Quark = curd?
Yeah, it's a kind of light curd cheese. Check out the link below for some more input from both sides of the Atlantic, but it seems you could substitute creme/fromage fraiche, light ricotta, or maybe cream cheese if need be. Check out a European market if there are any around, but I don't see why any of the suggested subs, especially greek yogurt or neufchatel, wouldn't work. Never worked with it myself though, just enjoyed it's delicious creations.