EES 2021 LAB 3 – CARBONATE ROCKS Name________________________________________________ Date_________________ Carbonate rocks are fundamentally different than siliciclastic rocks in terms of their depositional environments and chemistry. To deal with these differences, geologists have developed classification schemes and facies models specific to carbonate rocks. While the study of carbonate rock depositional environments and subsequent diagenetic alteration are subjects for at least a full semester class, we will attempt to cover some of the basics here. Components of Carbonate Rocks Carbonate rock grains, called allochems, can be divided into 4 broad categories: skeletal allochems, nonskeletal allochems, matrix, and cement. We will deal with skeletal, nonskeletal allochems and matrix compositions for this lab. Non-skeletal allochems Non-skeletal allochems are components which are not derived from the physical remains of a living organism (shell, coral bits, etc.). These include intraclasts, oolites, and pellets. Intraclasts are lithified (or at least partially lithified) aggregates of carbonate sediment. Rip-up clasts are an example of an intraclast grain. Oolites are spherical, concentric grains of carbonate sediment which coat a central grain which acts as a nucleus. Wave agitation on shallow shoals promotes the growth of oolites much like a snowball being rolled around in snow. Pellets are non-spherical, oblong shaped grains, which are usually the product of fecal material. Skeletal allochems Skeletal allochems are transported bits and pieces of marine organisms. This includes (but is not limited to) brachiopod values, pelecypod (bivalve) valves, gastropods, echinoderms, corals, and bryozoans. The degree of articulation (how “together” the organism is), abrasion, and diversity of skeletal allochems must be considered when interpreting a depositional environment. For example, a rock containing many broken brachiopod and coral fragments must have been deposited in a high-energy depositional environment, such as a reef margin or breaker zone. Conversely, a rock containing exclusively wellpreserved delicate organisms (such as bryozoans) must have been deposited in a low energy depositional environment, such as a lagoon. Sedimentary structures must also be incorporated into interpretations regarding depositional environments. Matrix Carbonate matrix composition can be divided into 2 classes: micrite and sparite. Micrite is essentially lime mud (analogous to the muddy matrix in a greywacke sandstone). Micrite appears “muddy” in hand sample and is a dull brownish color in thin section. Sparite has a crystalline texture (interlocking grains) and appears clear in thin section when viewed in plain polarized light. Classification Because the nature of the origin of carbonate rocks is almost unequivocally different than that of siliciclastic rocks, carbonate rocks are classified based on different characteristics than siliciclastic rocks. Folk’s (1959, 1962) scheme focuses on the composition of carbonate rocks. In Folk’s classification scheme, the type of allochem serves as a prefix to the cement type in the rock. For example, a rock containing mostly ooids in a sparry matrix would be classified as an oosparite. Folk’s classification further separates rocks which have formed in-situ. This is the biolithite category and refers to reef rocks and stromatolites. Rocks lacking allochems are simply called micrites or sparites (depending on the matrix). Rocks with cavities (usually micrites with sparite filled cavities) are referred to as dismicrites. Depositional Setting: Input the number that corresponds with the number on the figure. There are extra numbers than there are depositional environments, so be careful on which answer you select. SAMPLE # ROCK NAME DEPOSITIONAL SETTING 1 2 3 4 5 6 7 8 9 1 10 2 52 6 1 7 3 8 4 EES2021 – Sedimentary Environments COASTAL MARSH STRATIGRAPHY LAB Stratigraphy is a study of sediment layers (strata) that enables scientists to reconstruct the sequence of events or environmental changes as reflected in changing sediment types and vegetation patterns. In this part of the exercise, you will use the information collected from two areas (A and B) in a saltmarsh to reconstruct the past 200-500 years of environmental change. Materials: pencil and graph paper or you can use ant Graphics software. 1. Using the data from page 3, draw two geological cross-sections on the sheet of graph paper Area A on top, Area B below. Plot the cores as vertical columns by using the distance to each core from the next page (one square = 1 m of distance). Assume that the water table is horizontal and adjust all core elevations accordingly. 2. Draw cores A-1 and A-2 on the Area A section by using different symbols or colors for each layer (one square = 10 cm of depth). Write brief description of each layer next to the core section. Put cores B-1, B-2, B-3, and B-4 on the Area B section below using the core distances from the road (zero distance) as you did above. 3. Correlate the strata by connecting similar layers with lines. See the cross-sections below for an example of correlation (dashed lines). dune scar p 5 mar sh dune RO-2 RO-1 0 -5 5 MHW beach/washover 0 V.E. = 4 RO-3 0 20 m dune RO-4 mar sh tidal flat -5 MHW heavy m ineral concentr ations beach/washover RO-13 marsh ? Page 1 N EFFECTS OF STORMS ON COASTAL BARREIRS LANDWARD SAND TRANSFER storms: n unian) flood-tidal delta breach washovers SALTMARSH ZONATION (modified after John Norton) Page 2 Area A - marsh behind an old inlet Depth (cm) Description Vegetation from ground surface color, sorting, grain-size, Sp - Spartina patens Sa - Spartina alterniflora (most abundant listed first) Core A-1 50 m from road 0-18 18-40 Core A-2 dark-brown peat with S.p. gray, well-sorted medium sand 70 m from road 0-18 18-68 68-73 73-90 water table depth - 5 cm Sp none water table depth - 0 cm brown muddy peat dark brown peat brown muddy peat gray, well-sorted medium sand Sa, Sp Sp Sa, Sp none Area B - marsh behind a narrow barrier Saltmarsh starts 17 m from the road Core B-1 10 m from road 0-50 50-70 Core B-2 20 m from road water table depth - 50 cm dark gray medium sand brown coarse sand poison ivy none water table depth - 34 cm 0-8 8--70 brown-gray peaty sand brown coarse sand Sp none Transitional saltmarsh starts 23 m from the road Core B-3 30 m from road water table depth - 0 cm (water is at the surface) 0-20 20-36 36-40 Core B-4 40 m from road sandy brown peat peaty sand brown coarse sand Sp, Sa Sp none water table is 5 cm above core (this core is underwater) 0-22 22-40 40-50 brown fibrous peat dark brown sandy peat brown coarse sand Sa Sa, Sp none Page 3 COASTAL MARSH STRATIGRAPHY LAB – ANSWER SHEET REMEMBER TO ATTACH YOUR CORRELATED STRATIGRAPHIC SECTIONS Name___________________________________________ Date_________________ Area A – a saltmarsh behind a closed tidal inlet 1. What is the sandy feature underlying marsh peat? ______________________________ 2. The marsh that is represented by a mixture of Spartina patens and S. alterniflora plants is called a transitional marsh and occupies the boundary between the high and low marsh. What is the elevation of this type of marsh relative to tidal levels? (circle one of five answers) (Legend: MHT-mean high tide, MLT – mean low tide, MSL-mean sea level, SHT – spring high tide) MLT to MHT MHT MLT MSL to MHT MHT to SHT Area B – a saltmarsh behind a narrow barrier (not an old inlet site) 1. What is the sandy feature penetrated by all cores? ______________________________ 2. Briefly explain why S. alterniflora peat overlies a mixed (transitional) peat in core B-4 (for the same reason, mixed peat overlies S. patens peat in core B-3): (Hint: what must be happening to cause intertidal areas to take over supratidal ones?) _____________________________________________________________________________ Page 4
Purchase answer to see full attachment