Does Rock Thickness Compared to Time Represented Accurately?

Does Rock Thickness Compared To Time Represented offer a reliable measure of geological history? Discover the complexities of this relationship at COMPARE.EDU.VN, where we delve into the factors influencing rock formation and preservation. Understanding these variables is crucial for accurately interpreting the geological record and unravelling Earth’s past, including sedimentation rates and stratigraphic unconformities.

1. What Is the Significance of Unconformities in Geological Records?

Unconformities signify breaks in the geological record, representing periods of erosion or non-deposition. These interruptions are pivotal in understanding the geological history of a region, as the distribution and thickness of rock units at the unconformity’s interface often reveal more about past events than the rocks’ current orientation. Levorsen’s book (1960) provides excellent insights into paleogeologic maps, while Krumbein (1942) established 35 criteria for identifying subsurface unconformities.

• Types of Unconformities: Unconformities generally align with those identified by Billings (1946). Dunbar and Rodgers (1957, p. 119) introduced the term paraconformity for parallel beds with contacts indistinguishable from simple bedding planes.
• Major Breaks in Kansas: The Kansas rock section contains seven significant breaks: the present surface, pre-Tertiary post-Cretaceous, pre-Cretaceous post-Jurassic, pre-Triassic post-Permian, pre-Pennsylvanian post-Mississippian, pre-Mississippian post-Devonian, and pre-Paleozoic post-Precambrian.
• Time Placement: Designations such as “pre-Tertiary post-Cretaceous” are imprecise because land surfaces formed by erosion do not originate instantaneously. Their placement in geologic time is approximate, spanning late Cretaceous to early Tertiary periods.

2. How Does the Present Land Surface Serve as a Key to Geological History?

The present land surface in Kansas offers crucial insights into the area’s geological past (Jewett and Merriam, 1959). This surface gently slopes eastward across the High Plains, Plains Border, Smoky Hills, and Osage Plains. The highest point, Mount Sunflower, reaches 4,039 feet above sea level, while the lowest, in Montgomery County, is about 700 feet.

• Eastward Inclination: The eastward slope of the land surface contrasts with the regional structure of outcropping rocks older than Tertiary, suggesting the slope was established in late Tertiary time.
• Geologic Map: Figure 90 shows the areal geology of the present land surface, with older bedrock formations exposed progressively eastward. Igneous bodies crop out in Riley and Woodson counties.
• Structural Implications: Outcropping Pennsylvanian and Permian rocks in eastern Kansas dip gently westward at an average of 20 to 25 feet per mile, interrupted by the Nemaha Anticline.
• Prairie Plains Monocline: The monotonous westerly dip led to the name Prairie Plains Monocline (Prosser and Beede, 1904), more accurately termed the Prairie Plains Homocline, resulting from regional uplift centered in the Ozark area.
• Alluvial Terraces: Remnants of alluvial terraces, connected to glaciation, lie from a few feet to about 200 feet above valley floors in eastern Kansas (Jewett and Merriam, 1959).
• Glacial Deposits: Glacial till and outwash materials are prevalent in northeastern Kansas, with thick loess deposits bordering the Missouri and Kansas River valleys.
• Cretaceous Deposits: The eastern margin of Cretaceous deposits has retreated farther west in southern Kansas, exposing Permian rocks near the southern boundary.
• Smoky Hills and Blue Hills: The Smoky Hills, eroded from Dakota sandstone, mark the eastern edge of Cretaceous deposits, while the Blue Hills are eroded Cretaceous deposits at the eastern margin of the High Plains.
• High Plains: The High Plains consist of Pliocene beds and Pleistocene deposits, forming a thick wedge of unconsolidated material in western Kansas. The Arkansas River Lowland and “Equus beds” (McPherson Lowland) are notable features.

3. What Can Be Inferred from the Pre-Tertiary Post-Cretaceous Surface?

The pattern of sub-Cenozoic geology aligns with the regional surface geology of Kansas, as depicted in the Geologic Map of Kansas (Moore and Landes, 1937). Formations such as Cheyenne, Kiowa, and Dakota are grouped together, as are Greenhorn and Graneros. The Carlile, Niobrara, and Pierre formations are shown separately, along with a small area of Dockum? (Triassic) in the state’s southwestern corner.

• Subcrop Map: Figure 92 is partly a subcrop map, indicating the boundaries of various units in their current positions, regardless of Cenozoic cover.
• Outliers: Most Cretaceous formations have outliers east of the main outcrop belt, suggesting they once extended farther east, possibly to the flanks of the Ozarks.
• Pennsylvanian and Permian Systems: All units of the Pennsylvanian and Permian Systems must have extended farther eastward than they do now, though the extent of their westward retreat is uncertain.
• Erosion: Enormous quantities of rock material were removed from Kansas during Cenozoic time, contributing to Tertiary deposits of the Gulf Coast.

4. What Does the Pre-Cretaceous Post-Jurassic Surface Reveal About Past Environments?

The pre-Cretaceous post-Jurassic surface, entirely or partly pre-Cretaceous and post-Jurassic, is depicted in Figure 93. It is primarily a subcrop map, except in eastern Kansas where Cretaceous deposits are absent. The Permian-Pennsylvanian contact is shifted eastward to reflect its downdip migration since Jurassic time.

• Triassic Extent: The Triassic is shown extending beneath Jurassic deposits in southwestern Kansas, with its southeastern extent likely reduced by erosion.
• Lithological Evidence: Upper Cretaceous units show no evidence of shoreline or near-shore features, suggesting they extended farther east, though the exact distance is unknown.

5. Why Is the Pre-Triassic Post-Permian Surface of Major Importance?

The pre-Triassic post-Permian surface is of major importance due to abrupt changes occurring near the end of Permian time, which are reflected in the distribution of Paleozoic units. Figure 94 illustrates the sub-Mesozoic surface in western and central Kansas, where Mesozoic sediments are present, and an inferred pre-Mesozoic surface in eastern Kansas.

• Structural Influence: The outcrop pattern of Permian formations in western Kansas is controlled by the configuration of the Hugoton Embayment of the Anadarko Basin.
• Dip Analysis: Levorsen (1960) estimated the dip on the westward-dipping homocline in central Kansas to be approximately 7 feet per mile at the time of Mesozoic coverage.
• Outcrop Trends: In eastern Kansas, outcrop lines trend approximately north-south with gentle westward dips, reflecting the structure of the Ozarks and the Hugoton Embayment.

6. How Does the Pre-Pennsylvanian Post-Mississippian Surface Define Structural Provinces?

The pre-Pennsylvanian post-Mississippian surface is perhaps the most significant buried unconformity in Kansas. Its subcrop pattern defines the primary structural and petroliferous provinces in Kansas, resulting from crustal disturbances between mid-Mississippian and mid-Pennsylvanian time.

• Subcrop Recognition: It is possible to identify subcrops of Mississippian, Devonian, Silurian, Maquoketa, Viola, Simpson, Arbuckle, and Precambrian rocks on this surface (Fig. 95).
• Mississippian Distribution: Mississippian rocks are widespread beneath the Pennsylvanian, except on upwarped areas of the Cambridge Arch, Central Kansas Uplift, Pratt Anticline, and Nemaha Anticline.
• Precambrian Uplifts: Many areas of Precambrian rock underlie the Pennsylvanian in the cores of uplifts, often bounded by faults.
• Erosion and Sedimentation: Present distribution suggests Mississippian units once covered the entire state and have since been eroded from the uplifts (Fig. 96), with the resulting sediment forming lower Pennsylvanian strata.
• Basinal Distribution: Younger Mississippian rocks are found in the deepest parts of basins, with successively older beds encountered outward from the basinal centers.
• Controversial Cowley Formation: The area of the controversial Cowley Formation is shown in southern Kansas (Fig. 96), with its stratigraphic placement remaining problematic.

7. What Is the Significance of the Pre-Mississippian Post-Devonian Surface?

The Chattanooga Shale’s age in Kansas is debated as Devonian, Mississippian, or both. Due to the unconformity separating it from older rocks and its conformable relation with overlying rocks, it is convenient to consider it with the Mississippian. Figure 97 shows the distribution of rocks directly below the Chattanooga Shale or, where absent, next below Mississippian limestone.

• Structural Features: Crustal movements prior to Mississippian deposition significantly affected Kansas geology, influencing the Chautauqua Arch, ancestral Central Kansas Uplift, North Kansas Basin, and Southwest Kansas Basin.
• Inferential Distribution: No Mississippian or Chattanooga rocks remain on the northern part of the Nemaha Anticline or the Central Kansas Uplift structures, making the distribution of rocks on the pre-Mississippian surface inferential.
• Central Kansas Arch: The Central Kansas Arch is sometimes used to describe the combined ancestral Central Kansas Uplift and Chautauqua Arch, connected by a relatively lower positive element.

8. How Does the Pre-Paleozoic Post-Precambrian Surface Influence Paleozoic Formations?

The distribution of Precambrian rock types below the Paleozoic-Precambrian unconformity in Kansas has been discussed previously and illustrated in Figure 87. The configuration of this surface is explored under Present Structure.

• Paleozoic Overlap: Figure 98 shows Paleozoic formations resting on Precambrian rocks, including Arbuckle-Reagan (Cambrian-Ordovician), Simpson and Viola (Ordovician), and Cherokee, Marmaton, and Lansing-Kansas City (Pennsylvanian) rocks.
• “Worm’s Eye” Map: Levorsen (1960, p. 18) termed this type of map a “worm’s eye” or “lap-out” map, while the Paleotectonic Map Project uses “suprageologic.”
• Uplift and Erosion: Where formations younger than Arbuckle-Reagan are in contact with the Precambrian, it indicates either non-deposition of Arbuckle-Reagan rocks or their subsequent uplift and erosion.
• Buried Hills: Walters (1946) detailed the history of buried hills in central Kansas, noting that Precambrian rocks were locally exposed until Lansing-Kansas City time, when they were finally covered.

9. How Incomplete Is the Kansas Geologic Record?

Understanding geologic history requires preserved and interpreted rocks, including their organic remains. A complete rock record is ideal, but unattainable. By comparing the relative ages of rocks in Kansas with those elsewhere, it is possible to estimate the amount of missing rock record.

• Sequence Interpretation: The sequence of events is based on the relative positions of rock units. In the absence of exact age determinations, the amount of missing record can only be vaguely estimated.
• Correlation Charts: Geological correlation charts published by the Geological Society of America were used to compile data showing which geologic time divisions are represented in Kansas (Fig. 99).
• Missing Series: Many series are incompletely represented, including Lower and Middle Cambrian, Cayugan (Upper Silurian), Lower Devonian, Ochoan (Upper Permian), Lower and Middle Triassic, Lower and Middle Jurassic, and Paleocene, Eocene, and Oligocene (Tertiary).

10. Which Stratigraphic Units Are Missing from the Kansas Record?

Several series are not represented within Kansas, although all major divisions of the rock systems are present.

• Completely Absent Series: Lower and Middle Cambrian, Cayugan (Upper Silurian), Lower Devonian, Ochoan (Upper Permian), Lower and Middle Triassic, Lower and Middle Jurassic, and Paleocene, Eocene, and Oligocene (Tertiary) series are completely absent, represented only by unconformities.
• Incompletely Represented Series: Cincinnatian (Upper Ordovician), Upper Devonian, Upper Triassic, Comanchean (Lower Cretaceous), and Miocene (Tertiary) series are very incompletely represented.
• Most Complete Section: The most nearly complete rock section extends from the base of the Cherokee Group (Desmoinesian) to the top of the “Taloga Formation” (Guadalupian), though even this contains many disconformities.

11. How Do Geologic Time and Rock Thickness Relate in Kansas?

Radioactivity can now determine the age of rocks with reasonable accuracy, allowing for estimation of each geologic system’s duration (Table 4). New procedures have refined the geologic time scale (Kulp, 1961).

• Time Scale: Holmes (1959) presented composite maximum known thicknesses: Quaternary (6,000 feet), Tertiary (104,000 feet), Mesozoic (125,000 feet), and Paleozoic (217,000 feet), totaling about 452,000 feet for post-Precambrian rocks.
• Kansas Thickness: Approximate thicknesses in Kansas are: Quaternary (1,000 feet), Tertiary (800 feet), Mesozoic (3,650 feet), Paleozoic (10,500 feet), totaling about 16,000 feet. The thickest known section in Kansas is about 9,500 feet in the Hugoton Embayment.
• Unrepresented Time: By assuming each series represents one-third of its system’s duration, about 208 million years of geologic history are not represented in Kansas. Including incompletely represented series, approximately 290 million years, or half of post-Precambrian time, is unrepresented.

12. How Can Unrepresented Time Be Determined in Geological Records?

Estimating the amount of time not represented by the rock section in a given area can be done through several methods, though they only provide estimates.

• Ratio of Rock Thickness: Determining the ratio of total known rock thickness to thickness in the area concerned is unreliable due to varying sediment-accumulation rates.
• Subsidence Rates: Kay (1955) concluded that subsidence rates rarely exceeded 1,000 feet per million years, likely too high for Kansas.
• Sedimentation Rates: Using rates of sedimentation of different rock types in Kansas is also approximate (Barrell, 1917). Pettijohn (1949, p. 471) noted that sedimentation is seldom continuous.
• Shale Accumulation: Shales average about 0.5 mm per year or 700 years per foot.
• Cyclic Characteristics: Pennsylvanian and lower Permian rocks in eastern Kansas exhibit cyclic characteristics. Comparing the number of cycles to the total section can estimate the time required for each cycle.
• Cyclothem Analysis: Weller (1930) determined that an average cycle in Illinois was completed in about 400,000 years. About 35 percent of the sedimentary rocks in the upper Pennsylvanian are not represented.
• Basin vs. Uplift: The downwarped basins in Kansas are larger than the uparched areas by a ratio of about 6:1, with a larger volume of sediment. The most complete records are near the centers of the Forest City Basin, Cherokee Basin, Salina Basin, Sedgwick Basin, and the Hugoton Embayment.

13. What Factors Influence Sedimentation Rates?

Sedimentation rates are influenced by various factors, making it challenging to estimate precise accumulation times.

• Continuous vs. Intermittent Sedimentation: Sedimentation is seldom continuous, subject to numerous interruptions (Pettijohn, 1949).
• Rock Type Variations: Shales accumulate at rates greater than limestone but less than sandstones.
• Erosion and Non-deposition: Unconformities and disconformities indicate periods of erosion or non-deposition.
• Basin Subsidence: Subsidence rates affect sediment accumulation, though rates are variable and may not accurately reflect sedimentation time.

14. How Do Cyclic Sedimentary Patterns Help Estimate Missing Time?

Cyclic sedimentary patterns, particularly in Pennsylvanian and lower Permian rocks, can provide insights into missing time by comparing ideal cycles with what is actually present.

• Cyclothem Completeness: By analyzing the completeness of cyclothems, the amount of time not represented in the sedimentary record can be estimated.
• Average Cycle Duration: Comparing the number of cycles to the total section allows calculation of the average time for each cycle’s development.
• Unrepresented Time: By assuming each unit of the cycle required the same amount of time, the time not represented can be calculated, though this is not strictly correct due to varying development times and unrecorded cyclothems.

15. Can Basin Analysis Improve Estimates of Unrepresented Time?

Basin analysis can improve estimates of unrepresented time by examining the completeness of the rock record in different structural provinces.

• Basin Completeness: The rock record is most complete near the centers of basins such as the Forest City Basin, Cherokee Basin, Salina Basin, Sedgwick Basin, and the Hugoton Embayment.
• Structural Provinces: The proportion of time not represented by the rock record varies across structural provinces, with basins generally having more complete records than uplifts.
• Sediment Volume: Downwarped basins contain larger sediment volumes compared to uparched areas, reflecting more continuous deposition.

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