Here is my hypothesis: a fossilization in two times on the principle of the plaster moulding.
It would be a cone buried at the bottom of the Turonian sea and more precisely in the mud where it would have left part of its external imprint before decomposing.
The void left by the decomposition of the cone would have been filled by a new supply of calcareous mud. One can imagine that the walls of this cavity retained a thin layer of the organic matter that made up the cone, which would have prevented the external sediment and the internal sediment from welding together during the formation of the tuffeau (this one resulting from the transformation of calcareous mud into rock, by increasing the heat and the pressure generated by the burial under multiple layers of new sediments).
Millions of years later, the erosion cleared the tuffeau and freed this imprint in relief which reproduces the shape of the old cone.
The third photo accredits, in my opinion, the hypothesis of the supply of a new sediment in the cavity left by the decomposition of the cone. It should also be noted that less than half of the imprint of the cone has been preserved.
Among other things, the semblance of a peduncle at the top reminds me about a cycadal cone. But maybe you have another hypothesis to propose (sponge, crinoid, ...)? Don't hesitate to write me a personal message.
