I think the answer must be "no."
It is my understanding that no sin, no matter how grave, is ever "automatically" a mortal sin. That is because a mortal sin not only depends on the act involving grave matter, but it also depends on it being done with full knowledge and complete consent.
So, for example, murder is clearly a violation of the natural law, and is a grave sin. However, there may be circumstances when it is not committed with complete consent, and would therefore not be a mortal sin.
It may be accurate to say that all violations of the natural law are potentially mortal sins, but I leave that to other to determine.