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Eocene Chert Resources of Papua New Guinea

George J. Carman, V. P. St. John

The Eocene Port Moresby beds consist of chert, siliceous argillite, and calcarenite in a 3,000-m sequence. A northwest grain and severe topography at outcrop reflect 50°-80° structural dips, northeastward-inclined, low-angle thrust faults, and high-angle reverse faults. Within the fault slices, symmetrical folds plunge toward the northwest, and syntaphral folds occur intermittently.

The cherts are comprised of biogenic silica and rarer spherulitic, fibrous chalcedony in massive nodular concretions up to 2 m in diameter and as competent, repetitive, 10 to 30-cm thick beds. The calcarenites are richly nummulitic, partly silicified, and contain chert nodules. Radiolaria, planktonic foraminifera, and sponge spicules predominate over reworked fauna in the cherts. Microlaminations and sparse pyrite endorse the interpretation of sheltered, possibly deep, and rapid deposition in a reducing environment.

Chert and cherty limestone crop out again, 200 km northwest at Hell's Gate, Saw Mountains. Oil and gas seeps in the intervening Lakekamu-Moresby Embayment were first recorded 75 years ago, and seismic interpretation confirms the presence of buried thrust folds. Gas and Oil fluorescence was recorded in four vintage wells, but none have tested the Eocene.

Like the Monterey Formation, fractured radiolarian cherts have potential as a "resource" in terms of both reservoir and source. Moresby chert deposition is possibly related to opening of the Coral Sea. The chert diagenesis and thermal maturation were probably influenced by Paleogene overthrusting of oceanic crust 100 km east, Neogene uplift of the Mesozoic metamorphics in the 4,000-m Owen Stanley Range, and consequential southwesterly detachment folding of the Tertiary section.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.