MEGA-TSUNAMI DEPOSIT AT K/T BOUNDARY IN SOUTHEAST MISSOURI PORTION OF MISSISSIPPI EMBAYMENT

Recent trenching at a clay mine in southeast Missouri has exposed an unusual section of upper Cretaceous through lower Paleocene marine strata. The mine is located in the bootheel of Missouri along Crowley’s Ridge, an erosional remnant that contains latest Cretaceous and Paleogene marine sediments deposited in the northwestern portion of the Mississippi Embayment.

The mine floor rests on earliest Paleocene Clayton formation. Four trenches were excavated through Clayton clay and into Cretaceous Owl Creek formation. The deepest trench exceeded five meters and penetrated into Cretaceous McNairy formation. Each trench included an unusual section of Paleocene Clayton formation.

Paleocene Clayton formation is 185 cm thick and may be divided into four zones. The lowest zone is a 30 cm thick carbonate coquina containing a rich assortment of invertebrate and some vertebrate fossils. It is overlain by a 70 cm glauconitic sandy clay containing many fossils. Next up is a non-fossiliferous 65 cm gray clay and topped off with a 20 cm of non-fossiliferous indurated gray clay. Lowest coquina zone appears to have been deposited in scours within Cretaceous Owl Creek formation. Coquina filled scours have a boudinage appearance, are uniformly 30 cm thick by 30 cm to 50 cm across and are separated by 20 cm of dark clay. Scour tops are level.

One possible interpretation of this sequence is that it represents a very thick graded deposit resulting from a mega-tsunami generated by submarine slumps in the Gulf of Mexico following the Chicxulub impact event. Mississippi Embayment would have been a natural focus for a mega-tsunami bore ripping up sediment as it drove north from the Gulf. Some of those sediments contained microtektite layers.

Coquina zone resulted from heaviest elements settling out first, including rip-up clasts containing microtektites. The very green glauconitic clay above the coquina zone resulted from an organic rich fraction that settled out next. It is interesting to note that the fossil molds are of invertebrates that may have floated such as Turitella and ammonites.

Last to settle out were finest clay size particles and no fossils. A subsequent stillstand allowed some induration to occur before resumption of a transgressive phase resulting in the deposition of more than 50 meters of Porters Creek clay.