by Kord Ernstson & Ferran Claudin (July 2013)
Abstract. – The “round rocks” of the Weaubleau-Osceola impact structure have phenomenological counterparts in the Spanish Azuara/Rubielos de la Cérida impact structures where they occur within voluminous heavily brecciated rock units. Related nodular bodies within large monomictic movement breccias are observed also in the Ries impact structure. A process similar to the formation of monomictic impact breccias with rounded clasts as part of a mortar texture is suggested. A relation to the Weaubleau-Osceola “round rocks” may exist but not necessarily.

1 Introduction

The Weaubleau (or now Weaubleau-Osceola) circular feature in southwestern Missouri is a 19 km-diameter impact structure that formed in the Mid-Carboniferous about 330 million years ago (Evans et al. 2003).

A peculiar feature clearly restricted to and common throughout the Weaubleau structure are the “round rocks” called also “Missouri rock balls” or “Weaubleau eggs” (Figs. 1, 2). Originally considered to be of glacial origin they are in general attributed now to the impact event. The idea of a formation as mega-accrecionary lapilli has been discarded and a diagenetic formation from blasted siltstone clasts intermixed in the fallback breccia and subsequent silification is mostly discussed. Nonetheless, the process of formation is still poorly understood. Here, we present evidence of roughly similar nodules occurring in the Spanish large Azuara and Rubielos de la Cérida impact structures where different from the Weaubleau “round rocks” they can be observed how they developed in situ.

Fig. 1. Weaubleau “round rock”. The typical and most common size runs from golf-balls to grapefruit. Photo: Harmil, WIKIMEDIA COMMONS.

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Impact deposit at the Moneva reservoir, Azuara impact structure (Spain)

by Ferran Claudin, Daniel Gorgas & Kord Ernstson (March 2013)

In 2012 in the course of a field trip around the Moneva reservoir (Fig. 1, 2) one of the authors, Daniel Gorgas from Azuara who already in the past had frequently contributed important geologic observations to the Azuara impact research, came across a geologic setting that appeared to drastically deviate from the “normal” deposits well known to him as the extended young Tertiary sediments within the Azuara structure. Following his report we began to study the geological maps of the area around the reservoir (Fig. 2, 3) and then to investigate the outcrops in more detail.

Fig. 1. Location map for the investigated outcrops at the Moneva reservoir within the Azuara impact structure (roughly outlined by a dashed circle).  Continue reading →

by Kord Ernstson & Ferran Claudin (2012)

Shocked quartzite cobbles making up widely spread Triassic Buntsandstein conglomerates in Northern Spain have been reported (Ernstson et al. 1999, 2001) to be related to the Mid-Tertiary large Azuara multiple impact event with the formation of the Azuara impact structure and the Rubielos de la Cérida elongated impact basin (Hradil et al. 2001, Ernstson et al. 2001, 2002, Schüssler et al. 2002, Claudin & Ernstson 2003, Ernstson et al. 2003). The quartzite cobbles (and boulders) are peculiarly and intensively pockmarked and cratered (Figs. 1, 2) and show in general a closely spaced subparallel fracturing (Fig. 3). The cobbles’ characteristics become especially evident when they are found scattered in the field as a result of the conglomerate weathering (Fig. 4).

Fig. 1. Typically pockmarked, cratered and polished (the large boulder) quartzite cobbles and boulders from the Triassic Buntsandstein conglomerates.

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