WOOD, HEATHER K.¹, EVAN K. FRANSEEN², and ROBERT H. GOLDSTEIN^1
1University of Kansas, Lawrence, KS
²Kansas Geological Survey, Lawrence, KS

Abstract: Well-Preserved Geometries of Red Algal Boundstone Wedges: Transgressive or Highstand?

Upper Miocene (Tortonian-Messinian) carbonates near Fernan Perez, southeastern Spain, display a transgressive or early highstand systems tract deposited during a climate change from temperate to subtropical/tropical.

The El Ojo section exposes parts of two depositional sequences. The lower sequence is temperate-water heterozoan packstone and grainstone. These beds are truncated by an erosion surface. The surface is flat in proximal positions, steepens downslope in medial positions and flattens in distal positions. The second sequence begins above this surface with medial and proximal branching red algal boundstone. It maintains consistent red algal morphology along its entire trace. The algae branch upward in vertical or sub-vertical bushes 5-30 cm high with branches 2-5 mm in diameter. Similarly branching rhodoliths, 3 to 10 cm in diameter, appear in proximal positions. Medially, the boundstone covers 50 m of relief, consisting of several 1-2 m high backstepping wedges that onlap the sequence boundary and downlap distally. Minor volcanic-rich carbonates onlap these wedges and are overlain by prograding warmer-water forereef sediments containing Porites fragments.

Presence of the boundstone unit above the truncation surface and its backstepping wedge geometry suggest deposition during transgression. Assuming growth morphology is linked to water depth, the similarity along the entire surface indicates consistency of conditions, suggesting diachronous growth of algae during a relative sea-level rise. However, if morphology is not water-depth controlled, growth might indicate progressive colonization linked to highstand conditions. In either case, these strata provide important insight into transgressive or early highstand carbonate sedimentation during climate change.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas