Abstract: Parguera Limestone, Upper Cretaceous Mayaguez Group, Southwest Puerto Rico

By

Charles Coit Almy, Jr.

Rice University Ph.D. thesis, 203 p., June, 1965

The variations in the character of the Parguera Limestone reflect events occurring in the Puerto Rican portion of the Caribbean island arc in Santonian to Mestrichtian time.

In the area studied (Punta Melones-Ensenada), the Parguera Limestone consists of the following vertical sequence: (1) Bahia Fosforescente Member composed of basal tuffaceous calcarenites grading upwards into glauconitic, bioclastic limestone; (2) Punta Papayo Member composed of mudstones containing an abundant open-ocean microfauna, minor calcarenite, slump structures, and uncommon bedded cherts; (3) Isla Magueyes Member compose of volcanic conglomerates in which a bioclastic component increases upwards to form a coarse-grained bioclastic limestone. This section is found at Parguera and is 1150 meters (3770 feet) thick.

The section unconformably overlies the Bermeja Complex of Serpentinite and chert, the Rio Loco Formation, and the Sabana Grande Andesites. The pre- Parguera depositional surface slopes away from the major outcrops of the Bermeja Complex.

In the stratigraphic section given above, the Bahia Fosforescente Member grades upwards into the overlying Punta Papayo Member. The Isla Magueyes Member overlies the Punta Papayo Member with probable erosional disconformity.

From west to east (Punta Melones to Ensenada), the following stratigraphic changes occur: The Bahia Fosforescente and Punta Papayo Members thicken; a thick bioclastic limestone is found within the lower part of the Bahia Fosforescente Member; the Punta Papayo Member becomes somewhat tuffaceous; and the Isla Magueyes Member replaces the Melones Limestone. 

The units have been dated as follows: the Bahia Fosforescente Member is Santonian - Early Campanian; the Punta Papayo Member is Early - Late Campanian; and the Isla Magueyes Member is Late Campanian - Early Maestrichtian.

The source of the non-carbonate clastic material has a local easterly component and a regional northerly component. Chlorite, kaolinite, and mixed layer clays (primarily montmorillonite-illite) are the important clay minerals in the Bahia Fosforescente and Punta Papayo Members. In addition to the above clays, illite and montmorillonite are important in the Isla Magueyes Member.

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The source of the non-carbonate material was tectonically active in the Santonian - Early Campanian and Late Campanian - Maestrichtian times. The Parguera basin was very shallow marine at these times. In the Early - Late Campanian interval, the same area was deeper water marine shelf or slope. 

Detailed study indicates that the Parguera Limestone is nowhere overturned or overthrust as formerly believed. Movement during deposition was primarily vertical. Deformation after deposition was by vertical or strike-slip movement on faults or by folding as a sedimentary section draped over vertically shifting basement blocks.

Tectonic isolation of a Parguera block, bounded by vertical faults characterized by strike- slip or vertical movement, would permit the semi-independent geologic development of this block and its neighbors. Through such isolation the intrablock persistence of stratigraphic units, the mild structural deformation, and the thick limestone units in the Parguera area could have developed contemporaneously with the great volcanic sequences found elsewhere in Puerto Rico.

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