Chalk: all we need is a fracture log!

Chalk fracture logging is reviewed in the context of the broader geology needed to build conceptual ground models. Examples of drilling damaged core illustrate the many issues faced by core-loggers including identification of marker beds (marl seams, hardgrounds, flint bands, fossil shell beds) and the ‘interpretations’ necessary to complete a fracture log. Stratabound fractures impart a special style of fracturing to each Chalk formation. Lithology is a key factor in development of fracture style where marl seams control inclined conjugate fracture sets, development of listric growth faults and interbed slides. Lateral changes in lithology and thickness and consequent controls on fracture evolution are related to intra-Chalk tectonic episodes and tectonic setting with onshore interpretations supported by offshore seismic profiles. Strike-slip faults are illustrated in the Chalk cliffs of the Sussex coast. Fracture log reports should highlight special features such as shear zones and use annotated core photographs to illustrate issues requiring discussion.

First report of acrotretoid brachiopod shell beds in the lower Cambrian (Stage 4) Guanshan Biota of eastern Yunnan, South China

Brachiopod shell accumulations are abundant and diverse in the lower Cambrian strata of Yunnan Province, South China, but most commonly they are composed of linguloid and acrotheloid brachiopods. Here, we describe the first record of shell beds with high-density accumulations of microscopic acrotretoid brachiopods (usually <2 mm in width) in the muddy deposits of the Wulongqing Formation (Guanshan Biota, Cambrian Stage 4) in the Wuding area of Yunnan Province. The acrotretoid shell beds from the Wulongqing Formation vary from thin mm-thick pavements to more well-developed beds, several centimeters thick. The occurrence of remarkably rich acrotretoid shell beds indicates that microscopic lingulates began to exert an important role in hardening and paving the soft-substrate seafloor during the early Cambrian evolution of Phanerozoic “mixgrounds.” The new Guanshan material is referred to a new species, Linnarssonia sapushanensis n. sp., which differs from other species of Linnarssonia mainly in having a well-developed internal pedicle tube, as well as a relatively longer dorsal median septum. The occurrence of Linnarssonia sapushanensis n. sp. in the Wulongqing Formation in eastern Yunnan extend the oldest record of the genus on the Yangtze Platform of South China back to at least Cambrian Stage 4.UUID: http://zoobank.org/3e0c3878-6ce2-4eed-87bf-e39647c310c4

The Hettangian–Sinemurian (Lower Jurassic) strata of Redcar, Cleveland Basin, NE England: facies and palaeoecology

The foreshore at Redcar hosts the oldest Jurassic succession exposed on the Yorkshire-Cleveland Coast but has received little attention since the late nineteenth century. Temporary removal of beach sands by winter storms early in 2018 allowed for a sedimentological and palaeontological study of a nearly 60 m-thick foreshore section. The rocks are latest Hettangian to Early Sinemurian in age (Early Jurassic) and comprise five coarsening-upward cycles (parasequences) that grade from mudstones through siltstones into Gryphaea-rich shell beds. Ammonites are reasonably common along with a diverse benthos that includes abundant bivalves (e.g. Gryphaea, Cardinia, Luciniola, Plagiostoma and Oxytoma) and rarer serpulids, gastropods, foraminifera and solitary corals. In the upper part of the section thicker-shelled taxa are commonly bored by cirripedes and bryozoans. Despite relatively high benthic diversity and intense bioturbation, pyrite framboids are common with size distributions suggesting deposition under a moderately oxygen-restricted water column. The Redcar succession post-dates the end-Triassic mass extinction by as much as 4 Ma and the diverse marine assemblages indicate that recovery was substantially complete by this time.

PALEOENVIRONMENTAL IMPLICATIONS OF TIME-AVERAGING AND TAPHONOMIC VARIATION OF SHELL BEDS IN LAKE TANGANYIKA, AFRICA

ABSTRACT The lake bottom along structural platforms in Lake Tanganyika, Africa, is carpeted with numerous large shell beds, known to be of late Holocene age, but of uncertain assemblage process. The shell beds may be the result of sedimentological (physical) assembly processes, or biological processes, or both. Previous work focused on the distribution of shell-rich facies, and showed time averaging of the surficial shell bioclasts over the last ∼ 1600 calendar years BP. We focus on an extensive shell deposit along a deltaic platform in Kungwe Bay, Tanzania and examine time-averaging and taphonomy of Neothauma tanganyicense shells to constrain sedimentological and biological processes forming concentrations of shells. New radiocarbon dating indicates that Neothauma shells are time-averaged over the last ∼ 3000 calendar years. Younger shells predominate shallow-water and exhibit unimodal age distributions, while shells from deeper-water exhibit a broader age distribution. Taphonomic results indicate that water depth and distance from the delta river mouth influence shell abrasion and encrustation with more encrustation developing away from sediment input points. Shells with black coatings and reddish-orange oxidation patinas suggest local burial and exposure. The age-frequency distributions of the shells suggest production rates of the shells vary over time and with water depth, tracking climatically driven lake-level changes (e.g., Little Ice Age, ∼ 100–650 BP). In addition, age-distributions suggest that (1) mixing of different populations are more prevalent along the steeper deltaic slopes, and (2) recent decreasing production rates may reflect anthropogenic land-use change and attendant sedimentation, which has implications for Neothauma itself, and for organisms that are obligate occupants of the shell beds. These results suggest both climatic and depositional processes play unique roles in the distribution and accumulation of shell beds in Lake Tanganyika, which informs interpretation of similar paleoenvironments in the geologic record.