Accretionary lapilli

Accretionary lapilli in the suevitic basal breccia

Accretionary lapilli is a term originally solely related with volcanism. Accretionary lapilli are pellets that form by the accretion of fine ash around condensing water droplets or solid particles, particularly in steam-rich eruptive columns. Commonly, they exhibit a concentric internal structure, and, once formed, they can be transported and deposited by pyroclastic fall, surge, or flow processes (Allaby & Allaby, 1999; A Dictionary of Earth Sciences). Armored lapilli is the term that is especially used in the case the ash has accumulated around a small rock fragment. The armored-lapilli variety is frequently found in deposits of basaltic base surges.

Since similar processes are related with the turbulent explosion plume raising above the expanding excavation cavity in an impact cratering event, it is not surprising that accretionary lapilli have been found also in impact deposits. Graup (1981) describes accretionary lapilli to occur in the suevite fall-back breccia of the Ries impact structure. They are also reported for ejecta deposits of the K/T Chicxulub impact structure in Mexico, 

and Belize .

Concentrations of lapilli formed lapillistone that occurs as discontinuous, reworked clasts within the megabreccia related with the Late Devonian Alamo impact (

Accretionary lapilli are observed also in various outcrops in the Azuara and Rubielos de la Cérida impact structures where they contribute to the matrix of polymictic dike breccias and the suevitic basal breccia.

Near Fuentes Calientes

Image002Fig. 1. The basal breccia near Fuentes Calientes, eastern basin region. Close-up view (above hammer) in Fig. 2.

Image004Fig. 2. System of dikes composed of accretionary lapilli in a light-colored matrix is cutting through the basal breccia shown in Fig. 1.

Image006Fig. 3. Close-up of a lapilli-bearing dike penetrating the basal breccia near Fuentes Calientes. Note that many lapilli have the typical onion skin structure around a stony core.

Near Escriche – southeastern basin rim near Teruel

Image008Fig. 4. Large parts of the basal breccia outcropping near Escriche are composed of a lapillistone matrix with only few sharp-edged rock fragments, probably Muschelkalk limestone. Note that the sample shown here has the character of a matrix-within-matrix texture. Also note the matrix dike in the lower part penetrating the afore formed matrix giving evidence of a very peculiar lithification.

Image010Fig. 5. Close-up of the lapilli breccia exposed near Escriche. The field is 18 mm wide.

Near Corbatón, east of the Rubielos de la Cérida central uplift

Image012Fig. 6. Matrix-rich basal breccia near Corbatón. The darkish breccia clasts are probably Muschelkalk limestone.

Image014Fig. 7. Accretionary lapilli in the matrix of the basal breccia from near Corbatón. Field width 3 cm.

Image016Fig. 8. Accretionary lapilli from the Corbatón basal breccia in thin section. Photomicrograph, xx polarizers, field width 6.5 mm. The lapilli are basically carbonate with some accessory silicate material (e.g., quartz fragments in the large lapillo).

For comparison: lapillistone from a volcanic diatreme (Avon diatremes) in Missouri, USA

Image018Fig. 9. Sawed and polished sample of accretionary lapilli in the Avon kimberlitic diatremes, Missouri, USA. Field width 3.5 cm. Note the remarkable similarity of volcanic and impact accretionary lapilli rock texture not allowing to make a prompt distinction. Sample courtesy of John Townsend.

Here is a link to a short description of the Avon volcanic district: