Graduation Year

2002

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Geology

Major Professor

Ryan, Jeffrey G.

Keywords

meteorite impact, impact crater, astrobleme, cryptovolcanic, shock metamorphism, impactites, spherules, maskelynite, lithic breccias, asteroid impact, breccias, melt breccias, glass breccias, impacts

Abstract

The Gatun Structure, (Latitude N 09° 05ʹ 58.1", Longitude W 79° 47ʹ 21.8", situated in the triple-canopy rainforest 10 km to the WSW of the Gamboa and about 2 km south of the Isle of Barbacoas, Republic de Panama), is a partially inundated, quasi-concentric surface feature 2.2 - 3km in diameter, which appears in aerial photographs and in radar imagery as an arcuate chain of islands with a raised center. Although the structure has been heavily weathered and altered, it has retained morphology consistent with complex craters: an elevated circular central uplift 500-600 m in diameter and 50m high, and arcuate boundary ridges (a rim structure?) ranging from 50-100 meters high. Within the central peak, highly altered and fractured siltstone of the Gatuncillo (?) formation (Eocene) (+-) older rocks are uplifted and exposed through surrounding calcareous units of the Caimito formation (Oligocene) and the Las Cascadas formation (Miocene), the major target rocks in the region.

Lithologies in the structure include highly fractured siliciclastic rocks (siltstone, sandstones and greywackes), limestones with anomalous spherical glass inclusions, both black and white hypocrystalline glasses (possible melt rocks), lithic fragmental breccias, and melt-bearing breccias (possible impact melt breccias and suevites), some of which contain flow banding and evidence for selective melting of minerals. Three types of spherules (glass, fluid-drop and lithic), a pyroxene-quartz "necklace" disequilibrium structure (coronas), plagioclase feldspars exhibiting mosaicism and partially amorphization, possible liquid immiscibility between melts of calcite and felpspathic glass, as well as decomposition of titanomagnite or ulvospinel, are all petrographic indicators of a hypervelocity impact event.

The structure is crosscut by numerous dikes of unshocked basalt and basaltic andesite related to volcanism along the Panamanian segment of the Central American arc to the south. However, the lithologies of the Gatun Structure are chemically inconsistent with the regional volcanic rocks and the unshocked volcanic rocks that crosscut the structure. The lack of an igneous relationship between the Gatun structure and the explosive volcanism of Panamanian arc the presence of classical shock lithologies within the site, and the occurrence of spherules, maskelynite and other disequilibrium shock features in the rocks, an impact origin is our preferred interpretation for the Gatun structure.

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