Upper Cretaceous-Paleocene magnetic stratigraphy at Gubbio, Italy I. Lithostratigraphy and sedimentology MICHAEL A. ARTHUR ALFRED G. FISCHER j Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08540 ABSTRACT An essentially complete section of middle Cretaceous to Paleocene, calcareous, pelagic sediments is exposed at Gubbio, in the Umbrian Apennines of Italy. This, the first of five papers, presents the lithostratigraphic and sedimentological basis for sub- sequent discussions of biostratigraphy and magnetic stratigraphy. Biogenic coccolith-globigerinid oozes were deposited along with a constant but low background of fine terrigenous detritus at a mean sedimentation rate (compacted) of 6.6 m/m.y. from Albian to Eocene time; individual stages deviate from this by a factor of two. Short-term variations in carbonate input yielded a sequence of limestone beds punctuated by thin shale interbeds. These shale- limestone couplets accumulated in time spans between 20,000 and 100,000 yr, and the thin shales may represent up to one-half of the total time. Modification by diagenetic solution, tectonic shear, and weathering render shale interbeds unsuitable for magnetic work. The limestone beds, first bioturbated and then compacted and cemented during diagenesis, appear likely to have recorded magne- tic history at intervals on the order of 50,000 yr, sufficient to pro- vide a good record of polarity changes. Moderate to high states of oxidation prevailed except in Aptian-Albian and a portion of Turonian to Santonian times. INTRODUCTION This is the first of a series of five papers dealing with the middle Cretaceous to Paleocene sediments exposed in the Bottaccione gorge near Gubbio, Italy (Alvarez and others, this issue, Fig. 1; and Carta Geologica d'ltalia, sheet 116, "Gubbio"). The various au- thors have been carrying out an integrated study of lithostratig- raphy, biostratigraphy, sedimentology, geochemistry, and paleomagnetism. This paper presents the measured section and pre- liminary results of sedimentological-geochemical investigations still in progress — material necessary as a base for the subsequent pa- pers. LOCATION, PREVIOUS WORK, AND GEOLOGIC SETTING The Bottaccione section, located within a kilometre of the town of Gubbio in the Umbrian Apennines, promises to be of extraordi- nary importance for the understanding of the Cretaceous and Paleogene. Renz (1936, 1951) initially called attention to this ap- parently unbroken sequence of pelagic sediments, mainly lime- stone, of Tithonian or Neocomian through Eocene age. Studies by Luterbacher and Premoli Silva (1962, 1964) showed the presence of all standard Paleocene and Eocene foraminiferal zones, includ- ing a new, earliest Paleocene zone — that of Globigerirta eugubina, now recognized at several other localities. Cretaceous foraminiferal zones down to that of G. concavata have also been recognized (Premoli Silva, this issue). The geology of the Gubbio area was de- scribed by Barnaba (1958), and preliminary data on the paleomag- netism of the Gubbio sequence were presented by Alvarez and Lowrie (1974), Lowrie and Alvarez (1974, 1975), and Premoli Silva and others (1974). The Umbrian Apennines are probably parautochthonous, lying to the east of the allochthonous units of the northern Apennines (see Bortolotti and others, 1970, for general relations). As in cer- tain other Tethyan areas, such as the southern Alps, depositional environments in the Umbrian region evolved from a carbonate shelf setting in Triassic-Liassic time into a pelagic phase. This, commencing in the Jurassic and extending into the Paleogene, was succeeded by Miocene terrigenous turbidites. Main structural fea- tures in the Umbrian ranges are large anticlines, trending north- west, broken by normal faults. The Bottaccione gorge cuts through the northeastern limb of such a faulted anticline; bedding strikes average N50°W and northeastern dips range from 35° to 55°. Slump structures or intrastratal folds are visible on the northwest- ern wall of the gorge and in the Valle della Contessa farther west, but are insignificant on the southeastern side of the gorge where our section was measured and samples taken. Road cuts provide almost continuous outcrops of comparatively fresh rock, and occa- sional gaps can generally be filled by outcrops higher on the hill- side, especially along the Medieval aqueduct (the Bottaccione). The Cretaceous-Paleocene rocks have been buried to a depth of 1 to 2 km under Miocene flysch. Limestones lack porosity and break with a smooth conchoidal fracture: bedding-parallel and bedding- oblique stylolites are common, but no pervasive recrystallization is evident, and fossils are undistorted. Shales and marlstones, though compacted and fissile, show no tectonic cleavage. The region is free from intrusives and evidence of thermal alteration. LITHOSTRATIGRAPHY All paleomagnetic samples, biostratigraphic zones, and other points of observation in the succeeding four papers refer to a sec- tion measured by the authors. This commences at the contact of the massive, light-gray Maiolica Limestone, of Barremian-Aptian age, with dark, fissile shales and marls of the Aptian-Albian Scisti a Fuciodi. It extends from more than 400m, into Eocene rocks. A generalized version of this section is shown in Figure 1, along with the stage and zonal boundaries provided by Premoli Silva. Traditionally, this part of the section has been divided into three main lithic units or formations: the Scisti a Fucoidi, the Scaglia (pronounced Scahl-ya) Bianca, and the Scaglia Rossa. The distinc- tive characteristics of these units are described below, and certain chemical and mineralogical aspects of the major rock types are shown in Table 1. Scisti a Fucoidi This unit extends from 0 to 47 m in the measured section. It is composed primarily of shales and marlstones that show wide varia- Geological Society of America Bulletin, v. 88, p. 367-389, 11 figs., March 1977, Doc. no. 70305. 367 on February 14, 2016 gsabulletin.gsapubs.org Downloaded from