deposited in a setting with little or no syndepositional organic material. Trapping of methane and CO2 in the sandstone of the overlying Disi Formation bleached that unit. As CO2 dissolved in the formation waters, they became more dense and carried Fe++ and Mn++ downward from the Disi into the Umm Ishrin where the waters reached saturation for siderite and manganese carbonates. Methane was likely sourced by deeply buried late Cretaceous strata in the Dead Sea Fault Zone, and CO2 came from Neogene magmas. As uplift along the eastern shoulder of the fault zone continued, oxidizing ground water started to reach the iron and manganese carbonates, initiating the formation of the bands and dendrites. The bands exposed at Petra, no longer forming because they lie above the water table, are now being broken by active faults at the margins of the Petra graben.