LATE CRETACEOUS SILICEOUS SPONGES FROM EL RAYO FORMATION, PUERTO RICO

INTRODUCTION

THIS PAPER presents sponges from the Late Cretaceous El Rayo Formation, Puerto Rico. Siliceous sponges are common fossils in the Late Cretaceous of Europe (see for example Schrammen, 1910-1912; Moret, 1926; Weidenmayer, 1994; Pisera, 1999). So far only rare siliceous sponges from the Late Cretaceous of the Caribbean region have been reported: lithistid Uerea Lamouroux, 1821, hexactinellids Ventriculites Mantell, 1822, and Plocoscyphia Reuss, 1846 from Trinidad (Thomas, 1935; Trechmann, 1935), and Callopegma Zittel, 1878 from the Cariblanco Formation, Puerto Rico (Howell, 1966). The sponges studied by us are heavily silicifled and thus only their approximate determination was possible, but among bodily preserved sponges they are undoubtedly lithistids with tetraclone and rhizoclone desmas. Among loose spicule material, fragments of hexactinellid skeleton dominate, tetraclone and dicranoclone lithistids desmas are common, and hexactinellid lychniscosid skeletons are very rare.

GEOLOGICAL SETTING

The island of Puerto Rico is the easternmost island of the Caribbean Greater Antilles (Fig. 1.1), a complex island arc with accreted terranes. The island is composed of Lower Jurassic to Lower Cretaceous ophiolites, Lower Cretaceous to Eocene island arc volcanic and sedimentary rocks, Middle Oligocene to Pliocene terrigenous elastics, and limestone unconformably overlying the older rocks along the north and south coasts (Santos, 1999). The lire-Oligocene rocks are divided into the southwest, central, and northeast igneous provinces (Fig. 1.2).

The paleogeographic setting of the earliest island arc volcanism differed in each of the three igneous provinces (Santos. 1999). The southwest igneous province is separated from the rest of the island by the Great Southern Puerto Rico Fault Zone (Fig. 1.2). It contains a basal "Bermeja Complex." which is characterized by linear bodies of serpentinite, blocks of mafic metamorphic rocks, and cherts of Pleinsbachian to Aptian age (Schellekens et al.. 1993; Schellekens. 1998). This basal complex is unconformably overlain by a thick sequence of Santonian to Maastrichtian limestones and mudstones interbedded with volcanic rocks (Santos. 1999: Martínez, 2003).

Cretaceous rocks of southwestern Puerto Rico are dominated by volcanic and volcaniclastic facies, reflecting the back-arc setting of this depositional basin and proximity to active Cretaceous volcanic centers. Large-scale tectonic activity and rapid volcaniclastic sedimentation prohibited stabilization of structural platforms and carbonate sedimentation during much of the island history. But relatively short intervals of carbonate platform and slope development occurred episodically, commonly associated with limited reef growth. The last of the carbonate depositional episodes during the Late Cretaceous resulted in the deposition of the El Rayo Formation (Santos, 1999).

The El Rayo Formation is composed of basaltic to andesitic volcanic flows interbedded with volcaniclastic conglomerates, sandstones, and limestones (Slodowski, 1956; Mattson, 1960; Volckmann, 1983; Santos, 1999; Martínez, 2003). The limestone units are massive to bedded lenses interbedded with volcaniclastic breccias, conglomerates, and sandstones. The limestones represent a carbonate platform sequence deposited over a volcanic surface during a Maastrichtian transgressive event and are divided into four environmental facies based on rudist-bearing limestones: 1) nearshore transgressive; 2) lagoonal; 3) shelf margin; and 4) slope (Santos, 1999; Martínez, 2003).

The fossil sponges described herein were found in a light grayish limestone of the El Rayo Formation slope suite on the Sabana Grande Quadrangle (Fig. 1.2) (Martínez, 2003). The section where the sponges were found is 38.1 m thick (Fig. 2). These limestones are thin to medium bedded where the skeletal fragments are composed of calcareous algae, gastropods, and benthic foraminifera. The bedded limestones are composed of sequences of yellowish tan to light gray, interbedded mudstones, wackestones, and packestones with minor intercalated grainstones (Martínez, 2003). The packestones and grainstones are commonly composed of benthic foraminifera and fragments of calcareous algae, rudistids, echinoderms, gastropods, corals, and oysters. Volcanic clasts are occasionally present in the grainstones, making 10% of the composition of the rock (Martínez, 2003). The sponges are found in the middle of the section with Unit 11 presenting the greatest abundance (Fig. 2). Sponges are found in an upright position and as clusters of 5-20 specimens; in between such sponge groups are densely packed "conglomerates" consisting of sponge fragments of various sizes.

MATERIAL

Most of the sponges (seven more or less complete specimens and numerous broken fragments) have been obtained by dissolving a large block of limestone in acid and most are heavily secondarily silicified; the only nonsilicified specimen has been collected directly in the field. The calcareous specimen has no siliceous spicules preserved and shows only gross morphology. The silicified specimens also show a general sponge morphology and canalization. Polished slabs of the rock show that, in most cases, silicification occurs in the inner part of the sponge while the outer zone, usually thick, presents a calcareous preservation. In some rare cases spicules may be observed on the surface of the silicified portion of the sponge, but usually the process of silicification obliterates their shape. In many cases only fragments of sponges are preserved, clearly indicating a common breaking (Fig. 3). Lithistids, usually found as entire specimens, are strongly silicified, with spicules very poorly preserved (Fig. 4). Hexactinellids, on the contrary, only occur as small fragments and/or well-preserved fragments of the choanosomal dictyonal skeleton (Fig. 7). This preservation type only allows their approximate determination to a high taxonomic level.