Synsedimentary brecciation in the Eifelian (Middle Devonian) Baltic Basin: sudden catastrophe or diagenetic collapse

The Eifelian sedimentary record in the epeiric Baltic Basin contains an extensive brecciated interval known as the Narva Breccia. During recent decades, the origin of this breccia has been attributed to (1) palaeoseismicity, (2) extraterrestrial impact event or (3) diagenetic collapse. Our studies show the presence of two different breccia types that form 11 stratigraphic horizons and contain several laterally and stratigraphically unrelated breccias. The occurrence of Type-1 and Type-2 breccias, together with carbonate sabkha and carbonate inter- to supratidal tidal flat deposits on the basin margins, indicates synsedimentary brecciation formed in a sabkha environment. Type-2 breccia could also originate from wave-cycling loading.