Ediacaran–Cambrian phosphorites from the western margins of Gondwana and Baltica

Abstract

The late Ediacaran–mid Cambrian occurrences of phosphorites in the western Mediterranean region (West Gondwana) and southern Sweden (north‐west Baltica) are related to the poleward drift of West Gondwana and the northern drift of Baltica. As a result, these regions crossed subtropical and temperate palaeolatitudes of the Southern Hemisphere, in which oceanic upwelling and high organic productivity were the probable sources of phosphorus along their western margins. A parallel south‐west/north‐east migration trend of evaporites and delayed, time transgressive phosphorites point to distal upwelling environments associated with shallower arid conditions. Concentration of hardground‐derived phosphate accumulations took place by repeated alternations of low sedimentation rates and condensation, in situ early diagenetic and microbially mediated precipitation of amorphous and cryptocrystalline, carbonate‐bearing francolite, winnowing and polyphase reworking of previously phosphatized skeletons and hardground‐derived clasts. Thicker phosphorites of economic importance display a spiculate sponge‐thromboid consortium that points to a distinct benthic community developed on suboxic, slope to basinal substrates. For most phosphorite samples an up to ten‐fold enrichment in rare earth element concentration relative to post‐Archaean average Australian shale reflects significant incorporation of rare earth elements and yttrium after deposition. Considerable scavenging took place from anoxic pore waters, following reductive dissolution of iron oxides as evidenced from the medium rare earth element enrichment in most samples. This contrasts with the rare preservation of primary sea water features, such as cerium depletion and yttrium enrichment, in phosphatized thrombolites from the shallower realms (for example, Montagne Noire). The data are consistent with phosphogenesis after upwelling and transgression of nutrient‐rich, suboxic–anoxic, ferruginous waters over more oxygenated, rifting settings and inland portions of an evolving passive margin.

A cryptic record of Burgess Shale‐type diversity from the early Cambrian of Baltica

“…Possamentier & Allen ), where local sediment supply is outstripped by sea level rise (Nielsen & Schovsbo ; Álvaro et al . ). Time‐averaging during such episodes of sediment‐starvation could conceivably account for the increased bioturbation as successive communities of burrowers colonized the same sediment tier (Bromley ; McIlroy ).…”

“…During the early Cambrian, present‐day Scandinavia lay in an inverted position, facing the South Pole (Hartz & Torsvik ; Cocks & Torsvik ; Álvaro et al . ). Although once thought to oppose Laurentia across the Iapetus Ocean (see Hartz & Torsvik , fig.…”

Updated stratigraphic framework and biota of the Ediacaran and Terreneuvian in the Alcudia-Toledo Mountains of the Central Iberian Zone, Spain

“…The member is characterized by a finely developed lamination. In the Navalpino Anticline, the phosphorites can reach 500 m in thickness, and form an amalgamation of channels rich in lithoclasts, oncoids and phosphatic hardground-derived clasts (Gabaldón López et al, 1987;Santamaría et al, 1987a, b;Santamaría, 1988Santamaría, , 1996Gabaldón López & Hernández Urroz, 1989;Santamaría i Casanovas, 1995;Álvaro et al, 2016b).…”

Phosphogenesis associated with the Shuram Excursion: Petrographic and geochemical observations from the Ediacaran Doushantuo Formation of South China

“…2, 3). This suggests the occurrence of sediment reworking before final deposition (e.g., She et al, 2013;She et al, 2014;Cui et al, 2015;Álvaro et al, 2016). Thus, the 13 C-depleted authigenic calcite only records authigenetic history after the final deposition of phosphatic intraclasts.…”