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‘‘Distinguishing eustatic and epeirogenic controls on Lower‐Middle Cambrian boundary successions in West Gondwana (Morocco and Iberia)’’, by Ed Landing, Gerd Geyer and Wolfram Heldmaier, published in Sedimentology (2006), 53(4), 899–918 – discussion

As researchers working on the Cambrian of Gondwana, we welcome the contribution on the Moroccan Cambrian made by Landing, Geyer and Heldmaier (2006), who provide new data and stimulate constructive discussions, yet we think there are some issues to address. The lithostratigraphical and biostratigraphical model for the Lower/Middle Cambrian boundary of north-western Gondwana (Morocco, Germany, Turkey, Sardinia and Spain) consists of uppermost Lower Cambrian (Upper Bilbilian; Liñán et al., 1993b) siliciclastic, mixed or carbonate facies (with archaeocyathans and/or protolenid trilobites) followed by Middle Cambrian shales or carbonates with paradoxidid trilobites (such as the earliest Middle Cambrian species Acadoparadoxides mureroensis) (Sdzuy et al., 1999: figs 1 and 3). Thus the Daroca Formation (as well as the Daroca regression) and the lower part of the overlying Valdemiedes Formation in Spain are upper Lower Cambrian in age (Liñán et al., 2002). The main issue is that the authors follow the biostratigraphic scheme proposed by Geyer (1990) and place their Lower/Middle Cambrian boundary at the base of the Hupeolenus Zone; i.e. at a stratigraphic point below the classical Lower/ Middle Cambrian boundary identified in the Mediterranean region since the Nineteenth Century. The latter boundary is validated by most authors working on the Cambrian of northwestern Gondwana (Choubert, 1953: fig. 3; Hupé, 1960; Hutchinson, 1962; Sdzuy, 1971; Cowie et al., 1972; Dean & Özgül, 1994; Loi et al., 1995; Sdzuy et al., 1999; Wotte et al., 2004: fig. 2) and is currently under discussion by the International Subcommission on Cambrian Stratigraphy (ISCS). Actually, the international Cambrian correlation chart given by Geyer, Peng and Shergold (Geyer & Shergold, 2000) – which (with slight differences) is currently widely used (waiting until the ISCS completes its decisions on the subdivision of the Cambrian) – correlates the Hupeolenus Zone of Morocco with Lower Cambrian rocks of Spain, Baltica, Siberia, Kazakhstan, North and South China, Australia, Laurentia and Western Avalonia. Consequently, by proposing to place the Lower/ Middle Cambrian boundary at the base of the Hupeolenus Zone, the authors date the Daroca Formation and the lower part of the overlying Valdemiedes Formation as Middle Cambrian in age. Nevertheless, the Protolenus FAD (First Appearance Datum) in Cadenas Ibéricas (a Lower Cambrian index) lies within the arkosic siliciclastics of the Daroca Formation. Moreover, the lower part of the Valdemiedes Formation yielded the Lower Cambrian trilobites Protolenus dimarginatus, Protolenus jilocanus, Protolenus pisidianus, Protolenus termierelloides, Kingaspis campbelli, Hamatolenus (Hamatolenus) ibericus, Onaraspis altus and Alueva undulata, well below the Valdemiedes event (Dies et al., 2004). On the other hand, the co-existence of the last representatives of protolenids with Acadoparadoxides mureroensis in Turkey (Dean & Özgül, 1994) and Morocco (Geyer & Landing, 2004) reinforces the classic view of the Lower/Middle Cambrian boundary in the north-western margin of Gondwana, rather than rejecting it. The latest Lower Cambrian Valdemiedes event is also a topic of controversy with the authors. The reference section for this (‘rambla de Valdemiedes 2’ section, near the village of Murero, Zaragoza province, NE Spain; Liñán & Gozalo, 1986) lies within the upper Valdemiedes Formation. At the reference section, Liñán et al. (1993a) noted changes in sediment mineralogy and trace fossil contents in the event beds, as well as the disappearance of numerous trilobite species and dwarfing of the brachiopod fauna. In this section, the Valdemiedes event is recorded in an expanded, monofacial succession (in a transgressive context), consisting of mudstones and siltstones, which (according to illite crystallinity values) did not surpass the anchizone. Thus, the cryptic unconformity at the Valdemiedes event in the Cadenas Ibéricas proposed by Landing et al. (2006) is poorly defined, if at all. Furthermore, Sedimentology (2008) 55, 723–724 doi: 10.1111/j.1365-3091.2007.00929.x