--> Abstract: The Dilemma of Gaps in Carbonate Stratigraphic Sequences: A Case History from the Jurassic of the Venetian Alps, Italy, by W. G. Zempolich; #91004 (1991)

Datapages, Inc.Print this page

The Dilemma of Gaps in Carbonate Stratigraphic Sequences: A Case History from the Jurassic of the Venetian Alps, Italy

ZEMPOLICH, WILLIAM G., Johns Hopkins University, Baltimore, MD

The Lower Ammonitico Rosso (L.A.R.) of the western Venetian Alps is a thin, red nodular limestone that overlies a thick Late Triassic to Middle Jurassic (Aalenian) shallow-water platform sequence. It is thought to represent a Middle Jurassic (Bajocian) drowning event whereby the Trento Platform became a submerged plateau. The L.A.R. is problematic in that it (1) is rich in ammonites and other pelagic fauna; (2) contains stromatolites, oncolites, and shallow-water sedimentary structures; and (3) directly overlies platform sediments that contain complex brecciated fabrics filled by cement and Bajocian-age sediment. These seemingly contradictory features have generated much debate as to the sequence stratigraphy of the Trento Platform. Does the formation record platform drowning or expos re and shallow-water deposition followed by drowning?New evidence suggests that the L.A.R. was deposited in a shallow-water environment following a period of subaerial exposure. In an east-west transect from the platform edge to platform interior, a clear transition is exhibited from high- and low-energy open-marine facies to restricted lagoonal facies. High-energy open-marine facies include well-sorted skeletal- and peloidal-rich sands possessing low-angle to planar cross stratification and thin, fairly sorted ammonite- and belemnite-rich gravels. Sands include both shallow-water and pelagic fauna; gravels commonly contain ripup clasts from underlying sediments. Low-energy open-marine facies consist of buff colored wackestones and packstones that contain ammonites, belemnites, pelagic b valves, and peloids. Open-marine facies grade laterally towards the platform interior into red nodular limestones that contain thrombolites, stromatolites, and oncolites. Transitions from planar to low amplitude LLH stromatolite morphologies and transitions from thin columnar thrombolites up into amalgamated domal stromatolites suggest short-term deepening and shallowing events, respectively. With respect to paleogeography, the abrupt transition from open-marine facies at the platform edge to lagoonal facies in the platform interior indicates that thrombolites, stromatolites, and oncolites accumulated in a shallow restricted environment.

In contrast to previous interpretations, results of this study indicate that the Lower Ammonitico Rosso was deposited in shallow water following a period of subaerial exposure. True "drowning" of the Trento Platform did not occur during the Bajocian, but much later, during the late Oxfordian-Kimmeridgian.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)