of North America) using data from drill-holes and from rock surface exposures to correlate and align rock layers in all the local sequences across North America (Lindberg 1986). The outcome was an overwhelming confirmation that the megasequences, as well as the erosional discontinuities between them, as described by Sloss two decades earlier, indeed were real. Subsequent investigations show that these megasequences are global in extent (Clarey and Werner 2017; Clarey 2020). A major question associated with the discovery of this large-scale structure in the fossil-bearing stratigraphic record pertains to the mechanism responsible for generating the erosional unconformities separating the megasequences. The secular community’s answer partly involves alternating periods of falling and rising sea level driven by fluctuations in the rate of seafloor spreading and consequent changes in the amount of warm, buoyant lithosphere adjacent to the mid-ocean ridges. During periods of more rapid seafloor spreading, the ocean bottom near the ridges is relatively higher due to the warmer rock beneath, the ocean basin volume is thereby reduced, and the global sea level is higher. During periods of relatively slower seafloor spreading the opposite prevails. The other part of the answer involves changes in what is known as dynamic topography that is driven by flow of rock within the mantle. In the framework of the Genesis Flood and the rapid plate tectonics which it implies, fluctuations in the cooling rate of the new ocean lithosphere being generated by rapid seafloor spreading becomes a serious candidate explanation. As emphasized in the preceding section, a mechanism of enhanced cooling beyond that of thermal conduction seems to be a logical necessity. Allowing this mechanism to occur in pulses instead of in a uniform manner would plausibly account for sudden drops in sea level and global episodes of erosion of the continental sediments. Let us now explore that possibility in a quantitative manner to address the questions of how much sea level rise is implied by CPT and how large must the sea level drops be? As mentioned above, 50,000 km of active subduction zones and an average plate speed of 2 m/s, implies a rate of creation of new ocean floor of (50,000 km) x (0.002 km/s) = 100 km2/s or 8.64x106 km2/ day. If we assume that the thermal structure of subducting ocean lithosphere and also for newly forming lithosphere at a mid-ocean ridge to be similar to that of today, we infer a difference in sea bottom height from near a mid-ocean ridge and near a subduction zone of about 1,500 m (Solomon and Toomey 1992). From these numbers we can compute an estimate for the rate of ocean volume change arising from the CPT creation and subduction of ocean lithosphere. That rate is (8.64x106 km2/day) x (1,500 m x 1 km/1,000 m) = 1.3x107 km3/day. One can express this rate of ocean volume change in terms of a rate of change in average sea bottom height. Since the deep ocean basins cover some 70% of earth’s total surface area of 5.1x108 km2, the rate of average sea bottom uplift corresponding to a global rate of ocean volume change of (1.3x107 km3/day) is given by (1.3x107 km3/day) / (0.7 x 5.1x108 km2) = 0.036 km/day = 36 m/day. Now let us consider the sort of sea bottom drops from episodes of lithospheric cooling that might be appropriate. Over a time span of 150 days, the total cumulative amount of sea bottom rise without cooling is (36 m/day) x (150 days) = 5,400 m. To account for all the erosional unconformities, we require seven episodes of sea level drop, one at the beginning, five separating the six megasequences, and one at the end. If for simplicity we make all the episodes of equal size, we find that by choosing each to result in a 700 m drop in the average sea bottom height, we account for 7 x 700 = 4,900 m, or most of the total sea bottom drop needed to match the total sea bottom uplift. To make these totals match, we include an additional 500 m to the final drop to make that drop, the one responsible for the Flood runoff, equal to a total of 1,200 m. Just prior to that final cooling episode beginning at day 150, the mean global sea level reaches its maximum value of 995 m. This is very close to the maximum height assumed in the initial topography as displayed in Fig. 5. We have incorporated these features into the MABBUL software. We model the first six of these sea level drops to occur at 25-day intervals beginning at time zero, with each unfolding during a 24-hour period. The final drop begins at a time of 150 days at a rate of 48 m/day and extends over 25 days, corresponding to the runoff stage of the Flood. Motivated by the large amount of seafloor with later Cenozoic age as well as flat-topped guyots also of later Cenozoic age, we include in the model ongoing seafloor spreading and tsunami generation between days 175 and 220, during which the resulting sea bottom rise is exactly compensated by sea bottom drop from cooling of the oceanic lithosphere. This results in a sea level at the end of 220 days that matches the modern one. H. Candidate mechanism for the Great Unconformity The case just presented for large pulses of cooling of the ocean lithosphere during the Flood brings with it a compelling candidate mechanism for the Great Unconformity. The Great Unconformity is the erosional discontinuity that marks the base of the Sauk Megasequence. This boundary is noteworthy in that it coincides with the abrupt appearance of fossils of multicellular organisms in the earth’s rock record. As such it logically represents the onset of the Genesis Flood. In many places this erosional boundary displays evidence of extremely high-energy water flow. One example is in Devil’s Lake State Park in south central Wisconsin as displayed in Fig. 6. Figure 5. Initial topography assumed for the supercontinent Pannotia. BAUMGARDNER AND NAVARRO Large tsunamis and Flood sediment record 2023 ICC 373
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