Upper Cretaceous Stratigraphy of the Island of Brac within the Geodynamic Evolution of the Adriatic Carbonate Platform

Abstract

Though news coming out of Yugoslavia these days is generally bad, this unusual book is a welcome exception. The work was stimulated by a project develop a new geologic map of Croatia; the result is a highly readable book, with the authors' senses of history, curiosity, and humor readily evident throughout. The purpose, as stated in the Introduction, is to rehabilitate the concept of (p. 128) from dull, descriptive cataloging of names of units an approach stratigraphy that is dynamic, interpretive and set in a global context. The authors recognized that their contribution could not be fully utilized by both local and international audiences unless their results were published in Croatian and in English. The book should be useful for geologists in at least four categories: (1) the work provides a synthesis of Upper Cretaceous stratigraphy of a sizeable portion of the Adriatic carbonate platform and will be a valuable resource for specialists; (2) the Croatian text will be an important educational tool in Yugoslavia, presenting both lithologic and paleontologic details and a philosophical approach stratigraphic research; (3) the English text provides the international audience with a demonstration of stratigraphy as an exciting, dynamic, and interpretive science; and therefore (4) will be useful as a thoughtprovoking reading in advanced stratigraphy courses. The book includes 160 pages of text (20 x 29 cm format) with 22 figures including three foldouts, and 20 plates. Figures, with both Croatian and English captions, are located in the 102 pages of Croatian text. The English text covers 32 pages. The first two sections, the Introduction and the Short Review of Previous Stratigraphic Research on the Cretaceous of Brac, are relatively brief and nearly comparable in length and content in both Croatian and English. Section 3, Description of the Lithostratigraphic Units, and the discussion sections that follow, are about 40 percent as long in English as in Croatian. Detailed arguments concerning history and choice of formation names are only available in Croatian. Furthermore, consistent with the educational and philosophical aim of the Croatian text, more extensive general discussions of sedimentological and paleoecological topics are included there than in English. In Section 3, lithofacies and biofacies of each of the six Upper Cretaceous formations outcropping on the island are described; brief descriptions of these units and the authors' interpretations of them are presented below. Biofacies descriptions and interpretations emphasize foraminifera, mollusks, algae, and cyanobacteria. Biostratigraphic interpretations are based upon foraminifera in cases where differences in age interpretations have arisen between diagnostic foraminifera and rudists. The authors utilize both modern and Cretaceous carbonate facies models, as well as ecological studies of modern foraminifera and mollusks, in developing paleoenvironmental interpretations of facies within formations. During the Upper Cretaceous, what is now the Island of Brac was located in the interior of the large Adriatic carbonate platform. The oldest of the Upper Cretaceous units on Brac is the Cenomanian age Milna Formation, which is on the order of 370 m in thickness. The unit is characterized by alternation of three predominant facies: grain-supported packstone/grainstone, peloidal-skeletal wackestone, and micritic and cryptalgal laminites. The paleoenviromental interpretation is restricted, muddy platform-interior setting as indicated by intertidal laminites and shallow subtidal Chondrodonta oyster beds and radiolitid biostromes, and by frequent storm beds. The predominantly mud-supported limestones of the Sveti Duh Formation, illustrated as 140 m in thickness, conformably overlie the Milna Fm. Where the transition is gradual, intercalations of micrite repeatedly alternate with cryptalgal laminites in the top 10 m of the Milna and the boundary is arbitrarily placed after the last occurrence of laminite. Sveti Duh mudstones contain calcispheres and small planktonic foraminifera, as well as other fossil debris in varying proportions. In some cases, concentrations of fossil debris form bioclastic calcarenites that appear be allochthonous gravity-displaced turbidites. The Sveti Duh Fm is interpreted represent hemipelagic deposition associated with the Latest Cenomanian-Early Turonian global eustatic sea level rise. Unfortunately the authors do not address two interesting aspects of this unit. They do not speculate on how hemipelagics might alternate with laminites, interpreted as intertidal. Neither do they suggest an origin for the gravitydisplaced shallow-water bioclastics. However, these are among the few such anomalies that they do not consider, which is partly why the book is so provocative. The third major unit, the Gornji