Eocene and early oligocene micropaleontology and paleoenvironments in SE Umbria, Italy

Abstract

A detailed study, based on calcareous nannofossils and planktonic and benthic Foraminifera, has been carried out on five sections in the “Scaglia” Group of the Umbrian region (Visso and Cascia areas, Central Italy). These sections are characterized by continuous pelagic sedimentation and are constituted mainly of marls, marly limestones and limestones of Early Eocene to Early Oligocene age. Calcareous turbidites, with shallow-water skeletal debris, discharged from the Latium-Abruzzi carbonate platform, are interbedded within the pelagic sequences. The biostratigraphic study, based on calcareous nannofossils and planktonic Foraminifera, confirmed the continuous succession of standard events and revealed some new ones. These results allow us to refine the biostratigraphic resolution, with a very good correlation between calcareous nannofossil and planktonic foraminiferal zonations. The major features and events recognized are: • The Early-Middle Eocene boundary (Zones P9–P10) is characterized by the first occurrence (FO) of hantkeninids, by abundant thick-walled Globigerinatheka (G. senni) and by spinose acarininids. A decrease has been noted in planoconvex morozovellids and an increase of low-spired subbotinids. • The benthic assemblage shows a great variability, with an increase in the Zone P11 of the large Heterolepa and Cibicidoides. This is an event of worldwide significance. • In the calcareous nannofossils, the FO of Nannotetrina sp. was used to define the lower boundary of Zone CP13 instead of N. fulgens, because poor preservation prevented identification at the specific level. • The FO of Reticulofenestra umbilica, marking the lower boundary of Zone CP14, occurs in the upper part of Zone P11. • The Zone P13 was recognized indirectly because of the absence of the index species Orbulinoides beckmanni. The base of the zone is close to the FO of Turborotalia cerroazulensis, and is defined by an increase of Catapsydrax sp. and by the last occurrence (LO) of Acarinina bullbrooki. The top of the zone is defined by an increase of the high-spired subbotinids of the S. corpulenta group and the disappearance of Morozovella spinulosa • The Middle/Late Eocene boundary (Zones P14/P15) is characterized by the extinction of the spinose acarininids and of the smooth-walled Planorotalites. From Zone P13 to Zone P15 there is a replacement in the benthic assemblages. In fact, Pleurostomella, Heterolepa and Anomalinoides capitata characterize these zones, bolivinids become abundant and uvigerinids appear around the P14/P15 boundary. • The Late Eocene is characterized by an increase of subbotinids with abortive chambers and of the Turborotalia cerroazulensis lineage. Hantkeninids and cribohantkeninids are rare, particularly C. inflata. The FO of Istmolithus recurvus has been noted at the base of Zone P16. Within this zone, Globigerinatheka index and Turborotalia cunialensis coexist. The LO of G. index is associated with a strong decrease of representatives of the genus Globigerinatheka. At the same level of the LO of G. index, the coeval extinctions of Discoaster barbadiensis and Discoaster saipanensis are noted. Just above this level, within Zone P17, the LO of T. cerroazulensis lineage ( T. cocoaensis and T. cunialensis) has been recognized. The Eocene/Oligocene boundary is defined by planktonic Foraminifera at the extinction level of hantkeninids and cribrohantkeninids, shortly following that of the T. cerroazulensis group.