Microfacies associations in Asbian carbonates: an example from the Urswick Limestone Formation of the southern Lake District, northern England

Abstract

Abstract Semi-quantitative analysis of allochems from the Urswick Limestone Formation (Asbian) of the southern Lake District area of northern England has revealed a distinctive cyclicity of the microfacies. Cycle-top grainstone microfacies contain an algal flora comprising Koninckopora, Anatolipora and Polymorphocodium , with Girvanella filmaents and Ortonella lumps. Other allochems include intraclasts, large peloids and thick-shelled bivalves and gastropods. The middles of cycles are mostly packstones and micro-grainstones and contain allochems dominated by small peloids and the algae Kamaeena, Kamaenella and Epistacheoides , with the microproblematicum Ungdarella and relatively high abundances of micritic-walled formainifera such as endothyrids. Cycle bases contain a diverse algal assemblage including Coelosporella and Stacheoides , with other allochems represented by trilobites, ostracodes, Saccamminopsis , foraminifera such as Archaediscidae, the base late Asbian guide Howchinia , the base early Asbian guides Gigasbia gigas and Vissariotaxis , bivalves, small gastropods, bryozoans, sponge spicules and bored grains. Other allochems are found throughout most cycles, decreasing only in the very shallowest (intertidal) facies, or have an irregular distribution, and include brachiopod debris, crinoid ossicles and coral fragments. There are significant variations in allochem distribution according to palaeogeography. Close to the shelf margin there are higher abundances in the cycle top grainstones of the algae Koninckopora and Anatolipora , and also of the calcified filaments Girvanella and Ortonella , with thick-shelled gastropods, intraclasts and coarse peloids. At cycle bases, echinoderm arm plates and bryozoans are particularly abundant in packstone-wackestone textures. Platform interior facies are differentiated into a diverse open-marine type, with a high total abundance of bioclasts in the cycle base pack-wackestones including trilobites, Coelosporella, Stacheoides, Kamaena and bored grains, grainstones are dominated by small peloids, Kamaenella and Ungdarella ), and a more restricted cycle type, in which total bioclast abundances are low.