A spur-and-groove system is developed on the seaward side of the fringing reefs. The grooves of the eastern coast are normal to the reef front, but those at the northern end of the western coast trend northwest, oblique to the reef front. The prevailing direction of wave approach on the eastern coast is normal to the reef front, but on the western coast the prevailing wave approach direction at the reef front is from the northwest, oblique to the reef front and parallel to the orientation of the spur-and-groove system. No evidence was found that indicates whether the grooves are erosionally or constructionally formed. The spur and groove system and the wave regime of Isla de Mona are further discussed by Kaye and Hernandez Avila.
Seaward of the fringing reefs, a shallow inclined platform extends to a depth of about 20 m, the same depth as that prevailing at the base of the cliffs of the north coast. Fringing reefs are generally lacking on the southwestern coast from the vicinity of Playa del Uvero to Punta Arenas, but the same 20 m inclined platform that is present seaward of the fringing reefs is continuous through this distance, except off Piedra del Carabinero where it is only 10 m deep. The 20 m platform ranges in width from a maximum of 1.2 km off Playa Sardinera to a minimum of 0.7 km off Playa del Uvero.
About 150 m off the coast at Piedra del Carabinero the bottom drops precipitously to a depth of about 80 m. This near-vertical cliff has what appears to be an ancient wave-cut terrace at a depth of about 40m. Another inclined platform with a maximum depth of 100 m is 2 km wide north of Isla de Mona and 3.5 km wide east of Isla Monito. Some of the faults that bound the topographic highs occupied by both islands occur at the edge of this 100 m platform.
The seafloor surrounding Isla de Mona and Isla Monito is a relatively shallow southwestern extension of the broad linear bathymetric high that connects Hispafiola and Puerto Rico. Southwest of Isla de Mona, beyond the mapped area, the bottom drops steeply into the head of a large broad valley that continues southward toward the Muertos Trough; the 2000 m depth contour is only about 12 km distant from the Isla de Mona. South and southeast from the lsla de Mona depths also increase toward the Venezuelan Basin, but less precipitously and with more local topographic variations, at least within the mapped area.
In the northern part of the area the west-northwest-trending Isla de Mona fault zone forms an elongate steep-walled depression alined with a series of linear troughs and ridges. This topographic trend continues both east and west of the mapped area. The maximum depth of the depression is not clear from the echo-sounder records, but appears to be approximately 680 m.
Twenty-five samples of unconsolidated beach, back-reef, and fore-reef sediment were collected. The predominant constituent is skeletal sand with up to 20 percent lithoclasts; the sand is derived mainly from encrustlng types of coralline algae. Lithoclasts include altered skeletal grains, dolomite and limestone fragments, phosphatized limestones, and iron-stained sub aerial crust fragments of the carbonate rocks that make up the islands. Insoluble residue averages 0.23 percent and consists of angular to subrounded quartz fragments, biotite flakes, and a debris of glass and wood splinters.
The foraminiferal fauna is typical of northern Caribbean reef environments. Rotorbinella rosea, which lives on algae in high-energy environments, is the most abundant living foraminifer and occurs both in the back-reef and fore-reef sediments. In the fore-reef sediments, Amphistegina gibbosa and in the back-reef sediments Archais angulatus are the most common dead foraminifers. Because both living and dead foraminifers characteristic of the fore-reef sediments were encountered in back-reef samples, it seems that wave-action may have transported them landward.
Repeated attempts to obtain grab samples of sediment on the platform north of Isla de Mona and east of the lsla Monito to depths of 150 m were typical of hard bottom; on the basis of sampling results and seismic records, the bottom is thought to be simply limestone or dolomitic bedrock. Dredging by R/V Eastward at the depression 11 km northeast of Isla de Mona showed the bedrock there to be deep-water mid-Tertiary limestone.The depth at which this limestone was deposited could be the same as the 500 m depth from which it was dredged. It thus fails to provide evidence of tectonic movement.
A major west-northwest-trending fault zone, here named the Isla de Mona fault zone, is strongly expressed in the northern part of the study area by steep escarpments and other linear topographic features; the fault zone is prominent in the seismic records, though its displacement cannot be measured. The downthrown block between the two bounding faults has steep escarpments in the eastern and western part of the mapped area but has little or no topographic expression in the central part of the area. Topographic and seismic evidence indicates that the fault zone extends for at least 16 km to the west and 10 km to the east of the mapped area; not enough evidence is available, however, to trace it across the whole width of the Mona Passage. This may be the conjectural fault shown by Glover. It is generally parallel to the great northern and southern fault zones of Puerto Rico.
Isla de Mona and Isla Monito lie on two upthrown structural blocks that are almost completely delineated by several apparently near-vertical faults. The blocks are separated by a shallow graben, as was suggested by Kaye. The eastern boundary of the structural block containing Isla Monito is a north-trending fault that projects south across the graben and appears to be the same as the prominant fault in the north-central cliffs of Isla de Mona that was mapped by Briggs and Seiders.
Seaward from the Piedra del Carabinero area, the nearly vertical cliff 80 m high, the longest and steepest slope on any side of Isla de Mona, is inferred to be the scarp of an arcuate northwest-trending fault. Including this inferred fault, the Isla de Mona structural block is almost completely surrounded by faults having their upthrown sides toward the island. The vertical cliff and navigational hazards prevented seismic investigation of the inferred fault.
Southwest of lsla de Mona a major northwest-trending fault has its down thrown block on the landward side, contrary to all others around the islands. This could be the cause of the 0.4 m/km up-to-the-north tilt of the Isla de Mona and Isla Monito structural blocks that is demonstrated by systematically different heights above present sea level of the elevated sea-level nip present at scattered localities on Isla de Mona and Isla Monito. Another possible explanation is that an axial uplift of the northern Antillean Island Arc occurred during the unknown but short time between the formation of the ancient elevated nip and the present sealevel nip.
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