The lower Ludlow formation and its graptolite-fauna

[摘要] The Lower Ludlow Shales, which form the lowest section of the Ludlow division of the Silurian System, are most typically developed in the neighbourhood of the picturesque old town of Ludlow. It was in this district that they were first defined and described by Murchison. Lithologically, they are here essentially an argillaceous group of strata, the lower and upper limits of which are well marked by two distinct calcareous beds - the Wenlock Limestone below and the Aymestry Limestone above. Palaeontologically, the Lower Ludlow Shales are rich in fossils: brachiopods, cephalopods, crustacea, and graptolites.When the Lower Ludlow Beds are followed from the Ludlow district north-eastward along their line of strike as far as the valley of the Severn, south-westward as far as Aymestry, and eastward as far as the Malverns and Mayhill, they are seen to retain their lithological characters unaltered, and the Wenlock and Aymestry Limestones are both well developed. But when these beds are traced from the typical area to the west and north-west, the limiting calcareous strata dwindle away and eventually disappear, until, finally, the Lower Ludlow Shales merge lithologically on the one band into the Wenlock Shales below, and on the other into the Upper Ludlow Flags above. It is clearly impossible, therefore, in these westerly districts to separate, on purely lithological grounds, the Lower Ludlow Beds as a distinct group from the formations which overlie and underlie them.The proofs that the Lower Ludlow Beds of Britain contain a characteristic graptolite-fauna have been gradually accumulated be the researches of Hopkinson, Lapworth, Watts, Marr, and Lake (see pp. 418-19); and the evidences of the existence of a similar fauna at a corresponding horizon in Europe have long been known from the discoveries of Continental geologists. But little or nothing has hitherto been worked out with respect to the vertical range of the individual graptolite-species within the limits of the formation itself, or with regard to their geographical distribution in the equivalent strata of Wales and the West of England.During the last few years I have devoted much of my leisure-time to the study of the graptolitic fauna, the rock-sequence, and the vertical distribution of the various graptolite-species in the recognisable subdivisions of the Lower Ludlow Shales of the typical Ludlow district and of their equivalents along the Welsh Border, in order to ascertain (1) what are the truly characteristic graptolites of Lower Ludlow age as distinct from those of the Wenlock formation, and (2) to what extent the Lower Ludlow Beds are capable of subdivision into graptolite-zones. My results and conclusions are embodied in the present paper.

[发布机构] University:University of Birmingham;Department:Department of Geology

[关键词] Q Science;QE Geology