Conference Agenda

A plausible, pragmatic eustatic curve provides a valuable tool for not only understanding Earth systems processes through time, but also for generating subsurface characterisation of lithological variation and heterogeneity. We present a simplified workflow that allows for the construction of a pragmatic short-term (“3rd order”) mid-Cretaceous eustatic curve, for which the results can be tested by process-modelling to determine plausibility.

Sedimentary architecture, vertical facies trends and changes in palaeobathymetry indicators can be interpreted using a consistent sequence stratigraphic approach, which reduces uncertainty in understanding sea-level trends. The examination of multiple suitable stratigraphic sections in a global dataset can be used to identify a commonality in the timing of major transgressive and regressive events, although some residual uncertainty will remain. A prerequisite to this is detailed work on biostratigraphic calibration between different fossil groups and other chronological techniques (e.g., δ¹³C excursions).

Having established timings of synchronous eustatic rise and fall, eustatic magnitude limits can be estimated from stratigraphic observations, geochemical proxies, or from a compilation of published magnitudes. These can then be integrated with an independently calculated long-term eustasy trend and the resultant curve analysed for plausibility. Here forward stratigraphic modelling can be powerful for assessing the impact of uncertainties in timing and magnitude on the generation of a plausible eustatic curve. We show that many published Cretaceous eustatic curves fail to adequately create plausible sedimentary models in that the pace of eustatic change depicted is unable to generate the partitioning of different systems tracts observed in the rock record.

The use of high-resolution biostratigraphy and chemostratigraphy to precisely correlate between successions in geographically widespread regions provides a valuable tool for the identification of eustatic events. In the Cretaceous, this is made possible by detailed variance in the global d¹³C srecord, in conjunction with rapid evolution of inoceramid bivalves and ammonites. This approach is utilised here to identify eustatic events in the Albian and Cenomanian stages.

1. Middle-Upper Albian boundary

The succession of ammonite migrations and rapid evolution of the bivalve Actinoceramus permits precise correlation between central Texas (USA), the Vocontian Basin (SE France) and the Anglo-Paris Basin (southern England, northern France). Evolution of A. concentricus parabolicus immediately predates rapid, global sea-level fall which caused extensive erosion on shelves; later onlap from a major transgression was approximately coincident with evolution of A. sulcatus sulcatus, widely marked by development of intraformational conglomerates.

1. Albian-Cenomanian boundary

Hardgrounds occur between the basal Cenomanian marker (T. globotruncanoides) and immediately beneath the lowest occurrences of Cenomanian ammonites of the M. mantelli/G. wacoense zones, present in Texas, North Africa and NW Europe. A hiatus, representing up to 7 myr, and including the uppermost Albian and lowermost Cenomanian, is widely developed.

1. Lower-Middle Cenomanian boundary

High-resolution correlation is provided by ammonites and carbon isotope positive excursion MCE1. In NW Europe and Texas, ammonites and carbon isotope records correlate, and place a sequence boundary beneath the Cunningtoniceras inerme Zone. This represents a major sea-level fall, with subsequent transgression low in the Turrilites costatus Subzone.

The Late Cretaceous Cenomanian-Turonian stratigraphic record on the margins of the Arabian Plate is characterized by high eustatic sea levels, punctuated by short-term fluctuations, as well as tectonically forced sea level changes. Despite favorable exposure, carbonate deposits of this age on the NW plate margin are relatively understudied compared to the NE margin (Oman). This study presents a high-resolution, multi-proxy sequence stratigraphic investigation of the latest Albian-Coniacian Ajlun Group in west-central Jordan. A >260 km proximal-distal transect along the western border of Jordan was studied, integrating detailed sedimentological observations, new carbon isotope stratigraphy and biostratigraphic data, covering the Cenomanian-Coniacian for this region.

Preliminary results reveal six regionally traceable sequence boundaries in Jordan (SBs), which can be correlated across the Arabian Plate to Oman. Particular emphasis is placed on SB 2 at the Early-Middle Cenomanian boundary, correlatable to the Top Natih E SB (Oman), showing m-scale erosive features in tidal channel deposits and complex rudist-bearing clinoforms/sandwaves. Additionally, the Middle Turonian SB 4, regionally equivalent to the K150SB and the Top Natih A (Oman), indicates a sea-level fall of 10-20 m based on stratigraphic data from southern Jordan, with carbon isotope records likely placing SB 4 just above the Round-Down carbon isotope event. A short duration of this event, followed by a quickly re-established carbonate factory and moderate incision depths, suggests a primarily eustatic control of this SB in Jordan, contrasting with a principally tectonic control in Oman. The results provide a refined high-resolution stratigraphic record for the NW Arabian Plate.

Detailed stratigraphic-sedimentological analyses of numerous lower Cenomanian–lowermost Turonian core sections from the Upper Cretaceous Elbtal Group of the Saxonian Cretaceous Basin (SCB, Germany) provide detailed insight into the stratigraphic architecture and facies development of the fluvial to marine strata infilling the N–S-directed Pirna palaeovalley. This palaeovalley is part of a northwards-discharging palaeo-drainage system cut into the basement of the Mid-European Island that limits the SCB in the southwest. Deposition was constrained by the palaeotopography and closely tracked early Cenomanian–earliest Turonian sea-level changes, reflecting a continuous up-dip shift of facies belts. Stratal architectures indicate shallow western and steep eastern palaeovalley flanks. Northern and middle segments of the palaeovalley were filled with lower Cenomanian fluvial siliciclastics (Niederschöna Formation), while southern parts were still bypassed. First marine influences (tracked by ichnofossils) were manifested in the northern palaeovalley in the Wurmsandstein Member of the upper Niederschöna Formation during the late early Cenomanian. The retrogradational facies development continued during the middle and Late Cenomanian when widespread shallow-marine sandstones of the Oberhäslich Formation draped most of the pre-existing palaeo-topography. The marine onlap was continued by the rapid plenus Transgression and the deposition of the fine-grained siliciclastic offshore deposits of the uppermost Cenomanian Pennrich Formation, culminating in a maximum-flooding interval (Lohmgrund Horizon) at the base of the lower Turonian Brießnitz Formation. The Cenomanian succession of the study area represents a textbook example of a retrogradational palaeovalley backfilling in the course of a major transgression and highlights the validity of Walther’s law of facies.

An allostratigraphic framework for the Cenomanian–Turonian boundary (CTB) succession in the Western Canada Foreland Basin is based on > 3000 well logs, supplemented by core and outcrop. A low-gradient, ramp-style depositional system existed across the foredeep, with nearshore sand grading into offshore mud. A mudstone-dominated section in the proximal foredeep preserves a 185-m-thick 2 ‰ δ¹³C_org positive carbon-isotope excursion, typical of OAE2, but ×10 thicker than most ‘expanded’ sections. Throughout OAE2, inner shelf heterolithic mudstones form stacked parasequences, 5–15 m thick, organized into larger-scale upward-shallowing packages. The carbon-isotope curve allows detailed correlation to the CTB reference section at Eastbourne (UK), to the SH#1 core (Utah), and to other CTB sites. The globally-recognized sub-plenus unconformity (SB Ce5) is replaced in the high-accommodation foredeep by six high-frequency, basinally-isolated lowstand sandbodies, offset up to 150 km from highstand shorelines. Associated erosion surfaces merge eastward towards the forebulge. This lowstand is followed by the S. gracile transgression, inferred to record 15 to > 40 m of eustatic change. Additional lowstands, determined from facies mapping, precede the latest Cenomanian N. juddi transgression, and the basal Turonian M. puebloensis transgression. Sea-level change of 20–30 m is suggested by facies offsets. Despite a background subsidence rate of 0.3–0.5 m/kyr, repeated forced regressions, and hence relative sea-level falls, imply sea-level drawdown occurred at an even greater rate. The rates and amplitudes of sea-level change inferred from facies cyclicity in the proximal foredeep exceed reasonable limits on thermo- and aquifer-eustasy, hinting at a glacio-eustatic component.

New carbon stable-isotope datasets from shallow-marine successions of Turonian age are evaluated in sedimentological and biostratigraphic context, from tectonically separate depocenters in the Western Interior Seaway (WIS) of North America, with the aim to: (i) establish the chronology and assess the regional versus potentially global nature of long-term (c. 1 Myr) cyclic changes in palaeo-sea level; and (ii) suggest which astronomical cycles and climatic mechanisms may have been involved in driving sea-level fluctuations in this greenhouse regime. While each of the sections studied shows specific features related to local subsidence and clastic supply history, comparison of palaeobathymetric indices reveals several conspicuous stratigraphic features that can be correlated over c. 1700 km along the western margin of the WIS. A broad correlation is found in three principal, long-term intervals of shallow-water conditions, characterized by dominantly regressive stratal patterns, separated by major transgressive events. Most of those are considered time-equivalent in all sections, within the resolution of isotope-based and biostratigraphic correlation. Long-term cyclicity demonstrated in the WIS Turonian and correlative time-equivalent records in Central Europe suggests a relationship between major sea-level fluctuations and long-term amplitude modulation of obliquity cycles. However, shorter-term sea-level cycles paced by the long-eccentricity (405-kyr) cycle have been documented as well.