Carbonate Sedimentology Laboratory and Michigan Basin Core Research Laboratory,. Western Michigan ... Mud banks of Florida Bay with calcareous algae, sponges, and sea grasses. .... Scipio Field, Hillsdale County, MI,. P# 21833.
Using Modern Analogs to Understand the Genesis of Ancient Carbonates: Trenton/Black River Formations, Michigan Basin Voice, Peter J., Anthony E. Sandomierski, William B. Harrison III, and G. M. Grammer Carbonate Sedimentology Laboratory and Michigan Basin Core Research Laboratory, Western Michigan University, Kalamazoo, MI
Abstract: In modern carbonate depositional settings, carbonate mud is generated as a result of several processes, including disintegration of calcareous algae, micritization of carbonate grains, inorganic precipitation, and direct precipitation of mud-sized tests of calcareous phytoplankton. In Florida Bay, mud production occurs in shallow protected waters inland of the Florida Keys, and is thought to form from the breakdown of calcareous thalli of codiacean algae including species of Rhipocephalus, Udotea, and Penicillus. Mud accumulates to form banks through the baffling of currents by the sea grass, Thallassia. Skeletal sand shoals composed of the remains of Halimeda, mollusks, and minor input of corals are found in lateral relationship with the mud banks. The activities of burrowing, mudingesting arthropods convert carbonate mud to sand-sized peloidal grains. Burrows may obscure the primary depositional fabric of the sediment but can also enhance the porosity of a unit. The Trenton/Black River Formations (Ordovician) of the Michigan Basin were deposited on a shallow carbonate shelf and are characterized by bioturbated peloidal wackestones and mud-lean packstones with local crossbedded skeletal grainstones. Vascular plants such as Thallassia were not present during the Silurian, however, crinoids served the role of current bafflers in the development of mud banks. In addition, the activities of arthropods and worms likely contributed to the stabilization of mud banks by pelletizing the mud. Understanding the genesis and stabilization of mud-rich facies in modern carbonate depositional environments can be useful in understanding both the origin and the distribution of ancient muddy carbonates, and may help to predict reservoir trends on both regional and local scales.
Modern Carbonate Environments: Florida Bay
10 miles
Scleractinian Coral Reefs Algal tidal flats and mangrove islands
Mud banks of Florida Bay with calcareous algae, sponges, and sea grasses. Note presence of Halimeda, a sand-size carbonate grain producer.
Carbonate Mud Production and Pelletization Carbonate Mud Production •Disintegration of calcareous algae •Micritization of carbonate grains •Accumulations of calcareous phytoplankton •Direct inorganic precipitation
Calcareous Algae: Penicillus Degradation of algal skeletal material is one of the primary mechanisms for mud production.
4182.0
2 mm
Fossiliferous peloidal wackestone from the Trenton Formation. From the Casler 5-30, Jackson Co., Mi.
From Scholle, Bebout, and Moore, 1983
Pellets within a Callianassa burrow
Callianassa burrows consisting of pelletal carbonates, which through diagenesis may become peloids.
(after Tucker and Wright, 1990)
Modes of Formation of Peloids, the primary grains in both Florida Bay and in the Trenton/Black River Cores
Bioturbation and Burrowing
Significant Points • may obscure primary depositional fabric • may enhance porosity and permeability • may serve as conduits for diagenetic fluids Modern Callianassa burrow. Note pencil for scale.
From Scholle, Bebout, and Moore, 1983
Burrowing alters the primary fabric of sediments during and after deposition. As discrete structures, burrows act as conduits for geologic fluids, which may alter the fabric further.
Example of Glossifungites ichnofacies in the Trenton Fm.
Mangrove root structures and bioturbation in cored material from Florida Bay
Diagenesis along burrow pathways may enhance the reservoir properties of a geologic unit. The nature of burrowing therefore, whether it be horizontal traces or vertical structures, may impact the nature of fluid flow through the reservoir.
From Pemberton, 1992
Skolithos Ichnofacies
Develop in relatively high energy environments Develop in muddy to sandy substrates Deep, more or less permanent structures
From Pemberton, 1992
Glossifungites Ichnofacies Develop on hardgrounds
Cruziana Ichnofacies
Usually found in subtidal environments Poorly sorted unconsolidated materials Below fairweather wavebase
Mud Bank Formation
After Ginsburg, 1956 From Scholle, Bebout, and Moore, 1983
Distribution of mud banks in Florida Bay
Generalized facies model of a mud bank
Mud bank development in Florida Bay is initiated on coarse skeletal lag deposits. This low-relief antecedent topography provides photosynthetic organisms, including calcareous algae and Thalassia, a more favorable habitat. Disintegration of algal skeletal material provides a source for the muddy sediment, which is further altered by the digestive tracts of arthropods and worms to form pellets and peloids. Currents flowing over Thalassia are baffled, trapping sediment around the plants. Over time these processes generate a bank of muddy material, two to three meters in relief and 10s of miles long. Mud banks that build up to sea level may be colonized by mangroves and develop into mangrove islands with associated algal tidal flats.
Thalassia sea grass, a baffler of currents
Calianassa burrows and Thalassia
Trenton/Black River: Mud dominated Facies (Mud banks?)
Modern Callianassa burrows
Thalassia sea grass, modern bafflers
Burrowed Peloidal packstone Bryozoan bafflestone
3997.5
2 mm
From the MOC-Skinner #1
From Scholle, Bebout, and Moore, 1983
3879
3 mm From the McClure-Hergert #2
3 mm
3957
From the MOC-Skinner #1
Bryozoan/coral grainstone storm bed
Mollusk grainstone lag deposits
Mud banks are discrete structures developed through baffling by frondose organisms and the accumulation of muddy sediments. Modern mud banks in the Florida Bay develop through the baffling of currents by various organisms including Thalassia sea grass, calcareous algae, and bryozoans. Mud is generally produced from the disintegration of algal skeletal material and is pelletized through the activities of arthropods. Bank growth is initiated on topographic highs, which consist of coarse skeletal lag deposits. A similar set of facies is found in the Trenton/Black River.
Trenton/Black River: Facies MOC Skinner #1, Sec. 23, T5S, R3W, Scipio Field, Hillsdale County, MI, P# 21833
Courtesy of Mi. DEQ
Facies Types Burrowed Peloidal Packstone-Grainstone Burrowed to Cross Laminated, Skeletal, Peloidal Packstone-grainstone Laminated Peloidal Skeletal PackstoneGrainstone Core description compiled by the Carbonate and Evaporite Depositional Course, WMU, Fall 2003
Skeletal Intraclastic Grainstone
Fenestral, Peloidal, grain-dominated Packstone Bryozoan Boundstone Fissile “Shale”/Ash
Conduits for Fluid Flow 1. 2. 3. 4.
Burrows Fractures Primary fabric: Skeletal packstones/grainstones Primary Porosity: fenestral porosity
Modified from Tedesco and Wanless, 1991
Burrowing and subsequent infilling produces a fabric different from the original fabric. Infill can be composed of coarser skeletal debris through the action of storm currents. Fluids will later preferentially migrate through the debris filled burrows.
Fabric elements such as burrowing, fenestral porosity, and skeletal-rich zones provide conduits for the flow of diagenetic fluids. The alteration generated by fluids can either occlude or preserve porosity. The current reasoning with the Trenton/Black River Formations, suggest that reservoir quality permeability is related primarily to fracture zones and the presence of other potential permeabilityenhanced zones are ignored.
Burrowed dolomitic Peloidal wackestone with chert nodules
Secondary Porosity: Fractures
Fenestral Porosity in peloidal mudstones/wackestones Dolomitized burrowed peloidal packstones and skeletal grainstones Dolomitized burrowed peloidal packstone
Laminated Skeletal Grainstones
Significance •Trenton/Black River deposition may have occurred in the form of mud banks. •Current production models in the Trenton/Black River infer that fracture-related dolomite is the only potential hydrocarbon reservoir-type. •Examination of cored material suggests that alternative models are possible and necessary for full development of these reservoirs. •Dolomitization is related to both primary sedimentary or depositional facies and to secondary diagenetic controls.
