Question: A profile (clay soil) was excavated to a depth of 100 cm in the north of Tunisia. We have found limestone nodules, colluvium and alluvium. What I would like to know is the nature of the bedrock of this soil. Can we assert that there is a limestone bedrock?

Answer: 
There are three possible sources for the reported limestone nodules from the clay horizon from colluvium and alluvium. First, as noted before, they could be either detrital gravel of limestone either eroded from limestone bedrock. Given that the bedrock source from which this gravel was eroded and transported from could be either local or regional, such gravels tells a person nothing about the composition of the bedrock at your site. Second, the "limestone nodules" might also be limestone gravel that was either brought in either as construction material or eroded and redeposited from larger limestone blocks at some ancient ruin. Again, such gravel could come from any source within either the local area or region. This means that the presence of such material tells a person nothing about of what the local bedrock consists.

Finally, it is interesting that the "limestone nodules" occur in a clay layer. Normally, gravel-size pieces of limestone are not deposited together with clay. This and the clay layer being part of a soil suggest that the limestone nodules" are caliche nodules / concretions that precipitated and formed in place within the soil as part of a Petrocalcic “K” horizon of a soil developed in clayey colluvium and alluvium. Since caliche (calcrete) forms in Quaternary colluvium and alluvium independently of the type of local bedrock, the presence of caliche tells a person nothing about the local bedrock might be. The Soil Map of Tunisia at <https://library.wur.nl/isric/kaart/origineel/afr_tnped.jpg> might tell a person whether such soils are present within the area. The presence of "limestone nodules" does not indicate what the local bedrock might be composed of as there are multiple possible origins for them that are independent of the underlying bedrock.

A possibly useful map is:

Ben Haj Ali, M., Jedoui, Y., Dali, T., Ben Salem, H. & Memmi, L. (1985): Carte géologique de la Tunisie, echelle 1:500,000.– Office Topographie Cartographie; Tunis

Petrocalcic Horizon - <https://en.wikipedia.org/wiki/Petrocalcic_Horizon>

caliche / calcrete - <https://en.wikipedia.org/wiki/Caliche>
I can offer a counter-example. What if your soil sample is taken from the site of some lost ancient Roman buildings? The Romans built plenty in northern Tunisia, and like using [travertine limestone](http://tigerprints.clemson.edu/cgi/viewcontent.cgi?article=1909&context=all_theses). A few long-destroyed buildings would explain the presence of limestone nodules in the soil while saying nothing about the bedrock.

The point is there are many possible sources of limestone that are not the underlying bedrock. I don't think you can make that assumption.