Petrographic Analysis and Depositional History of an Open Carbonate Lagoon: Rice Bay, San Salvador, Bahamas
James Lyle Stuby
B.S., Juniata College, 1994
M.S., Wright State University, 2000

ABSTRACT
(as published in thesis)

The Bahamas consist of an archipelago of carbonate platforms with surficial deposits and rocks of Middle Pleistocene and younger age. San Salvador is one of the smaller platforms. The Bahamian Field station is located on the northern end of the island, on the shore of a large lagoon called Graham's Harbour. Separated from Graham's Harbour by a peninsula of lithified dunes (North Point) is Rice Bay. The goal of this thesis was to develop a depositional history of Rice Bay using outcrop relationships and petrographic analysis (point counting) of sediments from the floor of Rice Bay (in a transect from Rice Bay beach to an island in the bay known as Man Head Cay - Transect A), sediments from three sediment cores from the lagoon, and rocks from Man Head Cay.

Most of the field work was performed during June 24-28, 1998, when I took a course called Carbonate Depositional Systems, run jointly by Dr. Mark Boardman of Miami University and Dr. Cindy Carney of Wright State University, who were two of my thesis advisors (the other was Dr. David Dominic of Wright State University). I was able to return to San Salvador about a year later by taking the same course again, when I focused much of my attention on Man Head Cay. I also returned to San Salvador for the 10th Symposium on the Geology of the Bahamas, June 8-12, 2000.

A geologic map of Rice Bay has been drawn to show the various formations that make up the rocks in the area, and also to show the location of Transect A, the sediment core locations, and the location of the rock samples from Man Head Cay. The geology is based on a smaller scale geologic map published in Carew and Mylroie (1995, p.27), with a few minor changes. A revised map was made following my trip to Man Head Cay for the 10th Symposium, and became a figure in the paper I wrote for the proceedings volume.

Click here to see a panoramic photograph of Rice Bay taken from Man Head Cay. It shows (from left to right) Hanna Bay, Northeast Point, Rice Bay Beach (a.k.a. Coast Guard Beach), North Point, and Cut Cay. The lighthouse (due South) is a white spot on the horizon behind Hanna Bay. The water tower of the Bahamian Field Station and adjacent buildings are visible on the central horizon. Dark patches in Rice Bay are seagrass beds and subtidal rocks. The rocks and vegetation in the foreground are part of Man Head Cay. West is to the right of the small building on North Point.

I took some photographs of the spectacular crossbedding at North Point. Some of this crossbedding extends below sea level.

Stephanie Bain for scale.
Click on image to enlarge

Randy Russel for scale. Also note shipwreck behind Randy.
Click on image to enlarge

Some of this information has been presented at the North Central GSA meeting in Champaign, IL, April 22-23, 1999 (Abstract), although my interpretation of the data has changed since that time. I have presented this work at the 10th Symposium on the Geology of the Bahamas on San Salvador, June 8-12, 2000 (Abstract). I lead a field trip to Man Head Cay at that time, which allowed me to focus my attention on the protosol there.

Transect A

46 sediment samples were collected from the sea floor of Rice Bay in a straight line from the top of the beachface on Rice Bay Beach (station 98-0) to the beach on Man Head Cay (station 98-450). This transect passed through barren sand, Callianassid mounds, and various densities of seagrass (Thalassia testudinum and Syringodium filiforme) and calcareous green algae (Halimeda sp., Penicillus sp., Udotea sp., etc.).

Cross section through Transect A showing variation in subtidal environments.
Click on image to enlarge

Unfortunately, only 18 of the 46 samples were properly impregnated with epoxy which is required to make thin sections from unconsolidated sediments. Point count data from Transect A indicate several trends. For example, percent ooids decrease from Rice Bay Beach (98-0) towards Man Head Cay (98-450). Also, a significant correlation between seagrass density and certain grain types was found. The statistical information will be added to this web page at a later time.

Several photographs were taken by Michelle Geissler below water in Rice Bay along Transect A:

Callianassid shrimp mounds.
Click on image to enlarge

Calcareous green algae, seagrass, and callianassid shrimp mounds.
Click on image to enlarge

Dense seagrass (Thalassia testudinum and Syringodium filiforme).
Click on image to enlarge

James Stuby collecting sediment in barren sand.
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James Stuby, Derek Dice, and Matt Brueseke collecting sediment in a seagrass bed.
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James Stuby collecting sediment at edge of seagrass bed.
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Sediment Cores

The three sediment cores from Rice Bay were collected using the vibrocore technique. Their locations are shown on the geologic map. The cores were logged for grain size, sorting, and color macroscopically, and then impregnated with epoxy. A facies log has been created, showing facies contacts, root penetration, and thin section locations:


A total of 26 thin sections were made from the three cores (3 from RB 6-26-97, 12 from RB 6-27-97, and 11 from RB 6-27-98). Each thin section was counted to at least 300 points, and grain type percentages were calculated. The data are shown graphically in the form of cummulative compostition logs for each of the cores:


RB 6-26-97
(incomplete core)
Click on image to enlarge

RB 6-27-97
(penetrates peat at base)
Click on image to enlarge

RB 6-27-98
(penetrates peat at base)
Click on image to enlarge

"algae" includes calcareous green algae (Halimeda) and calcareous red algae (Goniolithon).
"foraminifera" includes Peneroplid (Archaius angulatus), perforate, imperforate, encrusting, and agglutinated forams.
"mollusks" include undifferentiated mollusk fragments, bivalves, and gastropods.
"other skeletal grains" include echinoderms (mostly echinoid spines), corals, bryozoa, ostracoda, and worm tubes.
"ooids" include only ooids, which are mostly superficial.
"peloids" include only peloids (fecal material).
"clasts" include intraclasts and oolitic clasts.
"other non-skeletal grains" include mud, organic matter, and roots.

Notice how algae decreases with depth in the complete cores, and ooids increase with depth.

There is strong evidence that facies 6 (f6) at the base of core RB 6-27-98 is beach sand and that it may be the source of the dunes of North Point (or even an unlithified portion of the North Point dunes). It is approximately the right age (see C14 data), the composition matches that of North Point, the fine grain size and moderate sorting match that of a beach (and are at variance with overlying poorly sorted lagoon sediments - see grain size log below), and visual comparison of the sediments in thin section is compelling as well.

I performed a graphical analysis of sieve data acquired from 20 sediment samples taken from core RB 6-27-98. The samples were sieved by Rick Zimmerman, an undergraduate at Miami University in Oxford, OH. A log shows interesting trends in median grain size, graphic standard deviation (sorting), and graphic skewness. These variables are plotted against depth. Note particularly how sorting drops from 2-3 phi to less than 1 phi in the lower 1.5 m of the core (f6). This sorting supports the interpretation of this sediment package as a beach or eolianite.


Carbon 14 Dating of Peat

Peat deposits are an excellent sea-level indicator, because they are generated by mangroves which thrive in the intertidal zone. They can also be carbon-14 dated. By determining the depth of the peat deposits, one can plot sea level vs. time on a graph and get a sea level curve. C-14 dates were obtained from peat at the bottom of Cores RB 6-27-97 and RB 6-27-98. Boardman, et al. (1988) generated a sea level curve for the Bahamas. The curve is a third-order best fit line to dates plotted vs. depth, obtained from peat in sediment cores from San Salvador, Andros, and Abaco. The data from the two cores in this study are plotted on the same graph.

Man Head Cay

Point count data for the eight rock samples from Man Head Cay indicate that it is non-oolitic, consisting of skeletal grains, peloids, and intraclasts, with very little cementation (high porosity).

Chart showing grain type distribution of Man Head Cay.

During my trip to San Salvador in June of 1999, I lead a group of grad students to carry out the Man Head Cay Project.

Image of Man Head Cay as viewed from North Point (facing East) across Rice Bay.


Depositional History

The following table shows the Late Quaternary depositional History of San Salvador, with emphasis on events in Rice Bay. O.I.S. = Oxygen Isotope Stage. SL = sea level relative to present, in meters. Primary sources of data and interpretations are this thesis, Carew and Mylroie (1995, especially Fig. 3, p. 8, and 1999), and Boardman, et al. (1988).

 

O.I.S.

Date (BP)

SL (m)

Events

I

6

>132,000

~-80

Lowstand. Exposed platform. Development of paleosol overlying Owl’s Hole Formation.

II

5e

~132,000 to ~125,000

~-8 to +7

Transgression. Flooding of platform and production of carbonate sand. Sand accumulates in beaches and is blown into dunes, which lithify into eolianites of the French Bay Member of Grotto Beach Formation. Some French Bay rocks may be exposed on northeastern Man Head Cay.

III

5e

~125,000

+7 to +8

Highstand. Ooid production in interior of San Salvador, as well as development of Cockburn Town Reef. Many eolianites of the French Bay Member are submerged.

IV

5e

~125,000 to ~119,000

+7 to ~-8

Regression. Lagoon sands reactivated and some are deposited as eolianites of the Cockburn Town Member of the Grotto Beach Formation along the eastern coast of San Salvador, including Man Head Cay. Minor hiatus in dune formation (pause in regression?) leads to development of protosol on Man Head Cay. Extensive penetration of vegemorphs occurs as lithification of the eolianites proceeds.

V

5d to 5a

~119,000 to ~75,000

~-8 to ~-15

Minor highstands and lowstands that probably do not flood platform.

VI

4 to 2

~75,000 to ~15,000

~-30 to ~-60

Major lowstands (O.I.S. 2 and 4) and minor highstand (O.I.S. 3). (Wisconsin Glacial Period). Exposed platform. Isostatic subsidence of ~1 m. Development of paleosol overlying Grotto Beach Formation. End of Pleistocene.

VII

1

~7000 to 6000

-8 to —5

Early transgression. Platform edges are flooded. Man Head Cay and other rocks form a sill for restricted lagoon (Rice Bay) in which mangroves thrive. The mangroves are preserved as peat.

VIII

1

~6000 to 5000

-5 to -3

Middle Transgression. Sill is breached and more open exchange of sea water is possible in Rice Bay. Sand accumulates in Rice Bay, and beaches prograde over some peat deposits, providing a sediment source for dunes of North Point (North Point Member of Rice Bay Formation), which quickly lithify. Lagoonal sands accumulate in other parts of Rice Bay.

IX

1

5000 to present.

-3 to 0

Late Transgression. Rice Bay continues to be filled with lagoonal sands. Waves cut a platform into the eolianites of North Point, and corals grow on the platform. Oolitic clasts from North Point are deposited in Rice Bay and Grahams Harbour. Beachrock (Hanna Bay Member of Rice Bay Formation) develops along the mainland as sea level rises, especially at Northeast Point.




Copyright 2006 James L. Stuby. All Rights Reserved.