Impact-generated carbonate accretionary lapilli in the Late Devonian Alamo Breccia

Abstract

Carbonate accretionary lapilli occur in the Late Devonian Alamo Breccia of south-central Nevada. They provide evidence for the extraterrestrial impact origin of the breccia, and help unravel the complicated events that formed it. The accretionary lapilli (Alamo lapilli) are concentrated in lapilli beds, and portions of the latter occur as reworked clasts that are isolated within the upper half of the thick breccia. The Alamo lapilli resemble volcanic accretionary lapilli reported from both silicate and carbonatite volcanoes. They are variable in size and detail, but generally exhibit a nucleus interpreted to be altered target rock, an enveloping mantle of siltand sandsized particles, and a very fine grained peripheral crust. Spherule composition is entirely carbonate, except for sparse shocked quartz grains incorporated into the mantle and diagenetic iron oxides. Rare preservation of undeformed bed segments shows the stratigraphy of the accretionary lapilli; they were deposited in poorly sizesorted layers with varying proportions of matrix. Their preserved form is spherical, deformed, or broken, implying varying degrees of damage before or during deposition, before bed hardening, and during catastrophic reworking and dewatering of the Alamo Breccia. We propose that the carbonate accretionary lapilli were preserved when target carbonate formations were pulverized by impact pressure and calcinated by impact heat, creating quicklime. The lapilli evolved by adhesion of particles within the impact cloud; they were partially cemented in flight by hydration, and then precipitated as one or more beds over early ejectite, debris flows, and/or nearly contemporaneous tsunamites. The cementation process continued in the lapilli beds so that portions of them survived reworking and initial breccia settling and dewatering. Coherent, isolated fragments of lapilli beds and deformed bed masses are preserved as much as 25 m beneath the top of the breccia, indicating the thickness of rock reworked during the Alamo Event. Warme, J.E., Morgan, M., and Kuehner, H.-C., 2002, Impact-generated carbonate accretionary lapilli in the Late Devonian Alamo Breccia, in Koeberl, C., and MacLeod, K.G., eds., Catastrophic Events and Mass Extinctions: Impacts and Beyond: Boulder, Colorado, Geological Society of America Special Paper 356, p. 489–504. J.E. Warme, M. Morgan, and H.-C. Kuehner 490 Figure 1. Alamo carbonate accretionary lapilli exposed on weathered surface of lapilli bed showing sizes, proportions of nuclei, mantles, and crust, and grainy matrix in this sample. Lapilli are circular, flattened, and in various stages of breakage. INTRODUCTION Carbonate spheroidal particles (Fig. 1), interpreted as impact-generated carbonate accretionary lapilli, represent an important constituent of the Late Devonian Alamo impact breccia in Nevada. For brevity, and to distinguish them from other kinds of accretionary spherules and spherical impact products, we call them Alamo lapilli. Shocked quartz grains are dispersed in the Alamo Breccia, and are incorporated into the Alamo lapilli. The beautifully preserved lapilli and shocked quartz are the best observable physical evidence for the impact origin of the breccia, although they compose much less than 1% of the deposit. Probably within only minutes to a day, a series of impactrelated events (Alamo Event of Warme and Sandberg, 1995, 1996; Morrow et al., 1998) eroded and resedimented a thick interval of carbonate platform rock of the Guilmette Formation, which became the thick Alamo carbonate megabreccia. The Alamo lapilli occur in isolated clasts scattered within the Alamo Breccia, and provide a key to understanding the character and the timing of events that led to the Alamo Breccia formation. Alamo lapilli are interpreted to have accreted in a vapor-rich impact plume by adhesion of carbonate particles and other contemporaneous processes. Their structure resembles some kinds of armored accretionary volcanic lapilli, having a central nucleus interpreted as carbonate target rock, an enveloping mantle of sandand silt-sized carbonate fragments, and an outer crust of very fine siltand clay-sized carbonate particles. Alamo lapilli imply that the impact target was dominated by carbonate rock and was wet. Evidence suggests that they precipitated as one or more widespread beds. The beds were then dismembered, by tsunami erosion and/or other energetic impact processes, and redeposited as isolated clasts in the massive breccia. Preserved lapilli survived these events by rapid cementation processes that began in flight and continued in the precipitated bed(s). This early cementation caused segments of the beds to be durable enough to withstand subsequent catastrophic transportation and deposition. Other examples of ancient carbonate lapilli may have formed and been preserved by impact processes. Carbonate strata anywhere may contain unrecognized intervals of impact lapilli and lapillistone.