Elemental Chemical Zonation of Black Shales Revisited -- Application to Petroleum Source Rocks? Pat Wilde Pangloss Foundation Berkeley, California Mary S. Quinby-Hunt Pangloss Foundation Berkeley, California

MSG Redox Groups compared to Average Shale Group 4 - Lower in Mn, Fe - Higher in Ba, V, Zn, As, U, Mo, Sb Group 3 - Lower in Mn, Fe - Higher in As, Mo, Sb Group 2 - Higher in Mo, Sb Group 1 - Essentially same as Average Shale

During Lower Paleozoic (pre-Devonian) Atmospheric O 2 below Present Levels. Saturation of O 2 in marine waters lower than at present, therefore Bottom waters anoxic at relatively shallow depths. Non-glacial times: marine shelf area much wider and anoxic at shallower depths where Group 3 & 4 conditions at sediment surface. At high productivity zones (off upwelling coasts and at Equator and 60 N&S) Group 3 & 4 black shales would be common sediments.

Post Devonian: higher atmospheric levels of O 2 Ocean more ventilated Group 3 & 4 black shales now much less common in open ocean shelves. Can form today in limited conditions but very uncommon on open ocean shelves –Anoxia at the Oxygen Minimum Zone on the slope and not the shelf –Areal extent and geologic preservation on slope very unlikely.

Solve for oxidant as function of depth

Unresolved: original Org C With diagenesis and petroleum migration Org C content of G4 shales should be lower with time. What would be the signature elements indicative of original Org C content that would not migrate? Targets: V, Zn, Ni, Ba, As, U. Mo, Sb. Best bet so far is Mo. Need G4 ‘modern’ samples to discern regression.

Upper Carboniferous after Cruse and Lyons (2004)

Upper Carboniferous after Cruse and Lyons (2004)

Pleistocene Cariaco Basin after Piper and Dean (2002)

Summary MSG Redox Groups Revisited Group 1: Redox above Mn++ solubility or Eh = +0.5 (pH 6) to +0.1 (pH 8) lower boundary Group 2: Redox above Fe+++ to Fe++ transition Eh = +0.5 (pH 6) to +0.1 (pH 8) upper boundary Eh = -0.1 (pH 6) to -0.2 (pH 8) lower boundary Group 3: Redox Sulfate reduction below Eh = -0.1 (pH 6) to -0.2 (pH 8) upper boundary Group 4: G3 with High Organic Productivity

Summary MSG Redox Groups Revisited + Depth of water range and shelf extent for each Group predicable if age and climate known using Wilde’s (1987) application of Wyrtki’s (1962) redox profile algorithms. + Need recalulation of Wyrtiki’s algorithm as direct function of redox for given climatic contitions..

Summary MSG Redox Groups Revisited -More work needed to discern original Org C content of G4 ancient sediments to ascertain petroleum source potential. -Modern analogs of G3/4 areally rare in Pleistocene-Modern conditions. Maybe found on the slope off Peru-Chile, Central America or in the Arabian Sea. Potentially in Pre-Pleistocene Cenozoic, but less usefully with age.

From Rutgers Oceanic Primary Productivity site:

Elemental Chemical Zonation of Black Shales Revisited -- Application to Petroleum Source Rocks? Pat Wilde Pangloss Foundation Berkeley, California Mary S. Quinby-Hunt Pangloss Foundation Berkeley, California