File:Lockne impact resurge breccia (455-458 Ma, Late Ordovician; Lockne Impact Structure, Jamtland, Sweden).jpg

English: Lockne impact resurge breccia (cut surface; field of view 8.2 centimeters across) from Tandsbyn Gully, western edge of Lockne Impact Crater, west of Lake Lockne, Jämtland, central Sweden.

The Lockne Impact Crater of central Sweden is a 7.5 km-diameter (but may be larger), epicontinental marine target crater, with an original water depth estimated at ~500 to 700 meters. Deposits from marine impacts differ significantly from those generated by terrestrial impacts. The rock shown above is an impact resurge breccia formed during the Lockne Impact (the Swedes call this rock “Loftarsten” or “Loftarstone”), and found stratigraphically atop the Lockne Breccia itself.

The impact resurge breccia deposit was generated by water, carrying with it abundant rock fragments, surging into crater area after the initial splash (an impact resurge turbidity flow). The fragments in this rock are mostly granule- & sand-sized, and noticeably angular in shape. Published research has found that most of these small fragments are limestone and impact melt rock.

Dating work indicates that the Lockne Impact occurred at about 455-458 million years ago, during the early Late Ordovician (stage 5, sensu Gradstein et al., 2004).

A recent publication announced that extraterrestrial chromite (FeCr2O4) has been found in the Lockne impact resurge breccia. The chromite chemistry is consistent with an origin in an L-chondrite meteorite. Chondrites are the most common meteorite type found impacting on Earth. It’s been observed that there's a relative abundance of preserved Ordovician-aged impact craters on Earth, and that many L-chondrites that fall to Earth nowadays have CRE ages corresponding to the Ordovician (for example, the Park Forest Meteorite).

So, the situation seems to be this: the L-chondrite parent body in the asteroid belt was significantly disrupted (presumably by a large impact) back in the Ordovician (~470 million years ago). This resulted in an temporary increase in impact events on Earth (for example, the Lockne Impact). L-chondrites that fall today are small surviving fragments from that ancient disruption event in the asteroid belt. Neat story!

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Some info. from:

Alwmark & Schmitz (2007) - Extraterrestrial chromite in the resurge deposits of the early Late Ordovician Lockne crater, central Sweden. Earth and Planetary Science Letters 253: 291-303.

Gradstein et al. (2004) - A Geologic Time Scale 2004. Cambridge. Cambridge University Press. 589 pp.