LATE TURONIAN MICROMETEORITES FROM UPPER EAGLE FORD GROUP, NORTH CENTRAL TEXAS

We report the discovery of fossil micrometeorites in Late Cretaceous shale of the Eagle Ford Group. Thirty eight spherules of cosmic origin were recovered from the upper part of Arcadia Park Formation, 25 miles SW of Dallas, Texas. The age of the shale is Late Turonian (89 Ma) based on the presence of planktic foraminifera from the Marginotruncana schneegansi biozone. Samples are from twelve cm below the upper disconformity with the bottom bed of Atco Member, Austin Chalk.

We isolated the micrometeorites by digesting about 0.5 kg of shale in alkaline cleaning solution, followed by sieving, drying and using a magnet on the residue. The micrometeorites are remarkably spherical and range in diameter from 90 to 170 microns. Their exteriors consist of a mosaic of dendritic crystals identified as single-phase recrystallized magnetite and Fe-silicide, testifying to rapid crystallization of high T iron-rich metallic and oxide liquids. Approximately 20% of these micrometeorites contain internal spherical cavities due to dissolution or loss of a metallic bed of Fe-Ni, feature commonly found among modern Antarctic Iron-type (I-type) cosmic spherules. On the basis of textural analysis, the magnetite and Fe-silicide spherules are shown to be mostly I-type (dominated by magnetite and wüstite intergrowths) with some spherules interpreted as G- type (dendritic magnetite crystals in a groundmass of silica-rich glass). Initial rock magnetic tests include continuous susceptibility (Ms) vs. heating and cooling, which yields irreversible results (75 to 100 % increase in Ms) on whole rock material. In heating, there is a sharp increase in Ms between about 450 and 520, as well as a small inflection at about 580C. In cooling, a sharp increase in Ms between about 580 and 565C indicates the presence of magnetite. Ms increases to about 310C, when there is a 30% drop in Ms, interpreted to reflect wüstite/magnetite intergrowths. Repeat experiments in both air and argon clearly show the formation of magnetite, associated with a sharp Hopkinson peak.

The presence of fossil cosmic spherules in sedimentary rocks serves as a proxy for the overall abundance of past micrometeorites and together with estimates of Eagle Ford sedimentation rate could be used to estimate the flux of extraterrestrial dust to Earth in this geological time interval.