Geosciences 308

Paul Spur reef field trip

The purpose of this trip is to examine the fossils and paleoenvironments of an Early Cretaceous patch reef now exposed along U.S. Highway 80 near Paul Spur, about halfway between Bisbee and Douglas, AZ.

The Paul Spur reef is one of several small reefs (called "patch" reefs, to distinguish them from more extensive fringing or barrier reefs) that crop out in southeastern Arizona and southwestern New Mexico. The reefs formed during a time of relatively high sea level, approximately 113 million years ago, when a narrow arm of the then-opening Gulf of Mexico extended into this region. The reefs and carbonate sediments deposited during this time make up a rock unit known as the Mural Limestone.

Like reefs today, scleractinian corals were an important component of the Paul Spur reef. Unlike reefs today, rudist bivalves were also an important part of this Cretaceous reef community. Rudist bivalves are an extinct group of bivalves in which the regular symmetry is lost as one valve (shell) is modified to form a cone or cup and the other valve forms a lid or cap on the lower valve. Also common in the Paul Spur reef, but uncommon in today's reefs, is the association of stromatolites with coral. The Paul Spur reef contains finely laminated carbonate mud atop portions of branching coral. Brachiopods (often silicifed) also occur in the reef.

Think of the major components of this fossil reef as falling into two major groups: fossils and matrix (the sand or mud between the fossils).

Common fossils

1. Branching platy coral - preserved as thin horizontal branches. Much of the delicate structure of the coral is lost because of the recrystallization of the primary aragonite into calcite. Preservation of corallites is uncommon.

Plan view

(looking down)

Cross-section

2. Branching finger coral - preserved as thin or thick vertical branching "fingers" of coral. Again, much of the delicate structure of the coral is lost because of recrystallization - preservation of corallites is uncommon.

Plan view

(looking down)

Cross-section

3. Unbranching, head coral - preserved as irregularly shaped masses of coral, up to about 1 meter in maximum dimension. Again, don't expect to see much detailed structure because of the poor preservation.

4. Caprinid rudists, or "caps" - solitary rudist bivalves with a characteristic honey-comb, or corrugated wall structure. In cross-section, they may look like this:

cross section plan view

5. Monopleurid rudists, or "monos" - gregarious rudist bivalves, often cementing their valves to their neighboring monos. Their wall structure is solid and often (though not always) dark in color.

plan view

6. Other. Lots of stuff falls into this category - brachiopods, echinoids, other kinds of rudists, oysters, and things you can't identify.

Matrix components

1. Carbonate sand - sand-sized grains of calcium carbonate, often consisting of small pieces of corals, molluscs, forams and brachiopods. The rock will look "gritty", especially if some of the grains are silicified.

2. Carbonate mud - clay-sized calcium carbonate, often derived from microscopic fragments of algae. Individual grains can not be seen with the naked eye. This matrix may appear as very light gray patches.

3. Stromatolitic carbonate mud - finely laminated carbonate mud.

We will walk around the outcrop and you will be shown each of these major types of fossils and matrix. Learn how to recognize them yourself.

Bibliography

James, N.P., and Bourque, P.-A., 1992. Reefs and mounds., pp. 323-348, in Walker, R.G., and Jones, N.P., ed.s. Facies Models. Response to Sea level Change. Geological Association of Canada. Good review of reefs from a geological and paleoecological perspective.

Scott, R.W., 1979. Depositional model of Early Cretaceous coral-algal-rudist reefs, Arizona. American Association of Petroleum Geologist Bulletin, v. 63, pp. 1108-1127.

-- a general discussion of the setting, fauna and facies of southeast Arizona reefs.

Scott, R.W., 1981. Biotic relations in early Cretaceous coral-alagal-rudist reefs, Arizona. Journal of Paleontology 55: 463-478. Available on electronic reserves under Geosciences 308, section 1. Password is Paleo

Roybal, G.H., 1981. Facies development in a Lower Cretacous coral-rudist patch reef (Mural Limestone, southeastern Arizona). Mountain Geologist, v. 18, p. 46-56.

-- a detailed map and analysis of the fossils and matrix of a nearby patch reef (not the same one as on this trip). Available on electronic reserves under Geosciences 308, section 1. Password is Paleo

The project:

After the walk-through, form up in teams of four and check out a tape and a compass. Then each team will make observations along one of the transects from the top of the cliff to the western-most extent of the outcrop.

Starting at the top of the cliff, mark out your first sample station.

Each sample station should measure about 1 meter on a side, a “quadrat” (note the spelling; it’s not “quadrant”). Within that quadrat, estimate the percentage of the area that is covered by each of the nine (six kinds of fossils and three kinds of matrix) components (Note: we will provide you with a handout to help with your estimations, PLEASE BRING IT WITH YOU ON THE FIELD TRIP). Make a sketch of the quadrat (be sure to note the quadrat number) and note any other observations about the fossils or matrix.

Then move ten meters to the west to the next sample station and repeat the procedure.

After you have reached and sampled your eighth and western-most sample station, walk around to the base of the cliff below your first sample station. There, sample another square meter. That quadrat will be oriented vertically rather than horizontally.

Your team’s field data should be a large table with nine rows (nine sample stations), with nine columns (each recording the percent cover by each of the reef components - fossils or matrix), nine sketches, and nine sets of additional observations.

In lab next week, each team will enter these data into a spreadsheet, sketches will be scanned-in, and notes entered onto a website that will then be available for everyone to use in preparing their own lab report.

Purpose of project:

· Map the distribution of the reef facies (groups of rocks that have similar fossils and sediment types).

· Interpret, based on the facies you mapped, the environments of the reef: water depth, water energy, direction toward open water.

· Identify the major components of the reef: framework builders, sediment producers, sediment binders

Each person must write their own, unique field trip report, due in class on Tuesday, Nov. 20.

The report should have -- AS A MINIMUM:

1. Names, name of your team-mates, photocopy of field data, field sketches and comments from your field work

2. Title

3. Abstract (succinct summary of content, approx 250 words)

4. Introduction

Purpose of work

Geographic and geologic setting

Brief review of previous work. Cite the geologic literature. We suggest having one person

on the team read one article and then report its findings to the other people on the team

5. Methods

Quadrats, transects, Excel

Estimating percentages, likely variation, how determined

6. Results (no interpretation here, just description)

Table listing fossils identified

Figure showing distribution of platy coral over entire outcrop

Figure showing distribution of monopleurid rudists over entire outcrop

Figure showing distribution of mud over entire outcrop

Figure – Your team’s transect, showing, with a pie diagram, the proportions of the major

constituents at each quadrat site

Discussion of each figure

Description of facies recognized (no interpretation here, just description). You should be

able to identify at least three facies (=rocks characterized by a distinctive fossil

content and lithology)

Map showing the distribution of the facies that you recognize

Cross-section showing the distribution of the facies that you recognize

7. Discussion

Environmental interpretation of each facies in terms of water energy, depth, direction of

open ocean. Where was the fore-reef, where was the crest, where was the back-

reef/lagoon?

Identification of framework builder(s) of the reef

Topography and orientation of the reef

An interpretive sketch showing the distribution of environments.

8. References cited

9. Appendix

Table that shows the composite data

Your group’s data and observations