all consolidated sedimentary rocks deposited by wind. • Carbonate eolianites are
deposited as coastal dunes adjacent to high energy beaches in warm climates.
EOLIAN DEPOSITIONAL ENVIRONMENTS Summary by: Jeannette Arce Carbonate Geology Class
GENERAL DESCRIPTION OF AN EOLIANITE
Figure 13A: Memoir 33
• Eolianite – this name was proposed (Sayles, 1931)for all consolidated sedimentary rocks deposited by wind. • Carbonate eolianites are deposited as coastal dunes adjacent to high energy beaches in warm climates.
GEOGRAPHY
Global Distribution of carbonate Eolianites
• • • •
Warm climate- CaCO3 production Nearby coastal areas Uncommon in interior deserts Can develop locally were alluvial fans provide carbonate sediment. • Areas with carbonate sand particles previously accumulated • Onshore winds to deliver coastal sands inland. • Some recent dunes have been formed on old shorelines. Figure 1: Memoir 33:
• Grain size- fine to medium • Sorting- very well sorted • Roundness- well rounded grains • Surface features- do not develop conspicuous surface features as polish, frosting or scratches. • Skewness- mean skewness nearly symmetrical (Lybian coast)
IMPORTANCE OF CARBONATE DUNES
Figure 16b: Memoir 33
• Localization of ancient strandlines and islands. • Eolian dunes exert control in future carbonate sediment distribution patterns. • Eolianites are potential reservoirs of oil and gas.
CLASSIFICATION CRITERIA
Figure 3: Memoir 33
Yucatan Peninsula: Exposure face parallel to strandline
• Geometry - Elongate bodies of carbonate grainstone trending parallel to the strandline. - variable thickness
• Grain size- fine to medium • sorting- very well sorted • Stratification- cross strata • Early stage cements • Trace fossils
DUNE TYPES
Figure 7: Memoir 33
• A-Transversal- relatively straight dune ridges parallel to the coast. • B-Barchanoid ridgeparabolic dunes with U and V shape sand hills. • C-Barchan- in areas of unidirectional wind and small sand supply • D-Blowout- common were vegetation is abundant and stabilizes the sand • E-Parabolic- common were vegetation is abundant
SEDIMENTARY STRUCTURES: CROSS STRATIFICATION Type of cross strata Planar
Figure 18: Memoir 33
Figure 16B: Memoir 33
- Onshore winds develop ridges parallel to the strandline. - Slipfaces dipping landward. - Form straight parallel sets - Several meters long
SEDIMENTARY STRUCTURES: CROSS STRATIFICATION Type of cross strata Tabular – Planar
Figure 22: Memoir 33
Figure 19: Memoir 33
Single big sand wave may form a large set of foreset cross beds consisting of tabular planar structures.
SEDIMENTARY STRUCTURES: CROSS STRATIFICATION Type of cross strata Wedge-Planar
Figure 20: Memoir 33
- Height of a few to 10 m or more. - Well represented in common transverse dune types
SEDIMENTARY STRUCTURES: CROSS STRATIFICATION Type of cross strata Festoon pattern - Have various scales - Develop in saddles between spillover lobes Figure 23A: Memoir 33
SEDIMENTARY STRUCTURES: CROSS STRATIFICATION Angles of foreset dips
Figure 16A: Memoir 33
Figure 19: Memoir 33
- Foresets of slipfaces are high angle (Bottom figure). About 340 degrees - In the windward side are low angle- less than 150.
SEDIMENTARY STRUCTURES: CROSS STRATIFICATION • Azimuth patterns and paleocurrent direction - Confirm consistency of sand transport direction.
Figure 23A: Memoir 33
• Thickness of laminae and beds - similar to siliceous dunes.
SEDIMENTARY STRUCTURES: CROSS STRATIFICATION • Measurement of strata - Thickness of Laminae - Different thickness, from millimeters to meters. • Traces of Foresets - High angle sloping surfaces may meet the underlying plane at an acute angle. - Curvature parallel to wind direction. - Convex upward Foresets due to early stabilization of the dunes.
MINOR STRUCTURES • Wind Ripple marks - Uncommon in eolian limestone – migration of dunes destroy them. - Characteristically low and have high indices. (Ratio of height to wavelength) - Parallel to the dip directions of the high angle foresets on the leeward side of the dune.
MINOR STRUCTURES • Contorted bedding - Slumping on high angle slipfaces. - Common feature in dunes. - In areas of high humidity and rainfall. Figure 25: Memoir 33
MINOR STRUCTURES • Rhizocretions or root traces
Figure 12: Memoir 33
- Are root cast structures of dune vegetation, preserved in the form of caliche. - Sometimes described as cemented plant root molds.
MINOR STRUCTURES
Figure 8A-8B: Memoir 33
• Bioturbation - Bioturbation by plant root systems is common. - Faunal bioturbation uncommon or not been recognized in many carbonate dunes. Due to early diagenesis or cementation. • Breccias and break apart - altered fragments of eolianite - Wet crusts on dry sand shatter into break aparts
Grain contact and meniscus cement • Cement precipitated at grain contacts where surface tension holds droplets of percolating water. • Generally fine to coarse crystalline calcite.
EARLY STAGE CEMENTS Pendulous cement • Precipitated on the bottom of the grains. • Common in eolianites.
Figure 28: Memoir 33
• Evidence of vadose zone cementation.
EARLY STAGE CEMENTS • •
• Figure 30: Memoir 33
•
Needle-fiber cement Needle like crystals of calcite cement. Needle fibers are composite crystals with strings of imbricated flattened rhombs of calcite Occur in intergranular pores and in intraparticle dissolution voids. Indicative of vadose zone cementation.
EARLY STAGE CEMENTS Microcrystalline- rind cement
Figure 32: Memoir 33
• Grains coated of irregular rinds of microcrystalline calcite (1-2µm) that grades poreward into bladed and rhombic crystals of calcite. • Few intergranular pores are completely filled.
EARLY STAGE CEMENTS Pore-occluding cement • Scattered pores totally filled with calcite cement, but adjacent pores remain cement free. • Crystal sizes and shapes: - finely crystalline anhedral blocky spar. - fine to coarse bladed druse. - large single crystals.
Diagenetic fabrics related to roots •
• • • Figure 33: Memoir 33
Rhizocretions Vertical or horizontal tabular structures of hard brown microcrystalline calcite. From mm to cm. Internal structure of wavy laminae of (caliche like) microcrystalline calcite. The core may preserve some of the cell structure of the root.
Diagenetic fabrics related to roots Root hair sheaths
Figure 34: Memoir 33
• Minute tubules of microcrystalline calcite abundant in interstitial pores. • Hollow sheaths of 515 µm diameter with wall thickness of 1-2 µm.
Diagenetic fabrics related to roots
Figure 37: Memoir 33
Microcodium • Diagenetic structures composed of calcite prisms arranged in spherical, elliptical or sheet like bodies. • Formed in some Pleistocene eolianites. • The produce of calcification of mycorrhizal(root plus fungus) associations.
Diagenetic fabrics related to roots
Figure 36: Memoir 33
Microscopic borings • Grains near rhizoconcretions riddled with tubular borings 1525m diameter. • Holes about the size of root hair sheaths. • May be result of root-hair and rootlet penetration of the carbonate grains.
Diagenetic fabrics related to roots Subaerial crusts
Figure 38-39: Memoir 33
• Caliche or calcrete crusts developed on and within Quaternary eolianites.
Progressive early diagenesis • More space is occupied by sparry calcite cement in progressively older Holocene eolianites. • Porosity ranges from about 25-40 percent in holocene eolianites. • Almost no dissolution occur in these calcarenites.
Ages of carbonate eolianites (Pleistocene Examples) • Most carbonate dunes and eolian limestones are classed Holocene or Pleistocene. • Theres no direct relationship between age and degree of lithification. • Most non lithified active dunes are of modern or Holocene age not Pleistocene. • Many Holocene dunes are partly cemented.
Ages of carbonate eolianites (Ancient Eolianites) • Few carbonate rocks from pre –Pleistocene have been interpreted as eolian limestones. • Few records from the Permian period have been interpreted as eolian carbonate rocks. • The great Oolite Series of Jurassic age in England have been interpreted as eolian.
