Sedimentary Characteristics of Modern Storm-Generated Sequences: North Insular Shelf, Puerto Rico
Published:January 01, 1986
Robert S. Brackett, David M. Bush, 1986. "Sedimentary Characteristics of Modern Storm-Generated Sequences: North Insular Shelf, Puerto Rico", Modern and Ancient Shelf Clastics: A Core Workshop, Thomas F. Moslow, Eugene G. Rhodes
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High-energy events (i.e., storms) play an important role in deposition of modern sediments on the north insular (island) shelf of Puerto Rico, a high-energy, mixed siliclastic/carbonate terrain. Individual storm sequences, such as the SS1, have been analyzed and correlated along the northwestern shelf in the area of Arecibo. The SS1 storm deposit is characterized by a distinctive sedimentary sequence representing three phases of deposition: (1) wave-generated basal lag deposit formed during "peak" storm conditions, (2) graded sand and silt unit redeposited from suspension during waning of storm energy, and (3) a poststorm flood deposited layer. The three units of this individual storm sequence were recognizable by variations in grain size, percent calcium carbonate, and primary physical structures.
A sharp, erosional contact and a well-developed lag marks the base of the SS1 sequence. The lag is much coarser grained and higher in calcium carbonate content than the underlying sediment. Both the high calcium carbonate and increased grain-size values are attributable to accumulations of shelly gravel and the selective removal of fine terrigenous sediment during peak storm winnowing. As the storm wanes, suspended sediment is deposited as a graded sand unit often containing possible hummocky cross-stratification. This portion of the SS1 sequence is characterized by decreasing-upward grain-size and carbonate values. The finest grain-size and lowest carbonate values of the SS1 deposit occur in the poststorm inundite. Between events, physical structures within the inundite and upper portion of the tempestite are obscured by bioturbation.
Faunal constituents within the SS1 basal lag change rapidly with increased distance from the Arecibo River. Faunal changes are correlatable with the change from fluvially derived (allochthonous) terrigenous sedimentation to shelf-derived (autochthonous) carbonate sedimentation. Near the river mouth (terrigenous-allochthonous sedimentation) the primary lag constituents are pelecypods and gastropods. The abundance of echinoids, bryozoans, and algal-encrusted fragments increases with the change to carbonate-autochthonous deposition away from the river mouth. A relative change in calcium carbonate content and median grain size also occurs.
The distance from the Arecibo River mouth seems to be the most important factor determining the characteristics of the SS1 event deposit. The following trends are observed in the SS1 sequence with increasing distance from the river mouth: (1) increased percentage of shelly carbonate in the basal lag deposit (from about 5% to over 70%), (2) increased thickness of the graded storm-sand unit (from 12 to 21 cm), (3) increased bioturbation levels (from rare to abundant burrows), (4) decreased preservation (as burrowing increases), (5) decreased thickness of the poststorm flood layer (from 16 to 4 cm), (6) decreased displacement of faunal constituents, (7) decreased frequency of hummocky cross-stratification and other wave-generated primary structures, and (8) decreased amalgamation of storm deposits.
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Modern and Ancient Shelf Clastics: A Core Workshop
The organizers of this core workshop opted for a broad, permissive definition of shelf clastics and contributors were encouraged to focus on their own interpretations rather than force-fitting their examples to a restrictive theme. As a result, the editors assembled a selection of core examples which range from standline to shelf edge. Given the dominance of petrophysical logs in subsurface studies, contributors to the workshop worked hard to make the necessary core-to-log comparisons which were essential if geologists are to more fully utilize wireline data in facies analysis. Contributors were also encouraged to place their studies within the context of energy prospects associated with environments of deposition. Hydrocarbon potential and reservoir quality dominate these discussions. The high degree of lithofacies variability and the effect of high-energy events on shelf deposits cause sediments formed within this environment to be particularly challenging exploration targets.