Identification and long-range correlation of bentonites in Turonian–Coniacian (Upper Cretaceous) chalks of northwest Europe

Abstract

A geochemical and mineralogical study of thin (2–20 cm) clay-rich beds found in Turonian–Coniacian chalks from the Anglo-Paris Basin has been undertaken. On the basis of their rare-earth element (REE) and mineralogical composition it is possible to subdivide beds into bentonites, characterized by a negative Eu anomaly and elevated smectite content, and detrital beds (characterized by a minimal or absent Eu anomaly and a greater proportion of illite). Six beds are considered to be bentonites (Glynde 1, Southerham 1, Caburn, Bridgewick 1, Lewes and Shoreham 2). Trace-element discriminant diagrams add support, as does comparison with similar studies undertaken in eastern England and northern Germany. Geochemical evidence demonstrates that the tephra was variably acidic in composition (rhyodacite–rhyolite–comendite/pantellerite), and was probably derived from crustal melting associated with initial rifting of the North Atlantic. Correlation of bentonites between the Anglo-Paris Basin, eastern England and northern Germany can be demonstrated and independently tested, establishing a tephro-event stratigraphy across northwest Europe for this period.

Secular variation in Late Cretaceous carbon isotopes: a new δ13C carbonate reference curve for the Cenomanian–Campanian (99.6–70.6 Ma)

“…7), confirming the lateral equivalence of these deposits to the Glynde Marls of southern England. The geochemical signatures of Barton Marl 1 and Glynde Marl 1, and the stratigraphically equivalent T C tuff of northern Germany (Wray, 1999), indicate that they represent the first in a series of Turonian-Coniacian bentonites (B1 in Fig. 7, Table 2) that provide a framework for international correlation in NW Europe.…”

“…1), the marker-bed terminology of Mortimore (1983) is used in preference to the North Downs lithostratigraphy of Robinson (1986) where marker-bed correlations throughout the area are well established. Marl seams, including geochemically distinct bentonites (Wray & Gale, 1993;Wray et al 1996;Wray & Wood, 1998;Wray, 1999;Wiese, Wood & Wray, 2004), have proven to be particularly reliable for regional correlation. Where appropriate, marker-bed names in general use prior to the work of Mortimore (1983) are retained.…”

“…Marl equivalence after Wray (1999). Marker beds named are from northern Germany, southern England, and northern England, respectively.…”

“…Isotope data sources: Salzgitter-Salder - Voigt & Hilbrecht (1997) ;Liencres Wiese (1999); Trunch -this study; Dover -Jenkyns, Gale & Corfield (1994). Stratigraphic data sources: Salzgitter-Salder -Ernst, Schmid & Siebertz (1983), Wood, Ernst & Rasemann (1984), Wiese & Kröger (1998); Liencres -Wiese (1997); Trunch -this study; Dover -Jenkyns, Gale & Corfield (1994), Gale (1996) to the Southerham Marl 1 in the southern province (Wray, 1999), the two marls equating to Bentonite B2 ( Table 2). The isotope profile at Trunch (Fig.…”

“…The Caburn Marl is equivalent to the Deepdale Lower Marl of northern England and the T D1 Tuff of northern Germany, representing Bentonite B3 (Figs 7,8, Table 2; Wray et al 1996;Wray, 1999).…”